Function kmeans_assign

Source/astcenc_find_best_partitioning.cpp:146–199

* @brief Assign texels to clusters, based on a set of chosen center points. * * @param blk The image block color data to compress. * @param texel_count The number of texels in the block. * @param partition_count The number of partitions in the block. * @param cluster_centers The partition cluster center colors. * @param[out] partition_o

 * @param[out] partition_of_texel   The partition assigned for each texel.
 */
static void kmeans_assign(
	const image_block& blk,
	unsigned int texel_count,
	unsigned int partition_count,
	const vfloat4 cluster_centers[BLOCK_MAX_PARTITIONS],
	uint8_t partition_of_texel[BLOCK_MAX_TEXELS]
) {
	promise(texel_count > 0);
	promise(partition_count > 0);

	uint8_t partition_texel_count[BLOCK_MAX_PARTITIONS] { 0 };

	// Find the best partition for every texel
	for (unsigned int i = 0; i < texel_count; i++)
	{
		float best_distance = std::numeric_limits<float>::max();
		unsigned int best_partition = 0;

		vfloat4 color = blk.texel(i);
		for (unsigned int j = 0; j < partition_count; j++)
		{
			vfloat4 diff = color - cluster_centers[j];
			float distance = dot_s(diff * diff, blk.channel_weight);
			if (distance < best_distance)
			{
				best_distance = distance;
				best_partition = j;
			}
		}

		partition_of_texel[i] = static_cast<uint8_t>(best_partition);
		partition_texel_count[best_partition]++;
	}

	// It is possible to get a situation where a partition ends up without any texels. In this case,
	// assign texel N to partition N. This is silly, but ensures that every partition retains at
	// least one texel. Reassigning a texel in this manner may cause another partition to go empty,
	// so if we actually did a reassignment, run the whole loop over again.
	bool problem_case;
	do
	{
		problem_case = false;
		for (unsigned int i = 0; i < partition_count; i++)
		{
			if (partition_texel_count[i] == 0)
			{
				partition_texel_count[partition_of_texel[i]]--;
				partition_texel_count[i]++;
				partition_of_texel[i] = static_cast<uint8_t>(i);
				problem_case = true;
			}
		}
	} while (problem_case);
}

/**
 * @brief Compute new cluster centers based on their center of gravity.