CL_FORMAT_YCbCr422DC and DM_IMAGE_PACKING_CbYCrYYY - Lurker's Guide - lurkertech.com
lurkertech.com Lurker's Guide CL_FORMAT_YCbCr422DC and DM_IMAGE_PACKING_CbYCrYYY

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CL_FORMAT_YCbCr422DC and DM_IMAGE_PACKING_CbYCrYYY

By Chris Pirazzi.

The section of the packings document called "Sampling Pattern Definitions" described the 4:2:0 sampling pattern.

There are two libraries which can represent data in this format: the the DM (digital media library), via DM_IMAGE_PACKING_CbYCrYYY, and the CL (Compression Library), via CL_FORMAT_YCbCr422DC (DC stands for "Duplicate Chroma"). The CL representation looks exactly like a 4:2:2 packing, except that each pair of image lines has identical Cr,Cb values. An example:

Say we have an image which is 4 pixels wide and 2 pixels high:

   1   2   3   4   
   5   6   7   8

Encoding that with the 420 DM_IMAGE_PACKING_CbYCrYYY packing, I'd get this in memory (increasing mem address):

   byte[ 0] = Cb(center of 1256) 
   byte[ 1] = Y(1) 
   byte[ 2] = Cr(center of 1256)
   byte[ 3] = Y(2) 
   byte[ 4] = Y(5)
   byte[ 5] = Y(6) 
   byte[ 6] = Cb(center of 3478) 
   byte[ 7] = Y(3) 
   byte[ 8] = Cr(center of 3478)
   byte[ 9] = Y(4) 
   byte[10] = Y(7)
   byte[11] = Y(8) 

With the CL_FORMAT_YCbCr422DC packing, what I get instead is this:

   byte[ 0] = Cb(center of 1256) 
   byte[ 1] = Y(1) 
   byte[ 2] = Cr(center of 1256)
   byte[ 3] = Y(2) 
   byte[ 4] = Cb(center of 3478) 
   byte[ 5] = Y(3) 
   byte[ 6] = Cr(center of 3478)
   byte[ 7] = Y(4) 
   byte[ 8] = Cb(center of 1256) AGAIN -- duplicate
   byte[ 9] = Y(5) 
   byte[10] = Cr(center of 1256) AGAIN -- duplicate
   byte[11] = Y(6) 
   byte[12] = Cb(center of 3478) AGAIN -- duplicate
   byte[13] = Y(7) 
   byte[14] = Cr(center of 3478) AGAIN -- duplicate
   byte[15] = Y(8) 
Say the image lines are numbered 0,1,2,3,4,.... If you are using CL_FORMAT_YCbCr422DC and the chroma on lines 0,2,4,6,... is not identical to that on lnies 1,3,5,7,..., then it is an error.

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CopyrightAll text and images copyright 1999-2017 Chris Pirazzi unless otherwise indicated.
Support This Site

I work on this site in my off hours. Please help me to push aside my day job and work on it more by supporting the site in one of these ways:
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Use your credit card or PayPal to donate in support of the site.
get anything from amazon.com
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Experience Thailand richly with my Talking Thai-English-Thai Phrasebook app.
get my chinese phrasebook app
Visit China easily with my Talking Chinese-English-Chinese Phrasebook app.
get thailand fever
I co-authored this bilingual cultural guide to Thai-Western romantic relationships.
Copyright

All text and images copyright 1999-2017 Chris Pirazzi unless otherwise indicated.