J420 (full range YUV 4:2:0) to ARGB conversion?

[Faraz Khan]

Hi guys,

First of all - thanks for libyuv and putting in the awesome ArgbToJ420 function! I find myself seriously missing the reverse (J420 to ARGB). We are a screensharing utility (Screenhero) and we rely on webrtc (and libyuv) heavily. On the mac we get around this by using openGl and a fragment shader - on windows we use libyuv for the I420 to ARGB conversion. I guess we could do the same in windows but if there is a magical way to get this functionality in libyuv it'll be great. I can help (with code) on this front but I dont really understand asm :\

Thanks guys!

[Frank Barchard]

Hi, I havent had time or strong need for the function, but it makes sense. 

The existing I420ToARGB code should easily adapt to J420ToARGB, given the new matrix values.

Mainly need to work out the coefficients.

We're really trying for jpeg compatible color space, hence the J, so the math is derived from jccolor.c

row_common.cc has comments about the J420 math:

// JPeg uses a variation on BT.601-1 full range
// y =  0.29900 * r + 0.58700 * g + 0.11400 * b
// u = -0.16874 * r - 0.33126 * g + 0.50000 * b  + center
// v =  0.50000 * r - 0.41869 * g - 0.08131 * b  + center

So the opposite would be in jdcolor.c

 *  R = Y                + 1.40200 * Cr
 *  G = Y - 0.34414 * Cb - 0.71414 * Cr
 *  B = Y + 1.77200 * Cb

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[Faraz Khan]

Frank - thanks! I think thats a good start. 

I'll give it a shot and let you know how that fares! I'm using the exact same matrix coeffs in my fragment shader (on the mac).

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Faraz Khan

Simple Collaboration Screensharing

www.screenhero.com