Demosaic Interpolator
Construct RGB pixel data from Bayer pattern pixels
- Library:
Vision HDL Toolbox / Conversions
Description
The Demosaic Interpolator block provides a Bayer pattern interpolation filter for streaming video data. The block implements the calculations using hardware-efficient, multiplier-free algorithms for HDL code generation. You can select a low-complexity bilinear interpolation, or a moderate-complexity gradient-corrected bilinear interpolation.
When you select bilinear interpolation, the block operates on a 3×3 pixel window using only additions and bit shifts.
When you select gradient correction, the block operates on a 5×5 pixel window. The calculation is performed using bit shift, addition, and low-order canonical signed digit (CSD) multiplication.
Ports
This block uses a streaming pixel interface with a
pixelcontrol bus for frame control signals. This interface enables the
block to operate independently of image size and format. All Vision HDL Toolbox™ blocks use the same streaming interface. The block accepts and returns a scalar
pixel value and a bus that contains five control signals. The control signals indicate the
validity of each pixel and its location in the frame. To convert a frame (pixel matrix) into a
serial pixel stream and control signals, use the Frame
To Pixels block. For a full description of the interface, see Streaming Pixel Interface.
Input
Output
Parameters
Interpolation algorithm — Algorithm used to calculate missing pixel values
Gradient-corrected linear (default) | Bilinear
Algorithm used to calculate missing pixel values, specified as one of the following:
Gradient-corrected linear— Bilinear average, corrected for intensity gradientBilinear— Average of the pixel values in the surrounding 3×3 neighborhood
Sensor alignment — Color sequence of the pixels in the input stream
RGGB (default) | GBRG | GRBG | BGGR
Select the sequence of R, G, and B pixels that correspond to the 2-by-2
block of pixels in the top-left corner of the input image. Specify the
sequence in left-to-right, top-to-bottom order. For instance, the default
sequence of RGGB represents an image with this pattern.
Line buffer size — Size of line memory buffer
2048 (default) | integer
Specify a power of two that accommodates the number of active pixels in a single horizontal line.
Choose a power of two that accommodates the number of active pixels in a horizontal line. If you specify a value that is not a power of two, the block uses the next largest power of two. The total memory size allocated depends on your selection of Interpolation algorithm:
Bilinear: 2-by-Line buffer size memory locationsGradient-corrected linear: 4-by-Line buffer size memory locations
Algorithms
The block pads the edges of the image with symmetric pixel values. For more details, see Edge Padding.
Latency
The block buffers one line of input pixels before starting bilinear interpolation calculations. The gradient correction calculation starts after the block buffers two lines.
The latency of the block is the line buffer latency plus the
latency of the kernel calculation. The line buffer latency includes edge padding by default. The
latency of the padding operation depends on the size of the kernel. If edge padding is not
necessary for your design, you can reduce the latency by setting the Padding
method parameter to None. When you use this option, the block
latency does not depend on your kernel size. To determine the exact latency for any
configuration of the block, measure the number of time steps between the input and output
control signals.
Note
When you use edge padding, use a horizontal blanking interval greater than twice the kernel width. This interval lets the block finish processing one line before it starts processing the next one, including adding padding pixels before and after the active pixels in the line. Standard streaming video formats use a horizontal blanking interval of about 25% of the frame width. This interval is much larger than the filters applied to each frame. When you disable edge padding, the horizontal blanking interval must be at least 12 cycles and is independent of the kernel size. If you are using a custom video format, set the horizontal blanking interval by using the Frame To Pixels block parameters. The horizontal blanking interval is equal to Total pixels per line – Active pixels per line or, equivalently, Front porch + Back porch. For more information, see Configure Blanking Intervals.
References
[1] Malvar, Henrique S., Li-wei He, and Ross Cutler. “High-Quality Linear Interpolation for Demosaicing of Bayer-Patterned Color Images.” Microsoft Research, May 2004. http://research.microsoft.com/pubs/102068/Demosaicing_ICASSP04.pdf.