Note: Use the class `"language-opencl"` for OpenCL kernel code. Host code is automatically highlighted with the `"language-c"` or `"language-cpp"` class.

## OpenCL host code

    // OpenCL functions, constants, etc. are also highlighted in OpenCL host code in the c or cpp language
    cl::Event KernelFilterImages::runSingle(const cl::Image2D& imgSrc, SPImage2D& imgDst)
    {
    	const size_t rows = imgSrc.getImageInfo<CL_IMAGE_HEIGHT>();
    	const size_t cols = imgSrc.getImageInfo<CL_IMAGE_WIDTH>();

    	ASSERT(rows > 0 && cols > 0, "The image object seems to be invalid, no rows/cols set");
    	ASSERT(imgSrc.getImageInfo<CL_IMAGE_FORMAT>().image_channel_data_type == CL_FLOAT, "Only float type images are supported");
    	ASSERT(imgSrc.getInfo<CL_MEM_FLAGS>() == CL_MEM_READ_ONLY || imgSrc.getInfo<CL_MEM_FLAGS>() == CL_MEM_READ_WRITE, "Can't read the input image");

    	imgDst = std::make_shared<cl::Image2D>(*context, CL_MEM_READ_WRITE, cl::ImageFormat(CL_R, CL_FLOAT), cols, rows);

    	cl::Kernel kernel(*program, "filter_single");
    	kernel.setArg(0, imgSrc);
    	kernel.setArg(1, *imgDst);
    	kernel.setArg(2, bufferKernel1);
    	kernel.setArg(3, kernel1.rows);
    	kernel.setArg(4, kernel1.rows / 2);
    	kernel.setArg(5, kernel1.cols);
    	kernel.setArg(6, kernel1.cols / 2);
    	kernel.setArg(7, border);

    	cl::Event eventFilter;
    	const cl::NDRange global(cols, rows);
    	queue->enqueueNDRangeKernel(kernel, cl::NullRange, global, cl::NullRange, &events, &eventFilter);
    }

## OpenCL kernel code

    // CLK_ADDRESS_CLAMP_TO_EDGE = aaa|abcd|ddd
    constant sampler_t sampler = CLK_NORMALIZED_COORDS_FALSE | CLK_ADDRESS_CLAMP_TO_EDGE | CLK_FILTER_NEAREST;
    typedef float type_single;

    type_single filter_sum_single_3x3(read_only image2d_t imgIn,
                                      constant float* filterKernel,
                                      const int2 coordBase,
                                      const int border)
    {
        type_single sum = (type_single)(0.0f);
        const int rows = get_image_height(imgIn);
        const int cols = get_image_width(imgIn);
        int2 coordCurrent;
        int2 coordBorder;
        float color;

        // Image patch is row-wise accessed
        // Filter kernel is centred in the middle
        #pragma unroll
        for (int y = -ROWS_HALF_3x3; y <= ROWS_HALF_3x3; ++y)       // Start at the top left corner of the filter
        {
            coordCurrent.y = coordBase.y + y;
            #pragma unroll
            for (int x = -COLS_HALF_3x3; x <= COLS_HALF_3x3; ++x)   // And end at the bottom right corner
            {
                coordCurrent.x = coordBase.x + x;
                coordBorder = borderCoordinate(coordCurrent, rows, cols, border);
                color = read_imagef(imgIn, sampler, coordBorder).x;

                const int idx = (y + ROWS_HALF_3x3) * COLS_3x3 + x + COLS_HALF_3x3;
                sum += color * filterKernel[idx];
            }
        }

        return sum;
    }

    kernel void filter_single_3x3(read_only image2d_t imgIn,
                                  write_only image2d_t imgOut,
                                  constant float* filterKernel,
                                  const int border)
    {
        int2 coordBase = (int2)(get_global_id(0), get_global_id(1));

        type_single sum = filter_sum_single_3x3(imgIn, filterKernel, coordBase, border);

        write_imagef(imgOut, coordBase, sum);
    }