Mailinglist Archive: radeonhd (288 mails)

[bugzilla-daemon@xxxxxxxxxxxxxxx]

http://bugs.freedesktop.org/show_bug.cgi?id=22901

           Summary: radeonhd Xv doesn't scale Y'CbCr (YUV) properly + Rec709
           Product: xorg
           Version: unspecified
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/radeonhd
        AssignedTo: lverhaegen@xxxxxxx
        ReportedBy: syscrash2k@xxxxxxxxx
         QAContact: xorg-team@xxxxxxxxxxx


Created an attachment (id=27928)
 --> (http://bugs.freedesktop.org/attachment.cgi?id=27928)
patch to fix YUV scaling and use Rec709 coefficients

radeonhd uses certain constants in the conversion from YUV to RGB in the Xv
code.
However, there's a minor scaling bug where 256 is used instead of 255 as the
maximum value of an 8-bit unsigned.

In addition, the entries of the matrix A used to perform the following
computation
[Y; Cb; Cr] = A * [R; G; B]
should probably be those given in Rec 709 (see
http://en.wikipedia.org/wiki/Rec._709).

I've attached a patch which does this.

I determined the Y_mul, Y_shift, C_mul, and C_shift values using the equation
given in the code comments, N' = N * N_mul + N_shift. For Y, N' is in the range
[0,1], but for C, it's in the range [-0.5,0.5].
Here's an example for Y:
When N = 16/255, we should have N' = 0. When N = 235/255, we should have N' =
1. The constants Y_mul and Y_shift are easily obtained by solving the system of
two equations. Here's a script for Maxima (http://maxima.sourceforge.net/) that
computes the coefficients:

---

range_min: 0;
range_max: 1;

value_min: 16;
value_max: 235;

A: matrix([value_min/255,1],[value_max/255,1]);
invert(A) . matrix([range_min],[range_max]);

----

Running this script (maxima -b path-to-script-file) gives 255/219 and -16/219,
which are Y_mul and Y_shift. To obtain the values for C_mul and C_shift, we
just need to change range_min to -0.5, range_max to 0.5, and value_max to 240.

In order to determine the entries of the matrix A used for the YUV to RGB
conversion, I used the following equations (from
http://en.wikipedia.org/wiki/YCbCr):

Y' = Kr * R'    + (1 - Kr - Kb) * G' + Kb * B'
Pb = 0.5 * (B' - Y') / (1 - Kb)
Pr = 0.5 * (R' - Y') / (1 - Kr)
....................................................
R', G', B' in [0; 1]
Y' in [0; 1]
Pb in [-0.5; 0.5]
Pr in [-0.5; 0.5]

and http://en.wikipedia.org/wiki/Rec._709 gives Kb = 0.0722, Kr = 0.2126. We
simply invert these equations to obtain the coefficients we want.

Without this patch, the white level (contrast control) test pattern from AVS HD
709 (explained at http://www.avsforum.com/avs-vb/showthread.php?t=948496 , MP4
version available at http://www.sendspace.com/file/kaotq5) was rather different
in mplayer -vo x11 (which does software YUV->RGB conversion) compared to
mplayer -vo xv. After the patch, they're pretty much indistinguishable (and it
looks just like it does on my PS3).

I should probably mention that a lot of the bars from the test patterns don't
flash on my PC monitors. I have my radeonhd box hooked up to an HDTV, where
they do flash (at least after applying the patch.)


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