move color encoding functions to GVMEncoder header

src/GVMEncoder.hh

@@ -20,3 +20,40 @@ enum class GVRDataFormat : uint8_t {
 };
 
 std::string encode_gvm(const Image& img, GVRDataFormat data_format);
+
+constexpr uint16_t encode_rgb565(uint8_t r, uint8_t g, uint8_t b) {
+  return ((r << 8) & 0xF800) | ((g << 3) & 0x07E0) | ((b >> 3) & 0x001F);
+}
+
+constexpr uint16_t encode_rgb5a3(uint8_t r, uint8_t g, uint8_t b, uint8_t a) {
+  if ((a & 0xE0) == 0xE0) {
+    return 0x8000 | ((r << 7) & 0x7C00) | ((g << 2) & 0x03E0) | ((b >> 3) & 0x001F);
+  } else {
+    return ((a << 7) & 0x7000) | ((r << 4) & 0x0F00) | (g & 0x00F0) | ((b >> 4) & 0x000F);
+  }
+}
+
+constexpr uint32_t encode_argb8888(uint8_t r, uint8_t g, uint8_t b, uint8_t a) {
+  return (a << 24) | (r << 16) | (g << 8) | b;
+}
+
+constexpr uint16_t encode_xrgb8888_to_xrgb1555(uint32_t xrgb8888) {
+  // In:  rrrrrrrrggggggggbbbbbbbbaaaaaaaa
+  // Out:                 -rrrrrgggggbbbbb
+  return ((xrgb8888 >> 9) & 0x7C00) | ((xrgb8888 >> 6) & 0x03E0) | ((xrgb8888 >> 3) & 0x001F);
+}
+
+constexpr uint32_t encode_rgbx8888_to_xrgb1555(uint16_t rgbx8888) {
+  // In:  rrrrrrrrggggggggbbbbbbbbxxxxxxxx
+  // Out:                 -rrrrrgggggbbbbb
+  return ((rgbx8888 >> 17) & 0x7C00) | ((rgbx8888 >> 14) & 0x03E0) | ((rgbx8888 >> 11) & 0x001F);
+}
+
+constexpr uint32_t decode_xrgb1555_to_rgba8888(uint16_t xrgb1555) {
+  // In:                  -rrrrrgggggbbbbb
+  // Out: rrrrrrrrggggggggbbbbbbbbaaaaaaaa (a is always FF)
+  return ((xrgb1555 << 17) & 0xF8000000) | ((xrgb1555 << 12) & 0x07000000) |
+      ((xrgb1555 << 14) & 0x00F80000) | ((xrgb1555 << 9) & 0x00070000) |
+      ((xrgb1555 << 11) & 0x0000F800) | ((xrgb1555 << 6) & 0x00000700) |
+      0x000000FF;
+}