I'm new to opencl but I have some experience using HLSL. In HLSL multiple passes are used when you need to finish a computation before moving on to the next step.

I would like to know how this sort of thing is done in opencl.

I would like to do something like

__kernel void imagingTest(__read_only  image2d_t srcImg, __write_only image2d_t dstImg)
    //DO Some stuff here

    // write the image
    write_imageui(dstImg, coord, bgra);     

I would like to apply this kernel multiple times


At the moment the C# code I have to call the kernel is

using System;
using System.Collections;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Runtime.InteropServices;

using Emgu.CV;
using Emgu.Util;
using Emgu;
using Emgu.CV.Structure;

using OpenCL.Net;

namespace HLSLTest
    public class Computations
        private Cl.Context _context;
        private Cl.Device _device;
        private Cl.Kernel kernel;
        private Cl.Kernel kernel2;

        private void CheckErr(Cl.ErrorCode err, string name)
            if (err != Cl.ErrorCode.Success)
                Console.WriteLine("ERROR: " + name + " (" + err.ToString() + ")");

        private void ContextNotify(string errInfo, byte[] data, IntPtr cb, IntPtr userData)
            Console.WriteLine("OpenCL Notification: " + errInfo);

        public void Setup()
            Cl.ErrorCode error;
            Cl.Platform[] platforms = Cl.GetPlatformIDs(out error);
            List<Cl.Device> devicesList = new List<Cl.Device>();

            CheckErr(error, "Cl.GetPlatformIDs");

            foreach (Cl.Platform platform in platforms)
                string platformName = Cl.GetPlatformInfo(platform, Cl.PlatformInfo.Name, out error).ToString();
                Console.WriteLine("Platform: " + platformName);
                CheckErr(error, "Cl.GetPlatformInfo");

                //We will be looking only for GPU devices
                foreach (Cl.Device device in Cl.GetDeviceIDs(platform, Cl.DeviceType.Gpu, out error))
                    CheckErr(error, "Cl.GetDeviceIDs");
                    Console.WriteLine("Device: " + device.ToString());

            if (devicesList.Count <= 0)
                Console.WriteLine("No devices found.");

            _device = devicesList[0];

            if (Cl.GetDeviceInfo(_device, Cl.DeviceInfo.ImageSupport, out error).CastTo<Cl.Bool>() == Cl.Bool.False)
                Console.WriteLine("No image support.");

            _context = Cl.CreateContext(null, 1, new[] { _device }, ContextNotify, IntPtr.Zero, out error); //Second parameter is amount of devices
            CheckErr(error, "Cl.CreateContext");

            //Load and compile kernel source code.
            string programPath = Environment.CurrentDirectory + "/../../../ImagingTest.cl";  //The path to the source file may vary

            if (!System.IO.File.Exists(programPath))
                Console.WriteLine("Program doesn't exist at path " + programPath);

            string programSource = System.IO.File.ReadAllText(programPath);

            using (Cl.Program program = Cl.CreateProgramWithSource(_context, 1, new[] { programSource }, null, out error))
                CheckErr(error, "Cl.CreateProgramWithSource");

                //Compile kernel source
                error = Cl.BuildProgram(program, 1, new[] { _device }, string.Empty, null, IntPtr.Zero);
                CheckErr(error, "Cl.BuildProgram");

                //Check for any compilation errors
                        out error
                    ).CastTo<Cl.BuildStatus>() != Cl.BuildStatus.Success
                    CheckErr(error, "Cl.GetProgramBuildInfo");
                    Console.WriteLine("Cl.GetProgramBuildInfo != Success");
                    Console.WriteLine(Cl.GetProgramBuildInfo(program, _device, Cl.ProgramBuildInfo.Log, out error));

                //Create the required kernel (entry function)
                kernel = Cl.CreateKernel(program, "imagingTest", out error);
                kernel2 = Cl.CreateKernel(program, "imagingTest", out error);
                CheckErr(error, "Cl.CreateKernel");


        public void ImagingTest(Image<Gray, Single> InputImage, out Image<Gray, Single> outputImage)
            Cl.ErrorCode error;

            int intPtrSize = 0;
            intPtrSize = Marshal.SizeOf(typeof(IntPtr));

            //Image's RGBA data converted to an unmanaged[] array
            byte[] inputByteArray;
            //OpenCL memory buffer that will keep our image's byte[] data.
            Cl.Mem inputImage2DBuffer;

            Cl.ImageFormat clImageFormat = new Cl.ImageFormat(Cl.ChannelOrder.RGBA, Cl.ChannelType.Unsigned_Int8);

            int inputImgWidth, inputImgHeight;
            int inputImgBytesSize;
            int inputImgStride;

            inputImgWidth = InputImage.Width;
            inputImgHeight = InputImage.Height;

            System.Drawing.Bitmap bmpImage = InputImage.ToBitmap();

            //Get raw pixel data of the bitmap
            //The format should match the format of clImageFormat
            BitmapData bitmapData = bmpImage.LockBits
                new Rectangle(0, 0, bmpImage.Width, bmpImage.Height),

            inputImgStride = bitmapData.Stride;
            inputImgBytesSize = bitmapData.Stride * bitmapData.Height;

            //Copy the raw bitmap data to an unmanaged byte[] array
            inputByteArray = new byte[inputImgBytesSize];
            Marshal.Copy(bitmapData.Scan0, inputByteArray, 0, inputImgBytesSize);

            //Allocate OpenCL image memory buffer
            inputImage2DBuffer = Cl.CreateImage2D
                Cl.MemFlags.CopyHostPtr | Cl.MemFlags.ReadOnly,
                out error

            CheckErr(error, "Cl.CreateImage2D input");

            //Unmanaged output image's raw RGBA byte[] array
            byte[] outputByteArray = new byte[inputImgBytesSize];            

            //Allocate OpenCL image memory buffer
            Cl.Mem outputImage2DBuffer = Cl.CreateImage2D
                Cl.MemFlags.CopyHostPtr | Cl.MemFlags.WriteOnly,
                out error

            CheckErr(error, "Cl.CreateImage2D output");

            //Pass the memory buffers to our kernel function
            error  = Cl.SetKernelArg(kernel, 0, (IntPtr)intPtrSize,  inputImage2DBuffer);
            error |= Cl.SetKernelArg(kernel, 1, (IntPtr)intPtrSize, outputImage2DBuffer);
            CheckErr(error, "Cl.SetKernelArg");

            //Create a command queue, where all of the commands for execution will be added
            Cl.CommandQueue cmdQueue = Cl.CreateCommandQueue(_context, _device, (Cl.CommandQueueProperties)0, out error);
            CheckErr(error, "Cl.CreateCommandQueue");

            Cl.Event clevent;

            //Copy input image from the host to the GPU.
            IntPtr[] originPtr = new IntPtr[] { (IntPtr)0, (IntPtr)0, (IntPtr)0 };  //x, y, z
            IntPtr[] regionPtr = new IntPtr[] { (IntPtr)inputImgWidth, (IntPtr)inputImgHeight, (IntPtr)1 }; //x, y, z
            IntPtr[] workGroupSizePtr = new IntPtr[] { (IntPtr)inputImgWidth, (IntPtr)inputImgHeight, (IntPtr)1 };
            error = Cl.EnqueueWriteImage(cmdQueue, inputImage2DBuffer, Cl.Bool.True, originPtr, regionPtr, (IntPtr)0, (IntPtr)0, inputByteArray, 0, null, out clevent);
            CheckErr(error, "Cl.EnqueueWriteImage");

            //Execute our kernel (OpenCL code)
            error = Cl.EnqueueNDRangeKernel(cmdQueue, kernel , 2, null, workGroupSizePtr, null, 0, null, out clevent);
            //Cl.WaitForEvents(1, new Cl.Event[] { clevent } );
            CheckErr(error, "Cl.EnqueueNDRangeKernel");

            //Wait for completion of all calculations on the GPU.
            error = Cl.Finish(cmdQueue);
            CheckErr(error, "Cl.Finish");

            //Read the processed image from GPU to raw RGBA data byte[] array
            error = Cl.EnqueueReadImage
                out clevent

            CheckErr(error, "Cl.clEnqueueReadImage");

            //Clean up memory

            Cl.ReleaseMemObject( inputImage2DBuffer);

            //Get a pointer to our unmanaged output byte[] array
            GCHandle pinnedOutputArray = GCHandle.Alloc(outputByteArray, GCHandleType.Pinned);
            IntPtr outputBmpPointer = pinnedOutputArray.AddrOfPinnedObject();

            //Create a new bitmap with processed data and save it to a file.
            Bitmap outputBitmap = new Bitmap(inputImgWidth, inputImgHeight, inputImgStride, PixelFormat.Format32bppArgb, outputBmpPointer);

            outputImage = new Image<Gray, Single>(outputBitmap);

            //outputBitmap.Save(outputImagePath, System.Drawing.Imaging.ImageFormat.Png);


I've been thinking that after the

error = Cl.EnqueueNDRangeKernel(cmdQueue, kernel , 2, null, workGroupSizePtr, null, 0, null, out clevent);

line I should add

Cl.WaitForEvents(1, new Cl.Event[] { clevent } );

Then enqueue another kernel but I'm not sure how to do it.

  • 1
    \$\begingroup\$ What OpenCL .NET library are you using? \$\endgroup\$ – S.Richmond Aug 15 '13 at 6:35
  • \$\begingroup\$ There are more than one? \$\endgroup\$ – sav Aug 15 '13 at 6:35
  • \$\begingroup\$ Yeah there are a few: openclnet.codeplex.com and opentk.com \$\endgroup\$ – S.Richmond Aug 15 '13 at 6:37
  • \$\begingroup\$ I'm using openclnet.codeplex.com \$\endgroup\$ – sav Aug 15 '13 at 6:38
  • \$\begingroup\$ Although I'm not particular about which one I use if another is easier \$\endgroup\$ – sav Aug 15 '13 at 6:38

I've figured it out. (I'm using cloo now)

I needed to do this in the opencl code.

I put in a barrier.


So for example

__kernel void AnisoDiff2D
    __global double* img1,
    __global double* img2
    const int x = get_global_id(0);
    const int y = get_global_id(1);
    int index   = CoordinateToIndex(  x, y  );

    //write some stuff to img2
    //img2[index] = ...

    //put in a barrier, this ensures all the processing is done before proceding

    //Set img1 based on some calculation from img2
    //img1[index] = f(img2[index]);

    //Repeat this as many times as you need
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