Overview - 1.1 English

Sensor Demosaic LogiCORE IP Product Guide (PG286)

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Images captured by a CMOS/CCD image sensor are monochrome in nature. To generate a color image, three primary colors (typically red, green, and blue) are required for each pixel. Before the invention of color image sensors, the color image was created by superimposing three identical images with three different primary colors. These images were captured by placing different color filters in front of the sensor, allowing a certain bandwidth of the visible light to pass through.

Kodak scientist Dr. Bryce Bayer realized that an image sensor with a Color Filter Array (CFA) pattern would allow the reconstruction of all the colors of a scene from a single image capture. The color filter array is manufactured as part of the image sensor as a layer laid over the phototransistors. Example CFA patterns are shown in This Figure . These are called Bayer patterns and are used in most digital imaging systems.

Figure 1-1: RGB and CMY Bayer Sensor Demosaic Patterns

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The original data for each pixel contains information only about one color, based on which color filter is positioned over that pixel. However, information for all three primary colors is needed at each pixel to reconstruct a color image. Some missing information can be recreated from the information available in neighboring pixels. This process of recreating the missing color information is called color interpolation or demosaicing, and might require dedicated hardware to process the image data in real-time.

There is no exact method to fully recover the missing information, as color channels have been physically sub-sampled by the CFA before proper low-pass filtering takes place, which might lead to aliasing between color channels.

Perfect recovery of the original signal might not be possible; however, the aliasing can be suppressed significantly by capitalizing on the temporal and spatial redundancies and structured nature of natural images/video sequences.

A variety of simple interpolation methods, such as Pixel Replication, Nearest Neighbor Interpolation, Bilinear Interpolation, and Bicubic Interpolation have been widely used for CFA demosaicing. Simple methods usually compromise quality, and more elaborate methods require the use of an external frame buffer. The Sensor Demosaic core was designed to efficiently suppress interpolation artifacts, such as the zipper and color aliasing effects, by minimizing Chrominance Variances in a 5x5 neighborhood, as illustrated in This Figure .

Figure 1-2: Xilinx Sensor Demosaic Block Diagram

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Image sensor data sheets specify whether the starting position, pixel(0,0) of the Bayer sampling grid is on a red-green or a blue-green line, and whether or not the first pixel is green. The Sensor Demosaic IP core supports all four possible Bayer phase combinations.