RidgeRun Auto exposure/Auto white balance library for DM368 and DM365

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= Introduction =
= Introduction =
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Video capture quality can be enhance with image processing, like auto white balance(AWB) and auto exposure algorithms(AE):
 
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*Auto exposure performs automatic adjustments of the image brightness according to the amount of light that reaches the camera sensor.
 
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*Auto white balance automatically compensates color differences based on lighting so white actually appears white.
 
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Some camera sensors don't include auto white balance and/or auto exposure processing, so RidgeRun offers a library with AE and AWB algorithms called librraew. This library has been developed for the DM365 platform. The DM365 VPFE has a module H3A designed to support control loops for auto focus, auto white balance and auto exposure by collecting statistics about the imaging/video data. There are two blocks in this module:
+
The DM365 and DM368 support the H3A hardware accelerator for supporting auto white balance (AWB) and auto exposure (AE).  CMOS or CCD sensor video capture quality can be enhanced with AWB and AE image processing:
-
* Auto focus engine
+
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* Auto exposure and auto white balance engine
+
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The librraew only use the auto exposure and auto white balance engine. This engine divides the frames into two dimensional blocks of pixels referred as windows. The engine can provide some image/video metrics:
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* Auto exposure performs automatic adjustments of the image brightness according to the amount of light that reaches the camera sensor.
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* Accumulation of clipped pixels along with all non-saturated pixels in each window per color.
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* Auto white balance automatically compensates color differences based on lighting so white actually appears white.
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* Accumulation of the sum of squared pixels per color.
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* Minimum and maximum pixels values in each window per color.
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The  AE/AWB engine can be configured to use up to 36 horizontal windows with sum + {sum of squares or min+max} output or up to 56 horizontal windows with sum output. Also can be configure to use up to 128 vertical windows. The width and height for the windows is programmable.
+
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By now, the librraew has only been tested  for the camera sensor mt9p031 but if you provide the appropriate functions for the library, it can works with any sensor. It is a plain C library and can be re-used with and integrated with any application. RidgeRun uses ipiped (see below) for testing and demonstration.
+
Some camera sensors don't include auto white balance and/or auto exposure processing, so RidgeRun offers a library with AE and AWB algorithms called '''librraew'''. This library was initially developed for the DM365/DM368 (DM36x) platform.  The DM36x video processing front end (VPFE) has an H3A engine designed to support control loops for auto focus, auto white balance and auto exposure by collecting statistics about the imaging/video data. There are two blocks in this module:
-
= Algorithms =
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* Auto focus engine
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== Auto white balance ==
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* Auto exposure and auto white balance engine
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When an image of a scene is captured by a digital camera sensor, the sensor response at each pixel depends on the illumination. Depending of the scene illumination, a distinct color cast appears over the captured scene. This effect appears in the captured image due to the color temperature of the light. If a white object is illuminated with a low color temperature light source, the object in the captured image will be reddish. Similarly, when the white object is illuminated with a hight color temperature light source, the object in the captured image will be bluish. The human eye is capable of compensate this color cast automatically through a characteristic known as color constancy, allowing the colors to be independent of the illumination. Auto white balance tries to simulate the color constancy for images capture.
+
-
 
+
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Many AWB algorithms follow a two-stage process:
+
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* Illumination estimation: can be done explicitly by chosen from a known set possible illuminations or implicitly with assumptions about the effect of such illuminations. The algorithms implemented in librraew use the implicitly estimation.     
+
-
* Image color correction: is achieved through an independent gain adjustment of the three color signals. Commonly only the blue an red gains are adjusted.
+
-
Bellow a brief description of the AWB algorithms for the librraew is presented:
+
The librraew library only uses the auto exposure and auto white balance hardware engine which requires the video frames to be in the Bayer color space. The DM36x does not allow the H3A engine to be used when the color space is YCbCr, which is common if you are using NTSC/PAL composite video input.
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;Gray World
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:The Gray World algorithm is based on the assumption that given an image with sufficient amount of color variation, the average reflectance of the scene is achromatic(gray). To be achromatic, the mean of the red, green, and blue channels in a given scene should be roughly equal. If the scene has a color that dominates, the results of the algorithm may not be satisfactory.
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;White Patch
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The H3A engine divides the frames into two dimensional blocks of pixels referred as windows. The engine provides image/video metrics:
-
:The White Patch algorithm assumes that the maximum response in an image is caused by a perfect reflector, that represents the color of the illumination. This white balancing attempts to equalize the maximum value of the three channels to produce a white patch. This algorithm must be used on scenes that ensure the captured image does not include saturated pixels.
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;White Patch 2
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* Accumulation of clipped pixels along with all non-saturated pixels in each window on a per color basis.
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:This is a variation of the White Patch that aims to resolve the problems with the saturated pixels. Uses an average of local maximums instead of absolute maximum. The results can change depending of the amount of windows in the image.
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* Accumulation of the sum of squared pixels per color.
 +
* Minimum and maximum pixels values in each window on a per color basis.
-
== Auto exposure ==
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The DM36x H3A engine can be configured to use up to 36 horizontal windows with sum + {sum of squares or min+max} output or up to 56 horizontal windows with sum output. The H3A engine can also be configure to use up to 128 vertical windows. The width and height for the windows is programmable.
-
One of the main problems affecting image quality, leading to unpleasant pictures, comes from improper exposure to light. The exposure is the amount of light that reaches the image sensor. So, the exposure determines how light or dark  the resulting image/video will be. If too much light strikes the image sensor, the image will be overexposed-washed out and faded. If too little light reaches the camera sensor produces an underexposed image, dark and lacking in details specially in shadow areas. The auto exposure algorithms tries the image be captured with a the best exposure. The best exposure is the exposure that enables to reproduce the most important regions (according to contextual or perceptive criteria) with a level of brightness, more or less in the middle of the possible range.
+
-
The AE algorithms process involves three process:
+
The librraew library was tested using an Aptina MT9P031 CMOS sensor. Support for other sensors was added later, thus validating the librraew design. If you provide the appropriate sensor-specific functions for the library, it can work with any sensor. The implementation is a plain C library and can be re-used with and integrated with any application capable of making C function calls. [https://github.com/RidgeRun/ipiped/wiki Image Pipe Daemon] uses librraew to provide auto exposure/Auto white balance.
-
* Light metering: its generally accomplished using the camera sensor itself or an external device as exposure detector.  
+
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* Scene a analysis: use brightness metering methods to estimate the scene illumination according to image metrics. Using this value, calculates the brightness adjustments to have the best exposure.
+
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* Image brightness correction: to ensure that the correct amount of light reaches the sensor image sensor parameters concerning to illumination and shutter time are adjusted. The CMOS sensor parameter is often called exposure time. The exposure time is defined as the amount of time that the sensor integrates light. In other words, it determines how long the sensor photo diodes array is exposed to light.  
+
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Librraew includes one AE algorithm that can use five brightness metering methods. The algorithm uses an electronic-centric approach based on the mid-tone idea. This algorithm uses metrics gotten for the sensor capture image as light metering. Calculates the required exposure time to get an optimal image brightness, using an expression that relates actual scene brightness, actual exposure time and the defined optimal image brightness. The optimal image brightness is defined as the value of the mid-tone.
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= License =
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All the metering systems define a pixel brightness as the average of the red, blue and green component. Bellow a brief description of the brightness metering methods is presented:
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RidgeRun auto-exposure/auto-white-balance library (C) Copyright 2010 - RidgeRun LLC.
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;Partial:
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:Averages the brightness of the pixels in a portion in the center of the frame, the rest of the frame is ignored. It is generally used when very bright or very dark areas on the edges of the frame would otherwise influence on a bad way the scene illumination.
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;Center weighted
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:Average the light information coming from the entire frame. Get brightness average of to regions: the pixels in a portion in the center and the pixels in the rest of the frame (background). The total brightness is calculated with emphasis placed on the center area. 75% of the total brightness is given by the center brightness and 30% by the background. This algorithm can be used when you want that the whole scene be well illuminated and not be affected for the small edges brightness variations. The subjects of the picture must be at the center of the image. But, if a back light is present into the scene the central part results darker than the rest of the scene and unpleasant underexposed foreground is produced.
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;Segmented:
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:This algorithm is designed for scenes that have a principal object in backlighting condition. Emphasize the luminance of the main object according to the degree of backlighting. Divides the frame on 6 pieces and weighting them.
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;Average
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:Average the light information coming from the entire frame without weighting any particular portion of the metered area. This algorithm can be used on scenes that not have a principal object and you want an average illumination. If the scene has a high contrast, the algorithm cause under or over exposure of parts of the scene.
+
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The center area for all the metering system is defined by a percentage of the image size and can be set by a librraew parameter.
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== Evaluation and Development License ==
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= Implementation Details =
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Subject to the terms and conditions of RidgeRun's SDK license, RidgeRun hereby grants to customer a product - based, non - exclusive, non - transferable, non - sublicensable, limited, worldwide license to install and use, for internal purposes only, an unlimited number of copies of the source and object code versions.
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Three applications are used to support the auto exposure and auto white-balance (AEW) adjustments in the RidgeRun's SDK:
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*Ipiped, a D-Bus server for controlling and configuring the camera sensor, the dm365 video processor and the aew library.
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*librraew, a library that includes auto white balance and auto exposure algorithms.
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*Ipipe-client, an D-Bus client that can be used to invoke any of the methods supported by the Ipiped.  
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== Running Ipiped ==
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== Distribution License ==
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Ipiped must run in background. If you are using RidgeRun's SDK and enabled ipiped it will be setup to start automatically when the system boots.
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-
<pre>
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-
ipiped &
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</pre>
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Ipiped registers with D-Bus and waits until ipipe-client requests to execute a method.
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Subject to the terms and conditions of RidgeRun's SDK license, RidgeRun hereby grants to customer a non-exclusive, non-transferable, non-sublicensable, limited, worldwide license to distribute RidgeRun Software in object code format only (no source code) in one product model sold by the customer.
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== Running Ipipe-client ==
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= Limitations =
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Ipipe-client is a Dbus client that use commands to invoke methods of the ipiped, so ipiped must be running to use ipipe-client. A command can required arguments depending of the functionality. Ipipe-client has two operation modes, you can ask to execute a single command or you can open an interactive console to execute a group of commands.   
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To execute a single command, you can use the following command line syntax 
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There are some not-so-obvious limitations when using the H3A engine:
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<pre>
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ipipe-client <command> <argument 1> ... <argument n>
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</pre>
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To get into the interactive console, you have to run ipipe-client without any command. Then to execute a command you only need to use the command and the required arguments.
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* AWB/AE correction limited to window sampling method listed above.
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<pre>
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* Can not use H3A engine with YCbCr color space, which includes NTSC/PAL composite video input.
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ipipe-client
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* Auto exposure can affect the video frame rate with dark images. Set a maximum exposure limit to keep the frame rate from dropping below an acceptable value.
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ipipe-client$ <command 1> <argument 1> ... <argument n>
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* Only tested with Linux 2.6.32 and the RidgeRun MT9P031 V4L2 driver.
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ipipe-client$ <command 2> <argument 1> ... <argument n>
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</pre>
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To quit the interactive console you can use quit or exit.
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In order to know the commands that are available run:
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= Algorithms =
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<pre>
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ipipe-client help
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</pre>
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or get into the interactive console and execute help.
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This shows a description of each command, as follows:
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== Auto white balance ==
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<pre>
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When an image of a scene is captured by a digital camera sensor, the sensor response at each pixel depends on the scene illumination. Depending of the illumination, a distinct color cast appears over the captured scene. This effect appears in the captured image due to the color temperature of the light. If a white object is illuminated with a low color temperature light source, the object in the captured image will have a reddish tint. Similarly, when the white object is illuminated with a high color temperature light source, the object in the captured image will appear somewhat blue instead of pure white. The human eye compensates for color cast automatically through a characteristic known as color constancy, allowing the colors to be independent of the illumination. Auto white balance tries to simulate the color constancy for captured images.
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Command                        Description
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 +
Many auto white balance algorithms follow a two-stage process:
 +
* Illumination estimation: this can be done explicitly by choosing from a known set of possible illuminations or implicitly with assumptions about the effect of such illuminations. The algorithms implemented in librraew use implicit estimation.     
 +
* Image color correction: this is achieved through an independent gain adjustment of the three color signals. Commonly only the blue and red gains are adjusted assuming the red gain is fixed.
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help                    Displays the help text for all the possible commands or a specific command.
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== Auto exposure ==
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set-debug              Enable/Disable debug messages.
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One of the main problems affecting image quality, leading to disappointing pictures, comes from improper light exposure. The image exposure is the amount of light that reaches the sensor. Exposure determines the lightness or darkness of the resulting image. If too much light strikes the image sensor, the image will be overexposed, washed out, and faded. If too little light reaches the camera sensor produces an underexposed image, dark and lacking in details especially in shadow areas. Auto exposure (AE) algorithms adjust the captured image in an attempt to reproduce the most important regions (according to contextual or perceptive criteria) with an average level of brightness, more or less in the middle of the possible range.
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init-aew                Initialize AEW algorithms.
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stop-aew                End AEW algorithm.
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shell                  Execute a shell command(shell_cmd) using interactive console.
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ping                    Show if ipipe-daemon is alive.
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quit                    Quit from the interactive console.
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exit                    Exit from the interactive console.
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get-video-processor    Show the video processor that is being used.
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get-sensor              Show the sensor that is being used.
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run-config-script      Execute a group of ipipe-client commands.
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set-previewer-mode      Configure previewer on continuous or one-shot mode.
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set-bayer-pattern      Sets R/Gr/Gb/B color pattern to the previewer.
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set-digital-gain        Sets red (R), green (G) and blue gains (G) on the ipipe.
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get-digital-gain        Returns the gain value for each color component(RGB).
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set-luminance          Brightness(Br) and contrast(C) adjustment.
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get-luminance          Returns the value of the Brightness(Br) and contrast(C) adjustment.
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flip-vertical          Flips the image vertically(on the sensor).
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flip-horizontal        Flips the image horizontally (on the sensor).
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set-exposure            Sets the effective shutter time  of the sensor for the light integration.
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get-exposure            Gets the exposure time of the sensor in us.
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set-sensor-gain        Sets red(R), green(G) and blue(B) gain directly on the sensor.
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get-sensor-gain        Gets sensor red(R), green(G) and blue(B).
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</pre>
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If you want more detailed information about a command execute:
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Auto exposure algorithms involves three processes:
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<pre>
+
* Light metering: this is generally accomplished using the camera sensor itself or an external device as exposure detector.
-
ipipe-client help <command>
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* Scene analysis: brightness metering methods use an estimation of the scene illumination according to image metrics. Using the oerall illumination value, brightness adjustments can be calculated to produce the best exposure.
-
</pre>
+
* Image brightness correction: this ensures that the correct amount of light reaches the image sensor by adjusting the illumination and shutter time parameters. The image sensor parameter is often called the exposure time. The exposure time is defined as the amount of time that the sensor integrates light. In other words, it determines how long the sensor photo diodes array is exposed to light.
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== Controlling librraew with ipipe ==
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= Documentation =
 +
* [[Librraew 1.1]] (current version)
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* [[Librraew 1.0]]
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Auto exposure and auto white balance adjustments can be started with an ipipe-client's command called init-aew. Init-aew requires some arguments to define the algorithms and other parameters. To see the arguments required you can request for help that show you the list as follows:
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= Using the demo version of librraew  =
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<pre>
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You can request a demo version of librraew in order to test the auto-white balance and auto-exposure algorithms to see if the technology meets your needs. This library will allow you to use all the features that comes with the full version of the librraew but with the following limitations:  
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Command: init-aew
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Syntax: init-aew <WB> <AE> <G> <EM> <T[us]> <fps> <seg> <width> <height>
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Description: Initialize AEW algorithms                                 
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Arguments:
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        WB: white balance algorithm, the options are:
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                G -for gray world algorithm
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                W -for retinex algorithm
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                W2 -for variant of retinex algorithm
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                N -for none
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        AE: auto exposure algorithm, the options are
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                EC -for electronic centric
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                N -for none
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        G: gain type, the options are:
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                S -to use the sensor gain
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                D -to use the digital
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        EM: exposure metering method, the options are:
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                P -for partial metering that take into account the light
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                information of a portion in the center and the rest of
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                the frame is ignored. The size  of the center depends of
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                of the parameter center_percentage
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                C -for center weighted metering that take into account
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                the light information coming from the entire frame with
+
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                emphasis placed on the center area
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                A -for average metering that take into account the light
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                information from the entire frame without weighting
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                SG -for segmented metering that divides the frame
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                on 6 pieces and weighting them to avoid backlighting
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        T: wait time in us, specifies the time between
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                algorithm adjustments, max value=1s=1000000us
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        fps: minimum frame rate
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        seg: frame segmentation factor, each frame is segmented into
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                regions, this factor represents the percentage of the
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                maximum number of possible regions
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        width: captured video/image horizontal size
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        height: captured video/image vertical size
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        center_percentage: defines the percentage of the image width
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                and height to be used as the center size
+
 +
*The algorithm will darken the image periodically.
 +
*After awhile the algorithm will stop working and the image capturing will be done with the last values calculated by the library. In order to test the library again you will need to restart the algorithm.
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</pre>
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[https://ridgerun.com/downloadcenter.shtml Go to download center]
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Also you can stop automatic adjustments with the command stop-aew
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[https://ridgerun.com/store/ Go to RidgeRun Store]
 +
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Some of the init-aew arguments need to be explained in more detail:
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= References =
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*T: the time between interactions defines how fast the algorithm can adjust the scene parameters. If you don't need fast changes you can use a greater time to get less CPU usage.
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*seg: this factor is related with the amount of CPU usage and the auto-adjustments precision. If you use a high segmentation percentage you will have greater CPU usage but you will get more precision on the adjustments.
+
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= Using librraew with other software rather than ipiped =
+
# Battiato, G. Messina, and A. Castorina. Exposure correction for imaging devices: an overview. In Single-Sensor Imaging: Methods and Applications for Digital Cameras, chapter 12. Rastislav Lukac, October 2008.
-
librraew is a plain C library and can be re-used and integrated with any custom application. Please contact RidgeRun for the documentation of the librraew API.
+
# Lee J.S., Jung Y.Y, Kim B.S., and Ko S.J. An advanced video camera system with robust af, ae, and awb control. IEEE Transactions on Consumer Electronics, 47:694–699, August 2001
 +
# Edmund Y. Lam. Combining gray world and retinex theory for automatic white balance in digital photography. Consumer Electronics, 2005. (ISCE 2005). Proceedings of the Ninth International Symposium on, pages 134–139, June 2005.
 +
# Edmund Y. Lam and George S. K. Fung. Automatic white balancing in digital photography. In Single-Sensor Imaging: Methods and Applications for Digital Cameras. Taylor & Francis Group, LLC, 2009.
 +
# Nitin Sampat, Shyam Venkataraman, Thomas Yeh, and Robert L. Kremens. System implications of implementing auto-exposure on consumer digital cameras. Proc. SPIE. Sensors, Cameras, and Applications for Digital Photography, 3650:100–107, March 1999
[[Category:Whitepaper]]
[[Category:Whitepaper]]

Revision as of 15:32, 9 April 2013

Contents

Introduction

The DM365 and DM368 support the H3A hardware accelerator for supporting auto white balance (AWB) and auto exposure (AE). CMOS or CCD sensor video capture quality can be enhanced with AWB and AE image processing:

Some camera sensors don't include auto white balance and/or auto exposure processing, so RidgeRun offers a library with AE and AWB algorithms called librraew. This library was initially developed for the DM365/DM368 (DM36x) platform. The DM36x video processing front end (VPFE) has an H3A engine designed to support control loops for auto focus, auto white balance and auto exposure by collecting statistics about the imaging/video data. There are two blocks in this module:

The librraew library only uses the auto exposure and auto white balance hardware engine which requires the video frames to be in the Bayer color space. The DM36x does not allow the H3A engine to be used when the color space is YCbCr, which is common if you are using NTSC/PAL composite video input.

The H3A engine divides the frames into two dimensional blocks of pixels referred as windows. The engine provides image/video metrics:

The DM36x H3A engine can be configured to use up to 36 horizontal windows with sum + {sum of squares or min+max} output or up to 56 horizontal windows with sum output. The H3A engine can also be configure to use up to 128 vertical windows. The width and height for the windows is programmable.

The librraew library was tested using an Aptina MT9P031 CMOS sensor. Support for other sensors was added later, thus validating the librraew design. If you provide the appropriate sensor-specific functions for the library, it can work with any sensor. The implementation is a plain C library and can be re-used with and integrated with any application capable of making C function calls. Image Pipe Daemon uses librraew to provide auto exposure/Auto white balance.

License

RidgeRun auto-exposure/auto-white-balance library (C) Copyright 2010 - RidgeRun LLC.

Evaluation and Development License

Subject to the terms and conditions of RidgeRun's SDK license, RidgeRun hereby grants to customer a product - based, non - exclusive, non - transferable, non - sublicensable, limited, worldwide license to install and use, for internal purposes only, an unlimited number of copies of the source and object code versions.

Distribution License

Subject to the terms and conditions of RidgeRun's SDK license, RidgeRun hereby grants to customer a non-exclusive, non-transferable, non-sublicensable, limited, worldwide license to distribute RidgeRun Software in object code format only (no source code) in one product model sold by the customer.

Limitations

There are some not-so-obvious limitations when using the H3A engine:

Algorithms

Auto white balance

When an image of a scene is captured by a digital camera sensor, the sensor response at each pixel depends on the scene illumination. Depending of the illumination, a distinct color cast appears over the captured scene. This effect appears in the captured image due to the color temperature of the light. If a white object is illuminated with a low color temperature light source, the object in the captured image will have a reddish tint. Similarly, when the white object is illuminated with a high color temperature light source, the object in the captured image will appear somewhat blue instead of pure white. The human eye compensates for color cast automatically through a characteristic known as color constancy, allowing the colors to be independent of the illumination. Auto white balance tries to simulate the color constancy for captured images.

Many auto white balance algorithms follow a two-stage process:

Auto exposure

One of the main problems affecting image quality, leading to disappointing pictures, comes from improper light exposure. The image exposure is the amount of light that reaches the sensor. Exposure determines the lightness or darkness of the resulting image. If too much light strikes the image sensor, the image will be overexposed, washed out, and faded. If too little light reaches the camera sensor produces an underexposed image, dark and lacking in details especially in shadow areas. Auto exposure (AE) algorithms adjust the captured image in an attempt to reproduce the most important regions (according to contextual or perceptive criteria) with an average level of brightness, more or less in the middle of the possible range.

Auto exposure algorithms involves three processes:

Documentation

Using the demo version of librraew

You can request a demo version of librraew in order to test the auto-white balance and auto-exposure algorithms to see if the technology meets your needs. This library will allow you to use all the features that comes with the full version of the librraew but with the following limitations:

Go to download center

Go to RidgeRun Store

References

  1. Battiato, G. Messina, and A. Castorina. Exposure correction for imaging devices: an overview. In Single-Sensor Imaging: Methods and Applications for Digital Cameras, chapter 12. Rastislav Lukac, October 2008.
  2. Lee J.S., Jung Y.Y, Kim B.S., and Ko S.J. An advanced video camera system with robust af, ae, and awb control. IEEE Transactions on Consumer Electronics, 47:694–699, August 2001
  3. Edmund Y. Lam. Combining gray world and retinex theory for automatic white balance in digital photography. Consumer Electronics, 2005. (ISCE 2005). Proceedings of the Ninth International Symposium on, pages 134–139, June 2005.
  4. Edmund Y. Lam and George S. K. Fung. Automatic white balancing in digital photography. In Single-Sensor Imaging: Methods and Applications for Digital Cameras. Taylor & Francis Group, LLC, 2009.
  5. Nitin Sampat, Shyam Venkataraman, Thomas Yeh, and Robert L. Kremens. System implications of implementing auto-exposure on consumer digital cameras. Proc. SPIE. Sensors, Cameras, and Applications for Digital Photography, 3650:100–107, March 1999
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