Calculate miplevels for image input data

mip.cpp

// Generate mipmap from input data
// adapted from https://github.com/ValveSoftware/openvr/blob/1fb1030f2ac238456dca7615a4408fb2bb42afb6/samples/hellovr_vulkan/hellovr_vulkan_main.cpp#L2271
template <typename PixelType, const size_t numChannels>
static void generate_mipmap( const PixelType *pSrc, PixelType *pDst, uint32_t const nSrcWidth, uint32_t const nSrcHeight, uint32_t *pDstWidthOut, uint32_t *pDstHeightOut ) {

	*pDstWidthOut = nSrcWidth / 2;
	if ( *pDstWidthOut <= 0 ) {
		*pDstWidthOut = 1;
	}
	*pDstHeightOut = nSrcHeight / 2;
	if ( *pDstHeightOut <= 0 ) {
		*pDstHeightOut = 1;
	}

	for ( uint32_t y = 0; y != *pDstHeightOut; y++ ) {
		for ( uint32_t x = 0; x != *pDstWidthOut; x++ ) {

			// We use floats to accumulate pixel values.
			//
			// TODO: if pixels arrive in non-linear SRGB format, we must convert them to linear when
			// we read them, and convert them back to non-linear when we write them back after averaging.
			//
			float channel[ numChannels ]{};

			uint32_t nSrcIndex[ 4 ]; // we reduce 4 neighbouring pixels to 1

			// Get pixel indices
			nSrcIndex[ 0 ] = ( ( ( y * 2 ) * nSrcWidth ) + ( x * 2 ) ) * 4;
			nSrcIndex[ 1 ] = ( ( ( y * 2 ) * nSrcWidth ) + ( x * 2 + 1 ) ) * 4;
			nSrcIndex[ 2 ] = ( ( ( ( y * 2 ) + 1 ) * nSrcWidth ) + ( x * 2 ) ) * 4;
			nSrcIndex[ 3 ] = ( ( ( ( y * 2 ) + 1 ) * nSrcWidth ) + ( x * 2 + 1 ) ) * 4;

			// Sum all pixels
			for ( uint32_t nSample = 0; nSample != 4; nSample++ ) {
				for ( uint32_t c = 0; c != numChannels; c++ ) {
					channel[ c ] += pSrc[ nSrcIndex[ nSample ] + c ];
				}
			}

			// Average results
			for ( uint32_t c = 0; c != numChannels; c++ ) {
				channel[ c ] /= 4.f;
			}

			// Store resulting pixels
			for ( uint32_t c = 0; c != numChannels; c++ ) {
				pDst[ ( y * ( *pDstWidthOut ) + x ) * numChannels + c ] = static_cast<PixelType>( channel[ c ] );
			}
		}
	}
}