Is there a way to capture Metal output as a bitmap or other format suitable for printing?

Question

I'm converting an app from OpenGL to Metal, using MTKit. The original app allowed the user to print the OpenGL-generated screen being viewed (I've also done this with Core Graphics). I'd like to do the same in Metal. I thought it would be easy to find a solution or at least an approach to the problem, but I'm stuck. Has anyone solved this, or does anyone have any inklings about what I should be looking for?

I'm a seasoned Mac programmer (would prefer an Objective-C solution but I do a bit of work with everything else) and am an advanced novice at Metal.

• MacOS 10.15.3, all MacBooks.
• Swift 5.
• Xcode 11.2.1.

Answer 1

I assume you want to save image from MTKView. But this function should work for any textures. Also don't forget to set: framebufferOnly = false; Property.

bool takeScreenshot = true;
/// Called whenever the view needs to render a frame
- (void)drawInMTKView:(nonnull MTKView *)view
{
    // Create a new command buffer for each render pass to the current drawable
    id<MTLCommandBuffer> commandBuffer = [_commandQueue commandBuffer];
    commandBuffer.label = @"MyCommand";

    // Obtain a renderPassDescriptor generated from the view's drawable textures
    MTLRenderPassDescriptor *renderPassDescriptor = view.currentRenderPassDescriptor;
    id<MTLTexture> currentSwapChainTexture = view.currentDrawable.texture;



    // Your render code...



    [commandBuffer addCompletedHandler:^(id<MTLCommandBuffer> cb)
     {
        if(takeScreenshot)
        {
            SaveTexture(currentSwapChainTexture);
            takeScreenshot = false;
        }

    }];

    // Finalize rendering here & push the command buffer to the GPU
    [commandBuffer commit];
    // CPU <-> GPU Synchronization
    if(takeScreenshot)[commandBuffer waitUntilCompleted];

---

Save texture function:

void SaveTexture(id<MTLTexture> texture)
{
    int width = (int) texture.width;
    int height = (int) texture.height;
    int bytePerPixel = 4;
    int bytesPerRow = width * bytePerPixel;
    int bytesCount = width * height * bytePerPixel;
    int bitsPerComponent = 8;
    int bitsPerPixel = 32;

    void *imageBytes = malloc(bytesCount);
    void *destBytes = malloc(bytesCount);

    MTLRegion mtlregion = MTLRegionMake2D(0, 0, width, height);
    [texture getBytes:imageBytes bytesPerRow:bytesPerRow fromRegion:mtlregion mipmapLevel:0];

    vImage_Buffer src;
    src.data = imageBytes;
    src.width = width;
    src.height = height;
    src.rowBytes = bytesPerRow;


    vImage_Buffer dest;
    dest.data = destBytes;
    dest.width = width;
    dest.height = height;
    dest.rowBytes = bytesPerRow;

    // BGRA -> RGBA (Swap)
    const uint8_t map[4] = {2, 1, 0, 3};
    vImagePermuteChannels_ARGB8888(&src, &dest, map, kvImageNoFlags);


    CGColorSpaceRef cgColorSpaceRef = CGColorSpaceCreateWithName(kCGColorSpaceDisplayP3); //kCGColorSpaceSRGB - For sRGB
    CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big;
    CGContextRef context = CGBitmapContextCreate(destBytes, width, height, bitsPerComponent, bytesPerRow, cgColorSpaceRef, bitmapInfo);
    CGImageRef cgImage = CGBitmapContextCreateImage(context);

    // Your NSImage
    NSImage * image = [[NSImage alloc] initWithCGImage:cgImage size:NSZeroSize];

    // Save to Photos
    [[PHPhotoLibrary sharedPhotoLibrary] performChanges:^
     {
        [PHAssetCreationRequest creationRequestForAssetFromImage: image];
    }

                                      completionHandler:^(BOOL success, NSError * error)
     {
        if(success) printf("Success \n");
    }];



    free(imageBytes);
    free(destBytes);
    CGColorSpaceRelease(cgColorSpaceRef);
    CGContextRelease(context);
    CGImageRelease(cgImage);
    texture = nil;
}