Add ordered dither mask generation to gfx

libs/gfx/include/psemek/gfx/dither.hpp

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <psemek/gfx/pixmap.hpp>
+
+namespace psemek::gfx
+{
+
+	basic_pixmap<std::uint8_t> ordered_dither(std::size_t size);
+
+}

libs/gfx/source/dither.cpp

@@ -0,0 +1,40 @@
+#include <psemek/gfx/dither.hpp>
+
+namespace psemek::gfx
+{
+
+	static std::uint8_t interleave_with_zeros(std::uint8_t input) {
+		std::uint8_t word = input;
+		word = (word ^ (word << 4 )) & 0x0f0f;
+		word = (word ^ (word << 2 )) & 0x3333;
+		word = (word ^ (word << 1 )) & 0x5555;
+		return word;
+	}
+
+	static std::uint8_t bit_interleave(std::uint8_t a, std::uint8_t b) {
+		return interleave_with_zeros(a) | (interleave_with_zeros(b) << 1);
+	}
+
+	static std::uint8_t bit_reverse(std::uint8_t b) {
+		b = (b & 0xF0) >> 4 | (b & 0x0F) << 4;
+		b = (b & 0xCC) >> 2 | (b & 0x33) << 2;
+		b = (b & 0xAA) >> 1 | (b & 0x55) << 1;
+		return b;
+	}
+
+	basic_pixmap<std::uint8_t> ordered_dither(std::size_t size)
+	{
+		if (size > 16)
+			throw std::runtime_error("8-bit dither map cannot be larger than 16x16");
+
+		basic_pixmap<std::uint8_t> result({size, size});
+
+		for (auto idx : result.indices())
+		{
+			result(idx) = bit_reverse(bit_interleave(idx[0] ^ idx[1], idx[0]));
+		}
+
+		return result;
+	}
+
+}