Difference between revisions of "JP4"

Revision as of 14:38, 4 November 2009

Note: the JP4 mode described here is referred as "JP46" in current 8.0 firwmare

JP4 Format

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

Different JP4 Modes in 8.X Software

only modes 0-2 can be processed with standard libjpeg:

• 0 - mono6, monochrome (color YCbCr 4:2:0 with zeroed out color componets)
• 1 - color, YCbCr 4:2:0, 3x3 pixels
• 2 - jp46 - original JP4 (from 7.X software), encoded as 4:2:0 with zeroed color components
• 3 - jp46dc, modified jp46 so each color component uses individual DC diffenential encoding
• 4 - reserved for color with 5x5 conversion (not yet implemented)
• 5 - jp4 with ommitted color components (4:0:0)
• 6 - jp4dc, similar to jp46dc encoded as 4:0:0
• 7 - jp4diff, differential where (R-G), G, (G2-G) and (B-G) components are encoded as 4:0:0
• 8 - jp4hdr, (R-G), G, G2,(B-G) are encoded so G2 can be used with high gain
• 9 - jp4fiff2, (R-G)/2, G,(G2-G)/2, (B-G)/2 to avoid possible overflow in compressed values
• 10 - jp4hdr2, (R-G)/2, G,G2,(B-G)/2
• 14 - mono, monochrome with ommitted color components (4:0:0)

Image decoding

JP4 image

1. Read image

I=imread('hdr02.jp4'); %read JP4 file like JPEG
  ,or online grab image from http like this:
I=imread('http://community.elphel.com/pictures/jp4.jpg');
  ,or cam:
I=imread('http://cam_ip/bimg'); %get online buffered image from cam

I=I(:,:,1);            %strip color data

Bayer CFA encoded image

II=deblock16x16(I);    %deblock image

%file deblock16x16.m
function y=deblock16x16(I)
y0=uint8(zeros(size(I)));
for x=1:16:size(I,1)
  for y=1:16:size(I,2)
    blk16=I(x:x+15,y:y+15);
      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);
        end
      end
    end
  end
y=y0;

Decoded image

J=demosaic(II,'gbrg');

3. Show image

imshow(J);

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:pattern=3  -vo gl
mencoder example: mencoder.exe test.avi -ovc lavc -lavcopts vcodec=mjpeg -o output.avi -vf demosaic=deblock=1:method=1,scale

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     
                     http://www-ise.stanford.edu/~tingchen/ 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                  
                     http://www-ise.stanford.edu/~tingchen/algodep/vargra.html            
                     Sources import from DCRAW by Frederic Devernay. DCRAW is a RAW       
                     converter program by Dave Coffin. URL:                               
                     http://www.cybercom.net/~dcoffin/dcraw/                              
- 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.

Pattern codes: pattern=0..3 -> [RGGB, BGGR, GBRG, GRBG]

JP4 HDR

Bayer pattern look like this

The remark: all kinds of bayer patterns can be received from initial RGGB by flipping on X and/or Y.

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.

Avisynth plugin

Avisynth plugin also available

AVS script example:

LoadCPlugin("jp4.dll")
DirectShowSource("test.avi")
JP4("AHD","RGGB")

Demosaicing/debayering links

A Study of Spatial Color Interpolation Algorithms for Single-Detector Digital Cameras. Ting Chen / Stanford University

Source code:

eLynx Image Processing SDK and Lab
libdc1394
Efficient, high-quality Bayer demosaic filtering on GPUs
http://svn2.assembla.com/svn/ge/libgedrawing/trunk/src/ImageBayer.cpp

JP4 to DNG conversion

Credits: Dave Coffin

This Linux comanndline tool allows conversion of JP4 files into a DNGs that dcraw and Adobe Photoshop can open.

Download LibTIFF v3.8.2 a

• extract the tar.gz (this should create a new folder called "tiff-3.8.2")

Apply this patch: in terminal (first cd to path of libtiff.patch):

patch -p0 < libtiff.patch  

build LibTIFF:

cd tiff-3.8.2
./configure
make
sudo make install

Then compile this C program with:

gcc -o elphel_dng elphel_dng.c -ltiff

Then use the created application:

Usage: ./elphel_dng "gamma" "input.jpg" "output.dng"
Example: ./elphel_dng 100 example_JP4.jpeg example.dng

Example files:

• Example JP4
• Example DNG
• Example JPG (converted)

See also:

• Demosaic on client side
•  Elphel camera under the hood: from Verilog to PHP - on LinuxDevices.com