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In general
==========
There are planar and packed modes.
- Planar mode means: you have 3 separated image, one for each component,
each image 8 bites/pixel. To get the real colored pixel, you have to
mix the components from all planes. The resolution of planes may differ!
- Packed mode means: you have all components mixed/interleaved together,
so you have small "packs" of components in a single, big image.
There are RGB and YUV colorspaces.
- RGB: Read, Green and Blue components. Used by analog VGA monitors.
- YUV: Luminance (Y) and Chrominance (U,V) components. Used by some
video systems, like PAL. Also most m(j)peg/dct based codecs use this.
With YUV, they used to reduce the resolution of U,V planes:
The most common YUV formats:
fourcc: bpp: IEEE: plane sizes: (w=width h=height of original image)
? 24 YUV 4:4:4 Y: w * h U,V: w * h
YUY2,UYVY 16 YUV 4:2:2 Y: w * h U,V: (w/2) * h
YV12,I420 12 YUV 4:2:0 Y: w * h U,V: (w/2) * (h/2)
YVU9 9 YUV 4:1:1 Y: w * h U,V: (w/4) * (h/4)
conversion: (some cut'n'paste from www and maillist)
RGB to YUV Conversion:
Y = (0.257 * R) + (0.504 * G) + (0.098 * B) + 16
Cr = V = (0.439 * R) - (0.368 * G) - (0.071 * B) + 128
Cb = U = -(0.148 * R) - (0.291 * G) + (0.439 * B) + 128
YUV to RGB Conversion:
B = 1.164(Y - 16) + 2.018(U - 128)
G = 1.164(Y - 16) - 0.813(V - 128) - 0.391(U - 128)
R = 1.164(Y - 16) + 1.596(V - 128)
In both these cases, you have to clamp the output values to keep them in
the [0-255] range. Rumour has it that the valid range is actually a subset
of [0-255] (I've seen an RGB range of [16-235] mentioned) but clamping the
values into [0-255] seems to produce acceptable results to me.
Julien (surname unknown) suggests that there are problems with the above
formulae and suggests the following instead:
Y = 0.299R + 0.587G + 0.114B
Cb = U'= (B-Y)*0.565
Cr = V'= (R-Y)*0.713
with reciprocal versions:
R = Y + 1.403V'
G = Y - 0.344U' - 0.714V'
B = Y + 1.770U'
note: this formule doesn't contain the +128 offsets of U,V values!
Conclusion:
Y = luminance, the weighted average of R G B components. (0=black 255=white)
U = Cb = blue component (0=green 128=grey 255=blue)
V = Cr = red component (0=green 128=grey 255=red)
Huh. The planar YUV modes.
==========================
The most missunderstood thingie...
In MPlayer, we usually have 3 pointers to the Y, U and V planes, so it
doesn't matter what is the order of the planes in the memory:
for mp_image_t and libvo's draw_slice():
planes[0] = Y = luminance
planes[1] = U = Cb = blue
planes[2] = V = Cr = red
Note: planes[1] is ALWAYS U, and planes[2] is V, the fourcc
(YV12 vs. I420) doesn't matter here! So, every codecs using 3 pointers
(not only the first one) normally supports YV12 and I420 (=IYUV) too!
But there are some codecs (vfw, dshow) and vo drivers (xv) ignoring the 2nd
and 3rd pointer, and use only a single pointer to the planar yuv image. In
this case we must know the right order and alignment of planes in the memory!
from the webartz fourcc list:
YV12: 12 bpp, full sized Y plane followed by 2x2 subsampled V and U planes
I420: 12 bpp, full sized Y plane followed by 2x2 subsampled U and V planes
IYUV: the same as I420
YVU9: 9 bpp, full sized Y plane followed by 4x4 subsampled V and U planes
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