How do I grab an image from my EAGLLayer ?

Question

I'm looking for way to grab the content of my opengl (as UIImage) and then save it into a file. I'm now giving glReadPixels a try though I'm not sure I'm doing the right thing as of what kind of malloc I should be doing. I gather that on OSX it's GL_BGRA but on the iPhone that doesn't work...

Answer 1

All OpenGL|ES complient GL-implementations have to support GL_RGBA as a parameter to glReadPixels.

If your OpenGL|Es supports the

GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES 

extension you can also query the native format. glReadPixels will understand this format as a parameter to glReadPixels and allows you to directly get the pixel-data in native format. This is usually a bit faster than GL_RGBA.

Take a look at the header-file or query the extension-support via glGetString.

Answer 2

Get the data out of the OpenGL ES view:

-(UIImage *) snapshot {
    NSInteger myDataLength = backingWidth * backingHeight * 4;
    // allocate array and read pixels into it.
    GLubyte *buffer = (GLubyte *) malloc(myDataLength);
    glReadPixels(0, 0, backingWidth, backingHeight, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
    // gl renders "upside down" so swap top to bottom into new array.
    // there's gotta be a better way, but this works.
    GLubyte *buffer2 = (GLubyte *) malloc(myDataLength);
    for(int y = 0; y <backingHeight; y++)
    {
        for(int x = 0; x <backingWidth * 4; x++)
        {
            buffer2[((backingHeight - 1) - y) * backingWidth * 4 + x] = buffer[y * 4 * backingWidth + x];
        }
    }
    free(buffer);
    // make data provider with data.
    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer2, myDataLength, NULL);
    // prep the ingredients
    int bitsPerComponent = 8;
    int bitsPerPixel = 32;
    int bytesPerRow = 4 * backingWidth;
    CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
    CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
    // make the cgimage
    CGImageRef imageRef = CGImageCreate(backingWidth, backingHeight, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
    // then make the uiimage from that
    UIImage *myImage = [UIImage imageWithCGImage:imageRef];
    return myImage;
}

Then to save the image:

-(void) saveImage:(UIImage *)image {}
    // Create paths to output images
    NSString  *pngPath = [NSHomeDirectory() stringByAppendingPathComponent:@"Documents/Test.png"];
    NSString  *jpgPath = [NSHomeDirectory() stringByAppendingPathComponent:@"Documents/Test.jpg"];

    // Write a UIImage to JPEG with minimum compression (best quality)
    // The value 'image' must be a UIImage object
    // The value '1.0' represents image compression quality as value from 0.0 to 1.0
    [UIImageJPEGRepresentation(image, 1.0) writeToFile:jpgPath atomically:YES];

    // Write image to PNG
    [UIImagePNGRepresentation(image) writeToFile:pngPath atomically:YES];
}

Answer 3

Look up "Technical Q&A QA1704 OpenGL ES View Snapshot", where Apple gives a worked example, including Nils' "better way" to unflip the image. It gets you to your UIImage, and from there follow Nils for a means of saving as a file.