Function pack_eac

external/basisu/transcoder/basisu_transcoder.cpp:15737–15891

Only checks 4 tables.

	// Only checks 4 tables.
	static void pack_eac(eac_block& blk, const uint8_t* pPixels, uint32_t stride)
	{
		uint32_t min_alpha = 255, max_alpha = 0;
		for (uint32_t i = 0; i < 16; i++)
		{
			const uint32_t a = pPixels[i * stride];
			if (a < min_alpha) min_alpha = a;
			if (a > max_alpha) max_alpha = a;
		}

		if (min_alpha == max_alpha)
		{
			pack_eac_solid_block(blk, min_alpha);
			return;
		}

		const uint32_t alpha_range = max_alpha - min_alpha;

		const uint32_t SINGLE_TABLE_THRESH = 5;
		if (alpha_range <= SINGLE_TABLE_THRESH)
		{
			// If alpha_range <= 5 table 13 is lossless
			int base = clamp255((int)max_alpha - 2);

			blk.m_base = base;
			blk.m_multiplier = 1;
			blk.m_table = 13;

			base -= 3;

			uint64_t packed_sels = 0;
			for (uint32_t i = 0; i < 16; i++)
			{
				const int a = pPixels[i * stride];

				static const uint8_t s_sels[6] = { 2, 1, 0, 4, 5, 6 };

				int sel = a - base;
				assert(sel >= 0 && sel <= 5);

				packed_sels |= (static_cast<uint64_t>(s_sels[sel]) << s_etc2_eac_bit_ofs[i]);
			}

			blk.set_selector_bits(packed_sels);

			return;
		}

		const uint32_t T0 = 2, T1 = 8, T2 = 11, T3 = 13;
		static const uint8_t s_tables[4] = { T0, T1, T2, T3 };

		int base[4], mul[4];
		uint32_t mul_or = 0;
		for (uint32_t i = 0; i < 4; i++)
		{
			const uint32_t table = s_tables[i];

			const float range = (float)(g_eac_modifier_table[table][ETC2_EAC_MAX_VALUE_SELECTOR] - g_eac_modifier_table[table][ETC2_EAC_MIN_VALUE_SELECTOR]);