Write (image) to byte array

[GeorgeR]

If it's not too much trouble, i'd love to see an example of writing an openCL program to a byte array (which System.Drawing.Bitmap and so on can convert to a bitmap). The program would simply write a flat color (orange?) to the image so you know you're not getting a false positive!

[nythrix]

I'm not sure I understood you correctly. You want to know how to generate an orange (or any color) Bitmap from a byte array using an OpenCL kernel?
If yes here goes:

using Cloo;
using System;
using System.Drawing;
using System.Drawing.Imaging;
namespace ClooTesting
{
    public class BitmapExample
    {
        public static void Main()
        {
            int imgWidth = 320;
            int imgHeight = 240;
            int channels = 4; // you should beware of the pixel format of the final image. ARGB in our case so there are 4 bytes per pixel.
            int bufferSize = imgWidth * imgHeight * channels;
            ComputeContextPropertyList cpl = new ComputeContextPropertyList(ComputePlatform.Platforms[1]);
            ComputeContext context = new ComputeContext(ComputeDeviceTypes.Default, cpl, null, IntPtr.Zero);
            ComputeCommandQueue commands = new ComputeCommandQueue(context, context.Devices[0], ComputeCommandQueueFlags.None);
            ComputeBuffer<byte> clBuffer = new ComputeBuffer<byte>(context, ComputeMemoryFlags.WriteOnly, bufferSize);
                        
            string clKernel = @"
kernel void BitmapKernel(global write_only uchar4* array)
{
    int gid = get_global_id(0);
// Even though the Bitmap format will be ARGB this array will be parsed as BGRA. So be careful with that. No, I don't know why it works like this. Ask Microsoft :)
    array[gid] = (uchar4)(0, 0, 255, 127); // This is full red/half transparent color.
}";
            ComputeProgram program = new ComputeProgram(context, clKernel);
            program.Build(null, null, null, IntPtr.Zero);
            ComputeKernel kernel = program.CreateKernel("BitmapKernel");
            kernel.SetMemoryArgument(0, clBuffer);
            commands.Execute(kernel, null, new long[] { imgWidth*imgHeight }, null, null);
            byte[] array = null;
            commands.ReadFromBuffer(clBuffer, ref array, true, null);
            
            Bitmap bmp;
            unsafe
            {
                fixed (byte* arrayPtr = array)
                    bmp = new Bitmap(imgWidth, imgHeight, channels * imgWidth, PixelFormat.Format32bppArgb, (IntPtr)arrayPtr);
            }
            bmp.Save("image.png");
            bmp.Dispose();
        }
    }
}

[nythrix]

However, there's a better way of getting the data directly into the Bitmap without using a temporary array.

Bitmap bmp = new Bitmap(imgWidth, imgHeight, PixelFormat.Format32bppArgb); 
BitmapData bmpData = bmp.LockBits(new Rectangle(0, 0, imgWidth, imgHeight), ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb); 
commands.Read(clBuffer, true, 0, clBuffer.Size, bmpData.Scan0, null); bmp.UnlockBits(bmpData);