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62 lines
2.5 KiB
62 lines
2.5 KiB
Consider three cases just to suggest the spectrum |
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of possiblities: |
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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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b) cubic upsample: each output pixel is a weighted sum |
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of 16 input pixels |
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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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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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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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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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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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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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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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