Paint app的OpenGL ES内容的屏幕截图

Question

我正在为iphone绘制应用程序。在我的代码中，我使用的是包含轮廓图像的imageView，我在其中将CAEAGLLayer用于在轮廓图像中填充颜色。

- (UIImage*)snapshot:(UIView*)eaglview{ 
GLint backingWidth1, backingHeight1; 

// Bind the color renderbuffer used to render the OpenGL ES view 
// If your application only creates a single color renderbuffer which is already bound at this point, 
// this call is redundant, but it is needed if you're dealing with multiple renderbuffers. 
// Note, replace "_colorRenderbuffer" with the actual name of the renderbuffer object defined in your class. 
glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer); 

// Get the size of the backing CAEAGLLayer 
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth1); 
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight1); 

NSInteger x = 0, y = 0, width = backingWidth1, height = backingHeight1; 
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 *image = UIGraphicsGetImageFromCurrentImageContext(); 

UIGraphicsEndImageContext(); 

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

return image;} 

使用功能相结合该屏幕截图与轮廓图：

- (void)Combine:(UIImage *)Back{ 


UIImage *Front =backgroundImageView.image; 


//UIGraphicsBeginImageContext(Back.size); 
UIGraphicsBeginImageContext(CGSizeMake(640,960)); 
// Draw image1 
[Back drawInRect:CGRectMake(0, 0, Back.size.width*2, Back.size.height*2)]; 

// Draw image2 
[Front drawInRect:CGRectMake(0, 0, Front.size.width*2, Front.size.height*2)]; 

UIImage *resultingImage = UIGraphicsGetImageFromCurrentImageContext(); 


UIImageWriteToSavedPhotosAlbum(resultingImage, nil, nil, nil); 


UIGraphicsEndImageContext(); 

}

保存此图像使用到画册现在我使用的功能考虑的OpenGL ES [CAEAGLLayer]呈现的内容截图功能

-(void)captureToPhotoAlbum { 
[self Combine:[self snapshot:self]]; 
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Success" message:@"Image saved to Photo Album" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil]; 
[alert show]; 
[alert release]; } 

上面的代码正在工作，但图像质量屏幕截图很差。在画笔的轮廓上，有一个灰色的轮廓。我已经上传了我的应用程序的截图，它是开放性内容& UIImage的组合。

有没有什么办法让opengles-CAEaglelayer内容Retina显示屏幕截图。

预先感谢您！

Answer 1

我不认为这个决议是你的问题在这里。如果屏幕上显示的图形上没有灰显的轮廓，则可能是您在保存过程中观察到压缩人为因素。您的图像可能被保存为质量较低的JPEG图像，其中伪像将显示在锐利的边缘上，如您的图形中的图像。

要解决这个问题，本·魏斯的答案here为迫使你的图片下面的代码保存到照片库为PNG：

UIImage* im = [UIImage imageWithCGImage:myCGRef]; // make image from CGRef 
NSData* imdata = UIImagePNGRepresentation (im); // get PNG representation 
UIImage* im2 = [UIImage imageWithData:imdata]; // wrap UIImage around PNG representation 
UIImageWriteToSavedPhotosAlbum(im2, nil, nil, nil); // save to photo album 

虽然这可能是解决你的问题，最简单的方法在这里，您也可以尝试采用多采样抗锯齿技术，正如Apple在iOS版OpenGL ES编程指南的“Using Multisampling to Improve Image Quality”部分所述。根据您的填充率限制情况，MSAA可能会导致您的应用程序出现一些放缓现象。

Answer 2

当您创建CG位图上下文时，您正在使用kCGImageAlphaPremultipliedLast。虽然我看不到你的OpenGL代码，但我认为你的OpenGL上下文并不能渲染预乘alpha。不幸的是，在IIRC中，不可能在iOS上创建非预乘积的CG位图上下文（它将使用kCGImageAlphaLast，但我认为这会使创建调用失败），因此您可能需要手动预乘数据从OpenGL中获取它并制作CG上下文。

另一方面，你的OpenGL上下文有alpha通道吗？你能不能让它变成不透明的白色，然后用kCGImageAlphaNoneSkipLast？