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Revision as of 07:46, 26 November 2008 by Kimstik (talk | contribs) (Image decoding)
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JP4 format

So we have added a special JP4 mode that bypasses the Demosaic in the FPGA and provides an image with pixels in each 16x16 macroblock that are rearranged to separate Bayer colors in individual 8x8 blocks, then encoded as monochrome. Demosaic will be applied during post-processing on the host PC. This section describe different algorithms and implementations used to provide this functionality.

Main goals:

- compression speed improvement
- possibility to obtain more high quality image (near to RAW)
- drasticaly lowering data size

Image decoding

JP4 format can be easy manipulated by Matlab
JP4 image

1. Read image

I=imread('hdr02.jp4'); %read JP4 file like JPEG
  ,or online grab image from http like this:
  ,or cam:
I=imread('http://cam_ip/bimg'); %get online buffered image from cam
I=I(:,:,1);            %strip color data
2. Remove block grouping
Bayer CFA encoded image

II=deblock16x16(I);    %deblock image
%file deblock16x16.m
function y=deblock16x16(I)
for x=1:16:size(I,1)
  for y=1:16:size(I,2)
      for dx=0:7
        for dy=0:7
          y0(x+2*dx  ,y+2*dy)   = blk16(dx+1,dy+1);
          y0(x+2*dx+1,y+2*dy)   = blk16(dx+9,dy+1);
          y0(x+2*dx  ,y+2*dy+1) = blk16(dx+1,dy+9);
          y0(x+2*dx+1,y+2*dy+1) = blk16(dx+9,dy+9);

2. Demosaic image (Decode from Bayer CFA (Color Filter Array) encoded image)
Decoded image

3. Show image


Stream decoding

JP4 stream can be decoded by mplayer. Use [this patch] for glue libdc1394 and MPlayer-1.0rc2 video filter frontend. Or download win32 binaries from sourceforge.

usage example: mplayer.exe test.avi -vf demosaic=deblock=1:method=7
Debayer (Demosaic) algorithm variants provided by libdc1394:
- Nearest Neighbor : OpenCV library
- Bilinear         : OpenCV library
- HQLinear         : High-Quality Linear Interpolation For Demosaicing Of Bayer-Patterned
                     Color Images, by Henrique S. Malvar, Li-wei He, and Ross Cutler,    
                        in Proceedings of the ICASSP'04 Conference.                      
- Edge Sense II    : Laroche, Claude A. "Apparatus and method for adaptively interpolating
                     a full color image utilizing chrominance gradients"                  
                        U.S. Patent 5,373,322. Based on the code found on the website     
            Converted to C and adapted to 
                     all four elementary patterns.                                        
- Downsample       : "Known to the Ancients"                                              
- Simple           : Implemented from the information found in the manual of Allied Vision
                     Technologies (AVT) cameras.                                          
- VNG              : Variable Number of Gradients, a method described in                  
                     Sources import from DCRAW by Frederic Devernay. DCRAW is a RAW       
                     converter program by Dave Coffin. URL:                               
- AHD              : Adaptive Homogeneity-Directed Demosaicing Algorithm, by K. Hirakawa  
                     and T.W. Parks, IEEE Transactions on Image Processing, Vol. 14, Nr. 3,
                     March 2005, pp. 360 - 369.


Bayer pattern look like this

R G1 R G1 R G1
G2 B G2 B G2 B
R G1 R G1 R G1
G2 B G2 B G2 B
R G1 R G1 R G1
G2 B G2 B G2 B

Some sensors have possibility to set independed scale to G1 and G2. Considering that the accessible optics does not give the full permission of a sensor resolution, it can be used for increase in a dynamic range of a image sensor.