为什么glReadPixels（）在iOS 6.0中的此代码中失败？

Question

以下是我用于从OpenGL ES场景中读取图像的代码：

-(UIImage *)getImage{

    GLint width;

    GLint height;

    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &width);

    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &height);


    NSLog(@"%d %d",width,height);

    NSInteger myDataLength = width * height * 4;

    // allocate array and read pixels into it.
    GLubyte *buffer = (GLubyte *) malloc(myDataLength);
    glReadPixels(0, 0, width, height, 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 < height; y++)
        {
        for(int x = 0; x < width * 4; x++)
            {
            buffer2[((height - 1) - y) * width * 4 + x] = buffer[y * 4 * width + x];
            }
        }

    // make data provider with data.
    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer2, myDataLength, NULL);

    // prep the ingredients
    int bitsPerComponent = 8;
    int bitsPerPixel = 32;
    int bytesPerRow = 4 * width;
    CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
    CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;

    // make the cgimage
    CGImageRef imageRef = CGImageCreate(width, height, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);

    // then make the uiimage from that
    UIImage *myImage = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    CGDataProviderRelease(provider);
    CGColorSpaceRelease(colorSpaceRef);
    free(buffer);
    free(buffer2);
    return myImage;

}

这适用于iOS 5.x及更低版本，但在iOS 6.0上，现在返回黑色图像。为什么glReadPixels()在iOS 6.0上失败？

Answer 1

CAEAGLLayer *eaglLayer = (CAEAGLLayer *) self.layer;
eaglLayer.drawableProperties = @{
    kEAGLDrawablePropertyRetainedBacking: [NSNumber numberWithBool:YES],
    kEAGLDrawablePropertyColorFormat: kEAGLColorFormatRGBA8
};

设置

kEAGLDrawablePropertyRetainedBacking = YES

（我不知道为什么这个小费很顺利..///)

Answer 2

请尝试使用此方法获取屏幕截图。输出图片为 MailImage

- (UIImage*)screenshot
{
    // Create a graphics context with the target size
    // On iOS 4 and later, use UIGraphicsBeginImageContextWithOptions to take the scale into consideration
    // On iOS prior to 4, fall back to use UIGraphicsBeginImageContext
    CGSize imageSize = [[UIScreen mainScreen] bounds].size;
    if (NULL != UIGraphicsBeginImageContextWithOptions)
        UIGraphicsBeginImageContextWithOptions(imageSize, NO, 0);
    else
        UIGraphicsBeginImageContext(imageSize);

    CGContextRef context = UIGraphicsGetCurrentContext();

    // Iterate over every window from back to front
    for (UIWindow *window in [[UIApplication sharedApplication] windows])
    {
        if (![window respondsToSelector:@selector(screen)] || [window screen] == [UIScreen mainScreen])
        {
            // -renderInContext: renders in the coordinate space of the layer,
            // so we must first apply the layer's geometry to the graphics context
            CGContextSaveGState(context);
            // Center the context around the window's anchor point
            CGContextTranslateCTM(context, [window center].x, [window center].y);
            // Apply the window's transform about the anchor point
            CGContextConcatCTM(context, [window transform]);
            // Offset by the portion of the bounds left of and above the anchor point
            CGContextTranslateCTM(context,
                                  -[window bounds].size.width * [[window layer] anchorPoint].x,
                                  -[window bounds].size.height * [[window layer] anchorPoint].y);

            // Render the layer hierarchy to the current context
            [[window layer] renderInContext:context];

            // Restore the context
            CGContextRestoreGState(context);
        }
    }

    // Retrieve the screenshot image
    Mailimage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return Mailimage;


}