Function WriteP4

Siv3D/src/Siv3D/ImageFormat/PPM/PPMEncoder.cpp:129–170

		}

		static bool WriteP4(const Image& image, IWriter& writer)
		{
			writer.write("P4\n", 3);
			WriteNumber(writer, image.width());
			writer.write(' ');
			WriteNumber(writer, image.height());
			writer.write('\n');

			const auto binarize = [&](const Color& c)->bool {return 128000 <= 299 * c.r + 587 * c.g + 114 * c.b; };

			for (int32 y = 0; y < image.height(); ++y)
			{
				int32 x = 0;
				for (; x + 7 < image.width(); x += 8)
				{
					const uint8 c =
						(binarize(image[y][x]) ? 128 : 0) +
						(binarize(image[y][x + 1]) ? 64 : 0) +
						(binarize(image[y][x + 2]) ? 32 : 0) +
						(binarize(image[y][x + 3]) ? 16 : 0) +
						(binarize(image[y][x + 4]) ? 8 : 0) +
						(binarize(image[y][x + 5]) ? 4 : 0) +
						(binarize(image[y][x + 6]) ? 2 : 0) +
						(binarize(image[y][x + 7]) ? 1 : 0);

					writer.write(c);
				}

				if (x < image.width())
				{
					uint8 c = 0;
					for (uint8 i = 128; x < image.width(); ++x, i >>= 1)
					{
						c += binarize(image[y][x]) ? i : 0;
					}

					writer.write(c);
				}
			}

			return true;
		}

		static bool WriteP5(const Image& image, IWriter& writer)
		{