Fast RGB => YUV conversion in OpenCL - alignment

Question

I know the following formula can be used to convert RGB images to YUV images. In the following formula, R, G, B, Y, U, V are all 8-bit unsigned integers, and intermediate values are 16-bit unsigned integers.
Y = ( ( 66 * R + 129 * G + 25 * B + 128) >> 8) + 16
U = ( ( -38 * R - 74 * G + 112 * B + 128) >> 8) + 128
V = ( ( 112 * R - 94 * G - 18 * B + 128) >> 8) + 128
But when the formula is used in OpenCL it's a different story.
1. 8-bit memory write access is an optional extension, which means some OpenCL implementations may not support it.
2. even the above extension is supported, it's deadly slow compared with 32-bit write access.
In order to get better performance, every 4 pixels will be processed at the same time, so the input is 12 8-bit integers and the output is 3 32-bit unsigned integers(the first one stands for 4 Y samples, the second one stands for 4 U samples, the last one stands for 4 V samples).
My question is how to get these 3 32-bit integers directly from the 12 8-bit integers? Is there a formula to get these 3 32-bit integers, or I just need to use the old formula to get 12 8-bit integer results(4 Y, 4 U, 4 V) and construct the 3 32-bit integers with bit-wise operation?

Answer 1

Even though this question was asked 2 years ago, i think some working code would help here. In terms of the initial concerns about bad performance when directly accessing 8-bit values, it's better to perform 32-bit direct access when possible.
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,j;
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, 16 };
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.

Answer 2

Like this? Use int4 unless your platform can use int3. Also you can pack 5 pixels into an int16 so you are wasting 1/16 instead of 1/4 of the memory bandwidth.
__kernel void rgb2yuv( __global int3* input, __global int3* output){
rgb = input[get_global_id(0)];
R = rgb.x;
G = rgb.y;
B = rgb.z;
yuv.x = ( ( 66 * R + 129 * G + 25 * B + 128) >> 8) + 16;
yuv.y = ( ( -38 * R - 74 * G + 112 * B + 128) >> 8) + 128;
yuv.z = ( ( 112 * R - 94 * G - 18 * B + 128) >> 8) + 128;
output[get_global_id(0)] = yuv;
}

Answer 3

Along with opencl specification data type int3 doesn't exists.
Page 123:
Supported values of n are 2, 4, 8, and 16...
In your kernel variables rgb, R, G, B, and yuv should be at least __private int4.
OpenCL 1.1 added support for typen where n = 3. However, I strongly recommend you don't use it. Different vendor implementations have different bugs, and it's not saving you anything.