#include "stdafx.h"

#include "SDL.h"
#include "SDL_dither.h"
#include "SDL_utils.h"

#define DitherMatDim 4

/* Fractions, 0/16 to 15/16:  */
static const int DitherMat[DitherMatDim][DitherMatDim] =
{
	{  0,  8,  2, 10 },
	{ 12,  4, 14,  6 },
	{  3, 11,  1,  9 },
	{ 15,  7, 13,  5 }
};

static int DitherMatCalc[DitherMatDim][DitherMatDim];
static bool DitherMatCalc_initted = false;

/* conv is the ratio from the input to the output. */
static Uint8 DitherPixel(int x, int y, int intensity,  int conv)
{
	/* The intensity matrix wraps.  This assumes the matrix dims are a power of 2. */
	x &= DitherMatDim-1;
	y &= DitherMatDim-1;

	/* Ordered dithering is scaling the old intensity range to the new, with
	 * the matrix values biasing to rounding down or up. Matrix values are in
	 * the range [0..1).  For example, converting the 8-bit value 100 to 4
	 * bits directly gives 6.25.  A matrix value of 0 means the pixel is not
	 * biased at all, which would cause it to be truncated to 6.  A value
	 * of 5/16 means that the value is biased to 6.5625, which is also truncated
	 * to 6.  A value of 15/16 biases to 6.1875, which causes it to be rounded
	 * up to 6.  So, a proportion  of pixels gets rounded up based on how close
	 * the number is to the next value. */

	/* Convert the number to the destination range. */
	int out_intensity = intensity * conv;
	
	/* Add bias. */
	out_intensity += DitherMatCalc[y][x];

	/* Truncate, and add e to make sure a value of 14.999998 -> 15. */
	return Uint8((out_intensity + 1) >> 16);
}

void SM_SDL_OrderedDither(const SDL_Surface *src, SDL_Surface *dst)
{
	if(!DitherMatCalc_initted) {
		for(int i = 0; i < DitherMatDim; ++i) {
			for(int j = 0; j < DitherMatDim; ++j) {
				/* Each value is 0..15.  They represent 0/16 through 15/16.
				 * Set DitherMatCalc to that value * 65536, so we can do it
				 * with integer calcs. */
				DitherMatCalc[i][j] = DitherMat[i][j] * 65536 / 16;
			}
		}
			
		DitherMatCalc_initted = true;
	}


	/* We can't dither to paletted surfaces. */
	ASSERT(dst->format->BytesPerPixel > 1);

	Uint32 src_cbits[4], dst_cbits[4];
	mySDL_GetBitsPerChannel(src->format, src_cbits);
	mySDL_GetBitsPerChannel(dst->format, dst_cbits);

	/* Calculate the ratio from the old bit depth to the new for each color channel. */
	int conv[4];
	for(int i = 0; i < 4; ++i)
	{
		int MaxInputIntensity = (1 << src_cbits[i])-1;
		int MaxOutputIntensity = (1 << dst_cbits[i])-1;
		/* If the source is missing the channel, avoid div/0. */
		if(MaxInputIntensity == 0)
			conv[i] = 0;
		else
			conv[i] = MaxOutputIntensity * 65536 / MaxInputIntensity;
	}

	/* Max alpha value; used when there's no alpha source.  */
	const Uint8 alpha_max = Uint8((1 << dst_cbits[3]) - 1);

	/* For each row: */
	for(int row = 0; row < src->h; ++row) {
		const Uint8 *srcp = (const Uint8 *)src->pixels + row * src->pitch;
		Uint8 *dstp = (Uint8 *)dst->pixels + row * dst->pitch;

		/* For each pixel: */
		for(int col = 0; col < src->w; ++col) {
			Uint8 colors[4];
			mySDL_GetRawRGBAV(srcp, src, colors);

			/* Note that we don't dither the alpha channel. */
			for(int c = 0; c < 3; ++c) {
				/* If the destination has less bits, dither: */
				colors[c] = DitherPixel(col, row, colors[c], conv[c]);
			}

			/* If the source has no alpha, the conversion formula will end up
			 * with 0; that's fine for color channels, but for alpha we need to
			 * be opaque. */
			if(src_cbits[3] == 0) {
				colors[3] = alpha_max;
			} else {
				/* Same as DitherPixel, except it doesn't actually dither; dithering
				 * looks bad on the alpha channel. */
				int out_intensity = colors[3] * conv[3];
	
				/* Truncate, and add e to make sure a value of 14.999998 -> 15. */
				colors[3] = Uint8((out_intensity + 1) >> 16);
			}

			/* Raw value -> int -> pixel */
			mySDL_SetRawRGBAV(dstp, dst, colors);

			srcp += src->format->BytesPerPixel;
			dstp += dst->format->BytesPerPixel;
		}
	}
}