glReadPixels crashes on specific render buffer widths

Question

I need to read content of CAEAGLLayer, which has various widths and heights (based on background image size). on iPad emulator everything is okay, but on device I have crashes or weird horizontal lines instead of content. Crashes not happens on width, for example, from 537 to 544 pixels

I'm using following code

- (UIImage*)image
{
        // Get the size of the backing CAEAGLLayer
    glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
    glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
    glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);

    NSInteger x = 0, y = 0, width = backingWidth, height = backingHeight;
    NSInteger dataLength = width * height * 4;
    GLubyte *data = (GLubyte*)malloc(dataLength * sizeof(GLubyte));

        // Read pixel data from the framebuffer
    glPixelStorei(GL_PACK_ALIGNMENT, 4);
    glReadPixels(x, y, width, height, GL_RGBA, GL_UNSIGNED_BYTE, data);

        // Create a CGImage with the pixel data
        // If your OpenGL ES content is opaque, use kCGImageAlphaNoneSkipLast to ignore the alpha channel
        // otherwise, use kCGImageAlphaPremultipliedLast
    CGDataProviderRef ref = CGDataProviderCreateWithData(NULL, data, dataLength, NULL);
    CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
    CGImageRef iref = CGImageCreate(width, height, 8, 32, width * 4, colorspace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast,
                                    ref, NULL, true, kCGRenderingIntentDefault);

        // OpenGL ES measures data in PIXELS
        // Create a graphics context with the target size measured in POINTS
    NSInteger widthInPoints, heightInPoints;
        // if (NULL != UIGraphicsBeginImageContextWithOptions) {
        // // On iOS 4 and later, use UIGraphicsBeginImageContextWithOptions to take the scale into consideration
        // // Set the scale parameter to your OpenGL ES view's contentScaleFactor
        // // so that you get a high-resolution snapshot when its value is greater than 1.0
        // CGFloat scale = eaglview.contentScaleFactor;
        // widthInPoints = width / scale;
        // heightInPoints = height / scale;
        // UIGraphicsBeginImageContextWithOptions(CGSizeMake( widthInPoints, heightInPoints), NO, scale);
        // }
        // else {
        // On iOS prior to 4, fall back to use UIGraphicsBeginImageContext
    widthInPoints = width;
    heightInPoints = height;
    UIGraphicsBeginImageContext(CGSizeMake(widthInPoints, heightInPoints));
        // }

    CGContextRef cgcontext = UIGraphicsGetCurrentContext();

        // UIKit coordinate system is upside down to GL/Quartz coordinate system
        // Flip the CGImage by rendering it to the flipped bitmap context
        // The size of the destination area is measured in POINTS
    CGContextSetBlendMode(cgcontext, kCGBlendModeCopy);
    CGContextDrawImage(cgcontext, CGRectMake(0.0, 0.0, widthInPoints, heightInPoints), iref);

        // Retrieve the UIImage from the current context
    UIImage *image1 = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

        // Clean up
    free(data);
    CFRelease(ref);
    CFRelease(colorspace);
    CGImageRelease(iref);

    return image1;
}

Answer 1

The view you are reading from, or your drawable area, must be a multiple of 32 (possibly 16, but I haven't tested this yet). Maybe you can try resizing your draw area to a multiple of 32? I had the horizontal line issue previously and resizing the section I was reading from 400x300 to 384x288 fixed everything.

Answer 2

Thanks a ton Ginamin! I was getting nightmares with this issue and your fix solved it.

I just wanted to know whether this a documented limitation by Apple/OpenGL or not? I didnt find this anywhere else.