blit.lut.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

#define RGB565_R_MASK 0b1111100000000000
#define RGB565_G_MASK 0b0000011111100000
#define RGB565_B_MASK 0b0000000000011111

const int ScreenWidth = 1024;
const int ScreenHeight = 768;
const int SpriteSize = 64;

typedef struct _RGB {
	float r;
	float g;
	float b;
} floatrgb_t;

floatrgb_t LUT[1<<16];

int min(int x, int y) {
	return x < y ? x : y;
}
int max(int x, int y) {
	return x > y ? x : y;
}

void bgra8888_to_floatrgba(uint8_t* bgra8888, float* floatrgba) {
	floatrgba[0] = bgra8888[2] / 255.f;
	floatrgba[1] = bgra8888[1] / 255.f;
	floatrgba[2] = bgra8888[0] / 255.f;
	floatrgba[3] = bgra8888[3] / 255.f;
}

void rgb565_to_floatrgb(uint16_t* rgb565, float* floatrgb) {
	floatrgb[0] = LUT[rgb565[0]].r;
	floatrgb[1] = LUT[rgb565[0]].g;
	floatrgb[2] = LUT[rgb565[0]].b;
}

float lerp(float from, float to, float time) {
	return (1-time)*from + time*to;
}

void floatrgb_to_rgb565(float* floatrgb, uint16_t* rgb565) {
	rgb565[0] =	(uint16_t)(floatrgb[0]*31) << 11 |
				(uint16_t)(floatrgb[1]*63) << 5  |
				(uint16_t)(floatrgb[2]*31);
}

void blitSprite(uint16_t* frameBuffer, uint8_t* sprite, int x, int y) {
	int start_i = max(-y, 0);
	int start_j = max(-x, 0);
	int end_i = min(ScreenHeight-y, SpriteSize);
	int end_j = min(ScreenWidth-x , SpriteSize);

	int spriteRowOffset = 4*(SpriteSize - (end_j-start_j));
	int bufferRowOffset = ScreenWidth - (end_j-start_j);

	uint8_t* spritePix = sprite + 4*(start_j + start_i*SpriteSize);
	uint16_t* bufferPix = frameBuffer + ScreenWidth*(y + start_i) + x + start_j;

	for(int i = start_i; i < end_i; ++i) {

		for(int j = start_j; j < end_j; ++j) {

			float over[4];
			float under[3];

			bgra8888_to_floatrgba(spritePix, over);
			rgb565_to_floatrgb(bufferPix, under);
			
			// Perform alpha blending
			under[0] = lerp(under[0], over[0], over[3]);
			under[1] = lerp(under[1], over[1], over[3]);
			under[2] = lerp(under[2], over[2], over[3]);

			floatrgb_to_rgb565(under, bufferPix);

			spritePix += 4;
			bufferPix += 1;
		}

		spritePix += spriteRowOffset;
		bufferPix += bufferRowOffset;
	}
}

void buildLUTs() {
	for(unsigned int i = 0; i < (1<<16); ++i) {
		LUT[i].r = ((i & RGB565_R_MASK) >> (6+5)) / 31.f;
		LUT[i].g = ((i & RGB565_G_MASK) >> 5)     / 63.f;
		LUT[i].b =  (i & RGB565_B_MASK)           / 31.f;
	}
}