Function encode_bc1

external/basisu/transcoder/basisu_transcoder.cpp:14904–15132

Good references: squish library, stb_dxt.

	// Good references: squish library, stb_dxt.
	void encode_bc1(void* pDst, const uint8_t* pPixels, uint32_t flags)
	{
		const color32* pSrc_pixels = (const color32*)pPixels;
		dxt1_block* pDst_block = static_cast<dxt1_block*>(pDst);
		
		int avg_r = -1, avg_g = 0, avg_b = 0;
		int lr = 0, lg = 0, lb = 0, hr = 0, hg = 0, hb = 0;
		uint8_t sels[16];
		
		const bool use_sels = (flags & cEncodeBC1UseSelectors) != 0;
		if (use_sels)
		{
			// Caller is jamming in their own selectors for us to try.
			const uint32_t s = pDst_block->m_selectors[0] | (pDst_block->m_selectors[1] << 8) | (pDst_block->m_selectors[2] << 16) | (pDst_block->m_selectors[3] << 24);
			
			static const uint8_t s_sel_tran[4] = { 0, 3, 1, 2 };
			
			for (uint32_t i = 0; i < 16; i++)
				sels[i] = s_sel_tran[(s >> (i * 2)) & 3];
		}
		else
		{
			const uint32_t fr = pSrc_pixels[0].r, fg = pSrc_pixels[0].g, fb = pSrc_pixels[0].b;

			uint32_t j;
			for (j = 1; j < 16; j++)
				if ((pSrc_pixels[j].r != fr) || (pSrc_pixels[j].g != fg) || (pSrc_pixels[j].b != fb))
					break;
						
			if (j == 16)
			{
				encode_bc1_solid_block(pDst, fr, fg, fb);
				return;
			}
			
			// Select 2 colors along the principle axis. (There must be a faster/simpler way.)
			int total_r = fr, total_g = fg, total_b = fb;
			int max_r = fr, max_g = fg, max_b = fb;
			int min_r = fr, min_g = fg, min_b = fb;
			for (uint32_t i = 1; i < 16; i++)
			{
				const int r = pSrc_pixels[i].r, g = pSrc_pixels[i].g, b = pSrc_pixels[i].b;
				max_r = basisu::maximum(max_r, r); max_g = basisu::maximum(max_g, g); max_b = basisu::maximum(max_b, b);
				min_r = basisu::minimum(min_r, r); min_g = basisu::minimum(min_g, g); min_b = basisu::minimum(min_b, b);
				total_r += r; total_g += g; total_b += b;
			}

			avg_r = (total_r + 8) >> 4;
			avg_g = (total_g + 8) >> 4;
			avg_b = (total_b + 8) >> 4;

			int icov[6] = { 0, 0, 0, 0, 0, 0 };
			for (uint32_t i = 0; i < 16; i++)
			{
				int r = (int)pSrc_pixels[i].r - avg_r;
				int g = (int)pSrc_pixels[i].g - avg_g;
				int b = (int)pSrc_pixels[i].b - avg_b;
				icov[0] += r * r;