row_sort_fun.cpp

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

#define JM_MATH_IMPLEMENTATION 
#define JM_MATH_USE_STDLIB
#include "jm_math.h"

#define STB_IMAGE_IMPLEMENTATION
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image.h"
#include "stb_image_write.h"

inline uint32_t get_blue(uint32_t color)  { return (color >> 16) & 0xFF; }
inline uint32_t get_green(uint32_t color) { return (color >>  8) & 0xFF; }
inline uint32_t get_red(uint32_t color)   { return (color >>  0) & 0xFF; }

//NOTE: Staight up comparison of pixel values
int pixel_compare(const void *a, const void *b)
{
	uint32_t color0 = *(uint32_t *)a; 
	uint32_t color1 = *(uint32_t *)b; 
	if(color0 > color1) return 1;
	if(color0 < color1) return -1;
	return 0;
}

inline uint32_t averaged_color(uint32_t color)
{
	uint32_t r = get_red(color); 
	uint32_t g = get_green(color);
	uint32_t b = get_blue(color); 
	return (r + g + b) / 3;
}

//NOTE: Comparison is done using the average of each channel(no alpha). This can provide a slightly smoother
//      appearance to the end result.
int pixel_compare_average(const void *a, const void *b)
{
	uint32_t avg0 = averaged_color(*(uint32_t *)a);
	uint32_t avg1 = averaged_color(*(uint32_t *)b);

	if(avg0 > avg1) return 1;
	if(avg0 < avg1) return -1;
	return 0;
}

//NOTE: human eyes tend to be more sensitive to green light and much less
//      sensitive to blue light and red light.
#define EYE_RED_COLOR_BIAS   0.2216f
#define EYE_GREEN_COLOR_BIAS 0.7152f
#define EYE_BLUE_COLOR_BIAS  0.0832f

inline float human_eye_color_intensity_bias(uint32_t color)
{
	//NOTE: map channels from 0-255 to 0.0f-1.0f
	float r = (float)get_red(color)   / UCHAR_MAX;
	float g = (float)get_green(color) / UCHAR_MAX;
	float b = (float)get_blue(color)  / UCHAR_MAX;
	return jmClamp01(jmDot(V3(r, g, b), V3(EYE_RED_COLOR_BIAS, EYE_GREEN_COLOR_BIAS, EYE_BLUE_COLOR_BIAS)));
}

int pixel_compare_eye_bias(const void *a, const void *b)
{
	float intensity0 = human_eye_color_intensity_bias(*(uint32_t *)a);
	float intensity1 = human_eye_color_intensity_bias(*(uint32_t *)b);

	if(intensity0 > intensity1) return 1;
	if(intensity0 < intensity1) return -1;
	return 0;
}

int main(void)
{
	int w, h;
	uint8_t *pixels = stbi_load("galaxy.jpg", &w, &h, 0, 4); 
	assert(pixels);

	//NOTE: sort each row separately
	for(int i = 0; i < h; ++i) 
	{
		qsort(&((uint32_t *)pixels)[i * w], w, sizeof(uint32_t), pixel_compare_eye_bias);
	}

	stbi_write_png("galaxy_sorted_05.png", w, h, 4, pixels, 0);

	STBI_FREE(pixels);
	return 0;
}