BC7 encoding for single-color blocks

bc7_single_color.cpp

void bc7_encode_single_color_block(U8 * output_bc7, const U8 rgba[4])
{
	U64 r = rgba[0];
	U64 g = rgba[1];
	U64 b = rgba[2];
	U64 a = rgba[3];

	const U64 bit6_mask = (0x40 << 8) | (0x40 << 22) | (0x40ull << 36);
	const U64 lo7_mask  = (0x7f << 8) | (0x7f << 22) | (0x7full << 36);
	U64 color_bits;

	// Color channels (see bc7solid/main.cpp)
	// this is the bit-hack version written to process 3 color chans at once
	color_bits = (r << 8) | (g << 22) | (b << 36);

	// The basic idea here is this: for every color channel, we get the right
	// results with
	//   e0 = val >> 1
	//   e1 = (val + (val < 128 ? 1 : -1)) >> 1
	// the bias in the e1 computation
	//   (val < 128 ? 1 : -1)
	// can be written as
	//   1 - ((val >> 6) & 2)
	// and the rest is just bit fiddling and accounting for the layout of mode 5.

	U64 t = color_bits << 6;
	color_bits = ((color_bits >> 1) & lo7_mask) - (color_bits & ~lo7_mask);
	color_bits += t + (t & bit6_mask);

	// alpha: just set endpoint 0 to target value then use all-0 inds
	color_bits |= a << 50;

	// insert mode bits
	color_bits |= 0x20; // mode 5, rotation=0

	RR_PUT64_LE_UNALIGNED(output_bc7 + 0, color_bits); // this covers 64 out of the 66 mode/color bits
	RR_PUT64_LE_UNALIGNED(output_bc7 + 8, 0xaaaaaaac); // last 2 color bits, color index bits, alpha index bits
}