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108 lines
4.6 KiB
Plaintext
108 lines
4.6 KiB
Plaintext
In general
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==========
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There are planar and packed modes.
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- Planar mode means: you have 3 separated image, one for each component,
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each image 8 bits/pixel. To get the real colored pixel, you have to
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mix the components from all planes. The resolution of planes may differ!
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- Packed mode means: you have all components mixed/interleaved together,
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so you have small "packs" of components in a single, big image.
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There are RGB and YUV colorspaces.
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- RGB: Read, Green and Blue components. Used by analog VGA monitors.
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- YUV: Luminance (Y) and Chrominance (U,V) components. Used by some
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video systems, like PAL. Also most m(j)peg/dct based codecs use this.
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With YUV, they used to reduce the resolution of U,V planes:
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The most common YUV formats:
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fourcc: bpp: IEEE: plane sizes: (w=width h=height of original image)
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444P 24 YUV 4:4:4 Y: w * h U,V: w * h
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YUY2,UYVY 16 YUV 4:2:2 Y: w * h U,V: (w/2) * h [MJPEG]
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YV12,I420 12 YUV 4:2:0 Y: w * h U,V: (w/2) * (h/2) [MPEG, h263]
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411P 12 YUV 4:1:1 Y: w * h U,V: (w/4) * h [DV-NTSC, CYUV]
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YVU9,IF09 9 YUV 4:1:0 Y: w * h U,V: (w/4) * (h/4) [Sorenson, Indeo]
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The YUV a:b:c naming style means: for <a> samples of Y there are <b> samples
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of UV in odd lines and <c> samples of UV in even lines.
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conversion: (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 (sorry, I can't call back his surname) suggests that there are
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problems with the above formula 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 formula 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 = Cr = red component (0=green 128=grey 255=red)
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Huh. The planar YUV modes.
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==========================
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The most misunderstood thingie...
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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 the order of the planes in the memory:
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for mp_image_t and libvo's draw_slice():
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planes[0] = Y = luminance
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planes[1] = U = Cb = blue
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planes[2] = V = Cr = red
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Note: planes[1] is ALWAYS U, and planes[2] is V, the fourcc
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(YV12 vs. I420) doesn't matter here! So, every codecs using 3 pointers
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(not only the first one) normally supports YV12 and I420 (=IYUV) too!
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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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Huh 2. RGB vs. BGR ?
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====================
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The 2nd most misunderstood thingie...
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You know, there are Intel and Motorola, and they use different byteorder.
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There are also others, like MIPS or Alpha, they all follow either Intel
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or Motorola byteorder.
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Unfortunately, the packed colorspaces depend on CPU byteorder. So, RGB
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on Intel and Motorola means different order of bytes.
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In MPlayer, we have constants IMGFMT_RGBxx and IMGFMT_BGRxx.
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Unfortunately, some codecs and vo drivers follow Intel, some follow Motorola
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byteorder, so they are incompatible. We had to find a stable base, so long
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time ago I've chosen OpenGL, as it's a wide-spreaded standard, and it well
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defines what RGB is and what BGR is. So, MPlayer's RGB is compatible with
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OpenGL's GL_RGB on all platforms, and the same goes for BGR - GL_BGR.
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Unfortunately, most of the x86 codecs call our BGR to RGB, so it sometimes
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confuse developers.
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If you are unsure, try the OpenGL driver (-vo gl). There is at least software
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OpenGL implementation for all major platforms and OS's.
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