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https://github.com/mpv-player/mpv
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52 lines
2.0 KiB
Plaintext
52 lines
2.0 KiB
Plaintext
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Huh. The planar YUV modes.
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==========================
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The most missunderstood thingie...
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Let's see: (some cut'n'paste from www and maillist)
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RGB to YUV Conversion:
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Y = (0.257 * R) + (0.504 * G) + (0.098 * B) + 16
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Cr = V = (0.439 * R) - (0.368 * G) - (0.071 * B) + 128
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Cb = U = -(0.148 * R) - (0.291 * G) + (0.439 * B) + 128
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YUV to RGB Conversion:
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B = 1.164(Y - 16) + 2.018(U - 128)
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G = 1.164(Y - 16) - 0.813(V - 128) - 0.391(U - 128)
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R = 1.164(Y - 16) + 1.596(V - 128)
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In both these cases, you have to clamp the output values to keep them in
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the [0-255] range. Rumour has it that the valid range is actually a subset
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of [0-255] (I've seen an RGB range of [16-235] mentioned) but clamping the
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values into [0-255] seems to produce acceptable results to me.
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Julien (surname unknown) suggests that there are problems with the above
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formulae and suggests the following instead:
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Y = 0.299R + 0.587G + 0.114B
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Cb = U'= (B-Y)*0.565
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Cr = V'= (R-Y)*0.713
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with reciprocal versions:
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R = Y + 1.403V'
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G = Y - 0.344U' - 0.714V'
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B = Y + 1.770U'
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note: this formule doesn't contain the +128 offsets of U,V values!
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Conclusion:
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Y = luminance, the weighted average of R G B components. (0=black 255=white)
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U = Cb = blue component (0=green 128=grey 255=blue)
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V = Cv = red component (0=green 128=grey 255=red)
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MPlayer side:
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=============
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In MPlayer, we usually have 3 pointers to the Y, U and V planes, so it
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doesn't matter what is they order in memory.
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But there are some codecs (vfw, dshow) and vo drivers (xv) ignoring the 2nd
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and 3rd pointer, and use only a single pointer to the planar yuv image. In
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this case we must know the right order and alignment of planes in the memory!
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from the webartz fourcc list:
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YV12: 12 bpp, full sized Y plane followed by 2x2 subsampled V and U planes
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I420: 12 bpp, full sized Y plane followed by 2x2 subsampled U and V planes
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IYUV: the same as I420
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YVU9: 9 bpp, full sized Y plane followed by 4x4 subsampled V and U planes
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