On 2019-06-19 11:12, Carl Karsten wrote:
I'm stuck on what constant to use.
framebuffer[i] = 0x80108010; /* black in YCbCr 4:2:2*/
Ah, magic constants.
That particular magic constant matches the YCBCR422_BLACK define in:
https://github.com/timvideos/HDMI2USB-litex-firmware/blob/eaab51fd65f2c7276c32e6973ca2c86beb188fb0/firmware/pattern.h#L6-L14
so at least the constant is consistent with the comment....
I'm trying to do this:
YCbCr (84, 130, 213) = debian red https://encycolorpedia.com/d70a53
hex: 54 82 d5
Debian red isn't exactly purple :-)
The pattern.h magic values (above) refer to:
https://github.com/timvideos/HDMI2USB-litex-firmware/blob/master/firmware/pattern.py
as how the values were calculated, and in particular:
https://github.com/timvideos/HDMI2USB-litex-firmware/blob/eaab51fd65f2c7276c32e6973ca2c86beb188fb0/firmware/pattern.py#L1-L12
seems to be the RGB -> Y Cb Cr -> packed Y Cb Cr calculations.
Hacking that script to put in the Debian Red RGB values, and some extra
print statements gives:
-=- cut here -=-
ewen@ashram:~$ python3 /tmp/pattern.py
215 10 83 RGB is 79 129 224 Y Cb Cr is 814fe04f
ewen@ashram:~$
-=- cut here -=-
So in theory something like:
framebuffer[i] = 0x814fe04f; // Debian Red
will get fairly close.
It looks like it's floor()ing the values in the RGB -> Y Cb Cr
conversion, rather than round()ing them, and assigning less to the Y
value than the online conversion, so you might want to finesse that a
bit more if the *exact* red matters to you as the colour model used
seems lightly different. But I suspect "colour temperature of the
projector" and "colour temperature of the room lighting" will probably
make more perceived difference.... :-)
From a glance at the code it looks like the pattern is:
Cb Y Cr Y
(in big endian format). Which means:
framebuffer[i] = 0x8254d554;
I think corresponds to the Y Cb Cr values you came up with.
Ewen
PS: I'm dubious about the accuracy of just repeating the 8-bit Y
component twice to give it 16 bits, as that seems like something that's
only really accurate at the extremes (Y = 0 / Y = 255). But off the top
of my head it's probably "close enough" to be recognisably like what you
were going for anyway.
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