Glenn Maynard
125 lines
3.8 KiB
C++
125 lines
3.8 KiB
C++
#include "stdafx.h"
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#include "SDL.h"
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#include "SDL_dither.h"
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#include "SDL_utils.h"
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#define DitherMatDim 4
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/* Fractions, 0/16 to 15/16: */
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static const int DitherMat[DitherMatDim][DitherMatDim] =
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{
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{ 0, 8, 2, 10 },
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{ 12, 4, 14, 6 },
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{ 3, 11, 1, 9 },
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{ 15, 7, 13, 5 }
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};
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static int DitherMatCalc[DitherMatDim][DitherMatDim];
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static bool DitherMatCalc_initted = false;
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/* conv is the ratio from the input to the output. */
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static Uint8 DitherPixel(int x, int y, int intensity, int conv)
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{
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/* The intensity matrix wraps. This assumes the matrix dims are a power of 2. */
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x &= DitherMatDim-1;
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y &= DitherMatDim-1;
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/* Ordered dithering is scaling the old intensity range to the new, with
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* the matrix values biasing to rounding down or up. Matrix values are in
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* the range [0..1). For example, converting the 8-bit value 100 to 4
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* bits directly gives 6.25. A matrix value of 0 means the pixel is not
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* biased at all, which would cause it to be truncated to 6. A value
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* of 5/16 means that the value is biased to 6.5625, which is also truncated
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* to 6. A value of 15/16 biases to 6.1875, which causes it to be rounded
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* up to 6. So, a proportion of pixels gets rounded up based on how close
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* the number is to the next value. */
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/* Convert the number to the destination range. */
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int out_intensity = intensity * conv;
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/* Add bias. */
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out_intensity += DitherMatCalc[y][x];
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/* Truncate, and add e to make sure a value of 14.999998 -> 15. */
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return Uint8((out_intensity + 1) >> 16);
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}
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void SM_SDL_OrderedDither(const SDL_Surface *src, SDL_Surface *dst)
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{
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if(!DitherMatCalc_initted) {
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for(int i = 0; i < DitherMatDim; ++i) {
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for(int j = 0; j < DitherMatDim; ++j) {
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/* Each value is 0..15. They represent 0/16 through 15/16.
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* Set DitherMatCalc to that value * 65536, so we can do it
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* with integer calcs. */
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DitherMatCalc[i][j] = DitherMat[i][j] * 65536 / 16;
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}
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}
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DitherMatCalc_initted = true;
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}
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/* We can't dither to paletted surfaces. */
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ASSERT(dst->format->BytesPerPixel > 1);
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Uint32 src_cbits[4], dst_cbits[4];
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mySDL_GetBitsPerChannel(src->format, src_cbits);
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mySDL_GetBitsPerChannel(dst->format, dst_cbits);
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/* Calculate the ratio from the old bit depth to the new for each color channel. */
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int conv[4];
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for(int i = 0; i < 4; ++i)
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{
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int MaxInputIntensity = (1 << src_cbits[i])-1;
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int MaxOutputIntensity = (1 << dst_cbits[i])-1;
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/* If the source is missing the channel, avoid div/0. */
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if(MaxInputIntensity == 0)
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conv[i] = 0;
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else
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conv[i] = MaxOutputIntensity * 65536 / MaxInputIntensity;
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}
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/* Max alpha value; used when there's no alpha source. */
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const Uint8 alpha_max = Uint8((1 << dst_cbits[3]) - 1);
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/* For each row: */
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for(int row = 0; row < src->h; ++row) {
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const Uint8 *srcp = (const Uint8 *)src->pixels + row * src->pitch;
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Uint8 *dstp = (Uint8 *)dst->pixels + row * dst->pitch;
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/* For each pixel: */
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for(int col = 0; col < src->w; ++col) {
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Uint8 colors[4];
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mySDL_GetRawRGBAV(srcp, src, colors);
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/* Note that we don't dither the alpha channel. */
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for(int c = 0; c < 3; ++c) {
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/* If the destination has less bits, dither: */
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colors[c] = DitherPixel(col, row, colors[c], conv[c]);
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}
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/* If the source has no alpha, the conversion formula will end up
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* with 0; that's fine for color channels, but for alpha we need to
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* be opaque. */
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if(src_cbits[3] == 0) {
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colors[3] = alpha_max;
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} else {
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/* Same as DitherPixel, except it doesn't actually dither; dithering
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* looks bad on the alpha channel. */
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int out_intensity = colors[3] * conv[3];
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/* Truncate, and add e to make sure a value of 14.999998 -> 15. */
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colors[3] = Uint8((out_intensity + 1) >> 16);
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}
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/* Raw value -> int -> pixel */
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mySDL_SetRawRGBAV(dstp, dst, colors);
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srcp += src->format->BytesPerPixel;
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dstp += dst->format->BytesPerPixel;
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}
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}
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}
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