lambda template usage for image between color type conversion with multiple possible bit depths

image.cpp

/*
	Copyright (c) 2022 Daniel Valcour
	Permission is hereby granted, free of charge, to any person obtaining a copy of
	this software and associated documentation files (the "Software"), to deal in
	the Software without restriction, including without limitation the rights to
	use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
	the Software, and to permit persons to whom the Software is furnished to do so,
	subject to the following conditions:
	The above copyright notice and this permission notice shall be included in all
	copies or substantial portions of the Software.
	THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
	IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
	FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
	COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
	IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
	CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

void fs::Image::Convert(const Image::ColorType color_type)
{
	if (this->IsEmpty()) 
	{
		throw fs::runtime_error("Can not convert image color type it is empty. convert_color_type: {}", magic_enum::enum_name(color_type));
	}
	if (color_type == Image::ColorType::None)
	{
		throw fs::runtime_error("Can not convert image to none color type. image_color_type: {}", magic_enum::enum_name(this->color_type));
	}

	std::size_t pixel_count = this->width * this->height * this->pages;
	std::size_t new_size = pixel_count * static_cast<std::size_t>(color_type);
	this->Reserve(new_size);

	auto convert = [&]<typename T>() {
		T* data_ptr = reinterpret_cast<T*>(this->data.get());
		switch (this->color_type)
		{ //loop through all pixels to convert. If input type has less bytes per pixel than output, the whole loop must be reversed. the channels per pixel must be set in reverse in this situation too unless the in color type has one channel per pixel
		case ColorType::G:
			switch (color_type)
			{
			case ColorType::G: // G -> G
				break;
			case ColorType::GA: // G -> GA
				for (std::size_t i = pixel_count; i-- != 0;)
				{
					std::size_t old_i = i;
					std::size_t new_i = i * 2;
					data_ptr[new_i++] = data_ptr[old_i];
					data_ptr[new_i] = 255;
				}
				break;
			case ColorType::RGB: // G -> RGB
				for (std::size_t i = pixel_count; i-- != 0;)
				{
					std::size_t old_i = i;
					std::size_t new_i = i * 3;
					data_ptr[new_i++] = data_ptr[old_i];  // G -> R
					data_ptr[new_i++] = data_ptr[old_i];  // G -> G
					data_ptr[new_i] = data_ptr[old_i];    // G -> B
				}
				break;
			case ColorType::RGBA: // G -> RGBA
				for (std::size_t i = pixel_count; i-- != 0;)
				{
					std::size_t old_i = i;
					std::size_t new_i = i * 4;
					data_ptr[new_i++] = data_ptr[old_i];    // G -> R
					data_ptr[new_i++] = data_ptr[old_i];    // G -> G
					data_ptr[new_i++] = data_ptr[old_i];    // G -> B
					data_ptr[new_i] = 255;                    // fill A
				}
				break;
			}
			break;
		case ColorType::GA:
			switch (color_type)
			{
			case ColorType::G: // GA -> G
				for (std::size_t i = 0; i < pixel_count; i++)
				{
					std::size_t old_i = i * 2;
					std::size_t new_i = i;
					data_ptr[new_i] = data_ptr[old_i];    // G -> G
				}
				break;
			case ColorType::GA: // GA -> GA
				break;
			case ColorType::RGB: // GA -> RGB
				for (std::size_t i = pixel_count; i-- != 0;)
				{
					std::size_t old_i = i * 2; // alpha is not needed so start on G channel
					std::size_t new_i = (i * 3) + 2;
					data_ptr[new_i--] = data_ptr[old_i];    // G -> B
					data_ptr[new_i--] = data_ptr[old_i];    // G -> G
					data_ptr[new_i] = data_ptr[old_i];      // G -> R
				}
				break;
			case ColorType::RGBA: // GA -> RGBA
				for (std::size_t i = pixel_count; i-- != 0;)
				{
					std::size_t old_i = (i * 2) + 1;
					std::size_t new_i = (i * 4) + 3;
					data_ptr[new_i--] = data_ptr[old_i--];    // A -> A
					data_ptr[new_i--] = data_ptr[old_i];      // B -> B
					data_ptr[new_i--] = data_ptr[old_i];      // G -> G
					data_ptr[new_i--] = data_ptr[old_i];      // R -> R
				}
				break;
			}
			break;
		case ColorType::RGB:
			switch (color_type)
			{
			case ColorType::G: // RGB -> G
				for (std::size_t i = 0; i < pixel_count; i++)
				{
					std::size_t old_i = i * 3;
					std::size_t new_i = i;
					std::size_t old_r = data_ptr[old_i++];
					std::size_t old_g = data_ptr[old_i++];
					std::size_t old_b = data_ptr[old_i];
					uint8_t gray = colorToGrayscale(old_r, old_g, old_b);
					data_ptr[new_i] = gray;
				}
				break;
			case ColorType::GA: // RGB -> GA
				for (std::size_t i = 0; i < pixel_count; i++)
				{
					std::size_t old_i = i * 3;
					std::size_t new_i = i * 2;
					std::size_t old_r = data_ptr[old_i++];
					std::size_t old_g = data_ptr[old_i++];
					std::size_t old_b = data_ptr[old_i];
					uint8_t gray = colorToGrayscale(old_r, old_g, old_b);
					data_ptr[new_i++] = gray;
					data_ptr[new_i] = 255;
				}
				break;
			case ColorType::RGB: // RGB -> RGB
				break;
			case ColorType::RGBA: // RGB -> RGBA
				for (std::size_t i = pixel_count; i-- != 0;)
				{
					std::size_t old_i = (i * 3) + 2;
					std::size_t new_i = (i * 4) + 3;                // these assignments are reversed to prevent errors on the second and third pixels
					data_ptr[new_i--] = 255;                      // A fill
					data_ptr[new_i--] = data_ptr[old_i--];      // B -> B
					data_ptr[new_i--] = data_ptr[old_i--];      // G -> G
					data_ptr[new_i] = data_ptr[old_i];          // R -> R
				}
				break;
			}
			break;
		case ColorType::RGBA:
			switch (color_type)
			{
			case ColorType::G: // RGBA -> G
				for (std::size_t i = 0; i < pixel_count; i++)
				{
					std::size_t old_i = i * 4;
					std::size_t new_i = i;
					std::size_t old_r = data_ptr[old_i++];
					std::size_t old_g = data_ptr[old_i++];
					std::size_t old_b = data_ptr[old_i];
					uint8_t gray = colorToGrayscale(old_r, old_g, old_b);
					data_ptr[new_i] = gray;
				}
				break;
			case ColorType::GA: // RGBA -> GA
				for (std::size_t i = 0; i < pixel_count; i++)
				{
					std::size_t old_i = i * 4;
					std::size_t new_i = i * 2;
					std::size_t old_r = data_ptr[old_i++];
					std::size_t old_g = data_ptr[old_i++];
					std::size_t old_b = data_ptr[old_i++];
					uint8_t gray = colorToGrayscale(old_r, old_g, old_b);
					data_ptr[new_i++] = gray;
					data_ptr[new_i] = data_ptr[old_i];
				}
				break;
			case ColorType::RGB: // RGBA -> RGB
				for (std::size_t i = 0; i < pixel_count; i++)
				{
					std::size_t old_i = i * 4;
					std::size_t new_i = i * 3;
					data_ptr[new_i++] = data_ptr[old_i++];    // R -> R
					data_ptr[new_i++] = data_ptr[old_i++];    // G -> G
					data_ptr[new_i] = data_ptr[old_i];        // B -> B
				}
				break;
			case ColorType::RGBA: // RGBA -> RGBA
				break;
			}
			break;
		}
	};

	switch (this->bit_depth)
	{
	case fs::Image::BitDepth::Octuple:
		convert.template operator()<std::uint8_t>();
		break;
	case fs::Image::BitDepth::Sexdecuple:
		convert.template operator()<std::uint16_t>();
		break;
	case fs::Image::BitDepth::Normalized:
		convert.template operator()<float>();
		break;
	default:
		break;
	}

	this->color_type = color_type;
}