Just submitted a post at stackoverflow which would also have a nice place here, so.. here we go 🙂

It was concerning the topic how to convert ARGB to YUV using the GPU. Some time ago I’ve developed and used the following OpenCL kernel to convert ARGB (typical windows bitmap pixel layout) to the y-plane (full sized), u/v-half-plane (quarter sized) memory layout as input for libx264 encoding.

__kernel void ARGB2YUV (
                        __global unsigned int * sourceImage,
                        __global unsigned int * destImage,
                        unsigned int srcHeight,
                        unsigned int srcWidth,
                        unsigned int yuvStride // must be srcWidth/4 since we pack 4 pixels into 1 Y-unit (with 4 y-pixels)
                    )
{
    int i;
    unsigned int RGBs [ 4 ];
    unsigned int posSrc, RGB, Value4 = 0, Value, yuvStrideHalf, srcHeightHalf, yPlaneOffset, posOffset;
    unsigned char red, green, blue;

    unsigned int posX = get_global_id(0);
    unsigned int posY = get_global_id(1);

    if ( posX < yuvStride ) { 
        // Y plane - pack 4 y's within each work item
        if ( posY >= srcHeight )
            return;

        posSrc = (posY * srcWidth) + (posX * 4);

        RGBs [ 0 ] = sourceImage [ posSrc ];
        RGBs [ 1 ] = sourceImage [ posSrc + 1 ];
        RGBs [ 2 ] = sourceImage [ posSrc + 2 ];
        RGBs [ 3 ] = sourceImage [ posSrc + 3 ];

        for ( i=0; i<4; i++ ) { 
            RGB = RGBs [ i ]; blue = RGB & 0xff; green = (RGB >> 8) & 0xff; red = (RGB >> 16) & 0xff;

            Value = ( ( 66 * red + 129 * green + 25 * blue ) >> 8 ) + 16;
            Value4 |= (Value << (i * 8)); 
        } 
        destImage [ (posY * yuvStride) + posX ] = Value4; 
        return; 
    } 
    posX -= yuvStride; yuvStrideHalf = yuvStride >> 1; 

    // U plane - pack 4 u's within each work item 
    if ( posX <= yuvStrideHalf )
        return;

    srcHeightHalf = srcHeight >> 1;
    if ( posY < srcHeightHalf ) {
        posSrc = ((posY * 2) * srcWidth) + (posX * 8);

        RGBs [ 0 ] = sourceImage [ posSrc ];
        RGBs [ 1 ] = sourceImage [ posSrc + 2 ];
        RGBs [ 2 ] = sourceImage [ posSrc + 4 ];
        RGBs [ 3 ] = sourceImage [ posSrc + 6 ];

        for ( i=0; i<4; i++ ) { 
            RGB = RGBs [ i ]; 
            blue = RGB & 0xff; green = (RGB >> 8) & 0xff; red = (RGB >> 16) & 0xff;

            Value = ( ( -38 * red + -74 * green + 112 * blue ) >> 8 ) + 128;
            Value4 |= (Value << (i * 8));
        } 
        yPlaneOffset = yuvStride * srcHeight; 
        posOffset = (posY * yuvStrideHalf) + posX; 
        destImage [ yPlaneOffset + posOffset ] = Value4; 
        return; 
    } 
    posY -= srcHeightHalf; 
    if ( posY >= srcHeightHalf )
        return;

    // V plane - pack 4 v's within each work item
    posSrc = ((posY * 2) * srcWidth) + (posX * 8);

    RGBs [ 0 ] = sourceImage [ posSrc ];
    RGBs [ 1 ] = sourceImage [ posSrc + 2 ];
    RGBs [ 2 ] = sourceImage [ posSrc + 4 ];
    RGBs [ 3 ] = sourceImage [ posSrc + 6 ];

    for ( i=0; i<4; i++ ) { 
        RGB = RGBs [ i ]; 
        blue = RGB & 0xff; green = (RGB >> 8) & 0xff; red = (RGB >> 16) & 0xff;

        Value = ( ( 112 * red + -94 * green + -18 * blue ) >> 8 ) + 128;
        Value4 |= (Value << (i * 8));
    }

    yPlaneOffset = yuvStride * srcHeight;
    posOffset = (posY * yuvStrideHalf) + posX;

    destImage [ yPlaneOffset + (yPlaneOffset >> 2) + posOffset ] = Value4;
    return;
}

This code performs only global 32-bit memory access while 8-bit processing happens within each work item.

Oh.. and the proper code to invoke the kernel

unsigned int width = 1024;
unsigned int height = 768;

unsigned int frameSize = width * height;
const unsigned int argbSize = frameSize * 4; // ARGB pixels

const unsigned int yuvSize = frameSize + (frameSize >> 1); // Y,U,V planes

const unsigned int yuvStride = width >> 2; // since we pack 4 RGBs into "one" YYYY

// Allocates ARGB buffer
ocl_rgb_buffer = clCreateBuffer ( context, CL_MEM_READ_WRITE, argbSize, 0, &error );
// ... error handling ...

ocl_yuv_buffer = clCreateBuffer ( context, CL_MEM_READ_WRITE, yuvSize, 0, &error );
// ... error handling ...

error = clSetKernelArg ( kernel, 0, sizeof(cl_mem), &ocl_rgb_buffer );
error |= clSetKernelArg ( kernel, 1, sizeof(cl_mem), &ocl_yuv_buffer );

error |= clSetKernelArg ( kernel, 2, sizeof(unsigned int), &height);
error |= clSetKernelArg ( kernel, 3, sizeof(unsigned int), &width);

error |= clSetKernelArg ( kernel, 4, sizeof(unsigned int), &yuvStride);
// ... error handling ...

const size_t local_ws[] = { 16, 32 };
const size_t global_ws[] = { yuvStride + (yuvStride>>1), height };

error = clEnqueueNDRangeKernel ( queue, kernel, 2, NULL, global_ws, local_ws, 0, NULL, NULL );
// ... error handling ...

Note: have a look at the work item calculations. Some additional code needs to be added (e.g. using mod so as to add sufficient spare items) to make sure that work item sizes fit to local work sizes.

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