我正在为iOS编写OpenGL应用程序，我需要获取渲染场景的应用程序截图。当我不使用多次采样时，一切正常。但是，当我打开多次采样时，glReadPixels不会返回正确的数据（正确绘制场景 - 多次采样时图形质量要好得多）。
我已经在SO和其他一些地方检查过一堆类似的问题，但是没有一个能解决我的问题，因为我已经按照提议的方式做了这些：
glReadPixels不会返回错误。kEAGLDrawablePropertyRetainedBacking设置为YES并在缓冲区显示后截屏 - 也不起作用。OpenGLES 1.x渲染API（使用kEAGLRenderingAPIOpenGLES1初始化的上下文）基本上我没有想法可能出错。在SO上发帖是我的最后一招。
这是相关的源代码：
创建帧缓冲区
- (BOOL)createFramebuffer
{
glGenFramebuffersOES(1, &viewFramebuffer);
glGenRenderbuffersOES(1, &viewRenderbuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
[context renderbufferStorage:GL_RENDERBUFFER_OES fromDrawable:(CAEAGLLayer*)self.layer];
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, viewRenderbuffer);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);
// Multisample support
glGenFramebuffersOES(1, &sampleFramebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, sampleFramebuffer);
glGenRenderbuffersOES(1, &sampleColorRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, sampleColorRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, 4, GL_RGBA8_OES, backingWidth, backingHeight);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, sampleColorRenderbuffer);
glGenRenderbuffersOES(1, &sampleDepthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, sampleDepthRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, 4, GL_DEPTH_COMPONENT16_OES, backingWidth, backingHeight);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, sampleDepthRenderbuffer);
// End of multisample support
if(glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES) != GL_FRAMEBUFFER_COMPLETE_OES) {
NSLog(@"failed to make complete framebuffer object %x", glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES));
return NO;
}
return YES;
}
解析缓冲区部分并拍摄快照
glBindFramebufferOES(GL_DRAW_FRAMEBUFFER_APPLE, viewFramebuffer);
glBindFramebufferOES(GL_READ_FRAMEBUFFER_APPLE, sampleFramebuffer);
glResolveMultisampleFramebufferAPPLE();
[self checkGlError];
//glFinish();
if (capture)
captureImage = [self snapshot:self];
const GLenum discards[] = {GL_COLOR_ATTACHMENT0_OES,GL_DEPTH_ATTACHMENT_OES};
glDiscardFramebufferEXT(GL_READ_FRAMEBUFFER_APPLE,2,discards);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
[context presentRenderbuffer:GL_RENDERBUFFER_OES];
快照方法（基本上是从apple docs复制的）
- (UIImage*)snapshot:(UIView*)eaglview
{
// 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);
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);
[self checkGlError];
glReadPixels(x, y, width, height, GL_RGBA, GL_UNSIGNED_BYTE, data);
[self checkGlError];
// 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;
}
答案 0 :(得分：2)
在将glResolveMultisampleFramebufferAPPLE绑定为绘制帧缓冲区并将viewFramebuffer绑定为读取帧缓冲区之后，通过执行sampleFramebuffer照常处理多重采样缓冲区。但是你还记得在viewFramebuffer之前将glBindFramebuffer(GL_READ_FRAMEBUFFER, viewFramebuffer)绑定为读取帧缓冲区（glReadPixels）吗？ glReadPixels将始终从当前绑定的读取帧缓冲区读取，如果在多重采样解析后未更改此绑定，则仍将是多重采样帧缓冲区而非默认帧缓冲区。
我还发现你的glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer) - 调用非常烦人，因为那并没有真正做任何有意义的事情，当前绑定的渲染缓冲区仅与渲染缓冲区上的函数相关（实际上只有glRenderbufferStorage）（但是也可能是ES对它做了一些有意义的事情，[context presentRenderbuffer:GL_RENDERBUFFER_OES]工作需要绑定它。但是，您可能认为此绑定还控制glReadPixels将读取的缓冲区，但这不的情况下，它将始终从当前的帧缓冲区绑定到GL_READ_FRAMEBUFFER。