-- draw a gradient in a raw file (512x512)
-- ugly LUA code by www.astrofra.com

------------------------------
function contrast(color_value)
------------------------------
	return ((color_value * color_value) / 256);
end

----------------------------
function invert(color_value)
----------------------------
	return (255 - color_value);
end

------------------------------------------------
function write_rgb_pixel(file_handler, r , g, b)
------------------------------------------------
	r, g, b = invert(contrast(r)), invert(contrast(g)), invert(contrast(b));
	
	if (r < 1) then r = 1; end;
	if (g < 1) then g = 1; end;
	if (b < 1) then b = 1; end;		
	io.write(string.format("%s%s%s", string.char(r),string.char(g),string.char(b)));
end

--------------------------------------------------
-- main
--------------------------------------------------
print("Gradient test");

file_name = "grad_out.raw";
file_handler = nil;

bmp_size_x = 512;
bmp_size_y = 512;

print("open file "..file_name.." in write mode");
bmp_handler = io.open(file_name,"wb");

if not(bmp_handler) then print("io.open returned error"); return; end

io.output(bmp_handler);

for i = 0, bmp_size_x-1, 1 do
	r = (i * 255) / bmp_size_x;		
	for j = 0, bmp_size_y-1, 1 do
		b = (j * 255) / bmp_size_y;
		g = (r + b) / 2;
		write_rgb_pixel(bmp_handler, r, g, b);
	end;
end;

io.close(bmp_handler);

print("Done");