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+Consider three cases just to suggest the spectrum
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+of possiblities:
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+
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+a) linear upsample: each output pixel is a weighted sum
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+of 4 input pixels
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+
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+b) cubic upsample: each output pixel is a weighted sum
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+of 16 input pixels
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+
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+c) downsample by N with box filter: each output pixel
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+is a weighted sum of NxN input pixels, N can be very large
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+
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+Now, suppose you want to handle 8-bit input, 16-bit
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+input, and float input, and you want to do sRGB correction
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+or not.
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+
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+Suppose you create a temporary buffer of float pixels, say
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+one scanline tall. Actually two temp buffers, one for the
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+input and one for the output. You decode a scanline of the
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+input into the temp buffer which is always linear floats. This
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+isolates the handling of 8/16/float and sRGB to one place
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+(and still allows you to make optimized 8-bit-sRGB-to-float
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+lookup tables). This also allows you to put wrap logic here,
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+explicitly wrapping, reflecting, or replicating-from-edge
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+pixels that would come from off-edge.
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+
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+You then do whatever the appropriate weighted sums are
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+into the output buffer, and you move on to the next
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+scanline of the input.
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+
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+The algorithm just described works directly for case (c).
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+Suppose you're downsampling by 2.5; then output scanline 0
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+sums from input scanlines 0, 1, and 2; output scanline 1
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+sums from 2,3,4; output 2 from 5,6,7; output 3 from 7,8,9.
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+Note how 2 & 7 get reused, but we don't have to recompute
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+them because we can do things in a single linear pass
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+through the input and output at the same time.
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+
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+Now, consider case (a). When upsampling, the same two input
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+scanlines will get sampled-from for multiple output scanlines.
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+So, to avoid recomputing the input scanlines, we need either
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+multiple input or multiple output temp buffer lines. Since
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+the number of output lines a given pair of input scanlines
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+might touch scales with the upsample amount, it makes more
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+sense to use two input scanline buffers. For cubic, you'll
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+need four scanline buffers, and in general the number of
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+buffers will be limited by the max filter width, which is
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+presumably hardcoded.
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+
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+You want to avoid memory allocations (since you're passing
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+in the target buffer already), so instead of using a scanline-width
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+temp buffer, use some fixed-width temp buffer that's W pixels,
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+and scale the image in vertical stripes that are that wide.
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+Suppose you make the temp buffers 256 wide; then an upsample
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+by 8 computes 256-pixel-width strips (from ~32-pixel-wide input
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+strips), but a downsample by 8 computes ~32-pixel-width
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+strips (from a 256-pixel width strip). Note this limits
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+the max down/upsampling to be ballpark 256x along the
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+horizontal axis.
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+
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+
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+
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Sean Barrett