OpenGL ES内容的Paint应用截图

Question

我正在为iPhone开发一款绘画应用。在我的代码中，我使用了一个包含轮廓图像的imageView，在其中放置了CAEAGLLayer以填充轮廓图像的颜色。现在，我正在使用以下函数对OpenGL ES [CAEAGLLayer]渲染内容进行截屏：

- (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;}

使用函数将此屏幕截图与轮廓图像结合：

- (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();  

使用函数将此图片保存到相册中

 -(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];    }

上述代码可以工作，但是截屏的图像质量较差。在笔刷的轮廓上，有灰色的轮廓。 我已经上传了一个应用程序的截图，其中结合了opengles内容和UIImage。 有没有办法获得opengles-CAEaglelayer内容的retina显示器截图呢？

预先感谢你！

- user392406

Answer 1

我不认为分辨率是你在这里遇到的问题。如果你在屏幕上看不到你的图纸上的灰色轮廓，那么很可能是在保存过程中出现了压缩伪影。你的图像可能被保存为低质量的JPEG图像，在锐利的边缘（例如你的图纸中的边缘）上会出现伪影。

为了解决这个问题，Ben Weiss在这里提供了以下代码，可以强制将你的图像作为PNG格式保存到照片库中：

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

虽然这可能是解决您问题的最简单的方法，但您也可以尝试使用多重采样抗锯齿技术，就像苹果在iOS OpenGL ES编程指南中的“使用多重采样改善图像质量”部分所描述的那样。根据您的填充速率限制情况，MSAA 可能会导致应用程序稍微变慢一些。

- Brad Larson

Answer 2

在创建CG位图上下文时，您正在使用kCGImageAlphaPremultipliedLast。尽管我看不到您的OpenGL代码，但我认为您的OpenGL上下文不太可能呈现预乘alpha。不幸的是，据我所知，在iOS上无法创建非预乘CG位图上下文（它将使用kCGImageAlphaLast，但我认为这只会使创建调用失败），因此您可能需要在从OpenGL获取数据并制作CG上下文之间手动预乘数据。

另一方面，您的OpenGL上下文是否有alpha通道的原因是什么？您可以将其设置为不透明白色，然后使用kCGImageAlphaNoneSkipLast吗？

- th_in_gs