glReadPixels returns null 多重采样

Question

我遇到了与 this one 相同的问题，但是根据这些提示，我仍然无法从 glReadPixels 获取数据。

我粘贴我的源代码，我的代码和之前的几乎一样one.And我在快照前设置了GL_READ_FRAMEBUFFER_APPLE，但是数据returns null。

创建我的帧缓冲区

- (void)createFrameBuffer {
glGenRenderbuffers(1, &colorRenderBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, colorRenderBuffer);
[_context renderbufferStorage:GL_RENDERBUFFER fromDrawable:_eaglLayer];

GLint backingWidth;
GLint backingHeight;
glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &backingWidth);
glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &backingHeight);

glGenFramebuffers(1, &defaultFrameBuffer);
glBindFramebuffer(GL_FRAMEBUFFER, defaultFrameBuffer);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,GL_RENDERBUFFER, colorRenderBuffer);

glGenFramebuffers(1, &sampleFramebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, sampleFramebuffer);

glGenRenderbuffers(1, &sampleColorRenderbuffer);
glBindRenderbuffer(GL_RENDERBUFFER, sampleColorRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER, 4, GL_RGBA8_OES, backingWidth, backingHeight);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, sampleColorRenderbuffer);

glGenRenderbuffers(1, &sampleDepthRenderbuffer);
glBindRenderbuffer(GL_RENDERBUFFER, sampleDepthRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER, 4, GL_DEPTH_COMPONENT16, backingWidth, backingHeight);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, sampleDepthRenderbuffer);

if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE){
    NSAssert(true, @"buffer is not complete");
    NSLog(@"Failed to make complete framebuffer object %x", glCheckFramebufferStatus(GL_FRAMEBUFFER));
}
}

渲染函数

- (void)render {
FC_PERFORMANCE_START();
if ([EAGLContext currentContext] !=_context) {
    [EAGLContext setCurrentContext:_context];
}
[self destoryFrameBuffer];
[self createFrameBuffer];
[self.director setup:self.frame.size contentScale:self.contentScaleFactor backgroudColor:self.backgroundColor];
[self.director mainloop];
glBindFramebuffer(GL_READ_FRAMEBUFFER_APPLE, sampleFramebuffer);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER_APPLE, defaultFrameBuffer);
glResolveMultisampleFramebufferAPPLE();
glBindRenderbuffer(GL_RENDERBUFFER, colorRenderBuffer);
const GLenum discards[]  = {GL_COLOR_ATTACHMENT0,GL_DEPTH_ATTACHMENT};
glDiscardFramebufferEXT(GL_READ_FRAMEBUFFER_APPLE,2,discards);
glBindFramebuffer(GL_FRAMEBUFFER, sampleFramebuffer);
[_context presentRenderbuffer:GL_RENDERBUFFER];
FC_PERFORMANCE_END("FCViewOpenGL Render");

}

屏幕截图功能

- (nullable UIImage*)snapShot{
__block UIImage *ret = nil;
dispatch_sync(dispatch_get_main_queue(), ^{
    glBindFramebuffer(GL_READ_FRAMEBUFFER_APPLE, defaultFrameBuffer);

    GLint backingWidth;
    GLint backingHeight;
    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &backingWidth);
    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &backingHeight);

    NSInteger x = 0, y = 0, width2 = backingWidth, height2 = backingHeight;
    NSInteger dataLength = width2 * height2 * 4;
    GLubyte *data = (GLubyte*)malloc(dataLength * sizeof(GLubyte));

    CHECK_GL_ERROR_DEBUG();
    glReadPixels((GLint)x, (GLint)y, (GLsizei)width2, (GLsizei)height2, GL_RGBA, GL_UNSIGNED_BYTE, data);
    GLenum attachments[] = {GL_COLOR_ATTACHMENT0, GL_DEPTH_ATTACHMENT   };
    glDiscardFramebufferEXT(GL_READ_FRAMEBUFFER_APPLE, 2, attachments);
    CHECK_GL_ERROR_DEBUG();

    CGDataProviderRef ref = CGDataProviderCreateWithData(NULL, data, dataLength, NULL);
    CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
    CGImageRef iref = CGImageCreate(width2, height2, 8, 32, width2 * 4, colorspace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast,
                                    ref, NULL, true, kCGRenderingIntentDefault);

    NSInteger widthInPoints, heightInPoints;
    if (NULL != UIGraphicsBeginImageContextWithOptions) {
        CGFloat scale = self.contentScaleFactor;
        widthInPoints = width2 / scale;
        heightInPoints = height2 / scale;
        UIGraphicsBeginImageContextWithOptions(CGSizeMake(widthInPoints, heightInPoints), NO, scale);
    }
    else {
        widthInPoints = width2;
        heightInPoints = height2;
        UIGraphicsBeginImageContext(CGSizeMake(widthInPoints, heightInPoints));
    }

    CGContextRef cgcontext = UIGraphicsGetCurrentContext();

    CGContextSetBlendMode(cgcontext, kCGBlendModeCopy);
    CGContextDrawImage(cgcontext, CGRectMake(0.0, 0.0, widthInPoints, heightInPoints), iref);

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    free(data);
    CFRelease(ref);
    CFRelease(colorspace);
    CGImageRelease(iref);
    ret = image;
});
return ret;
}

谁能帮我一下？

Answer 1

见OpenGL ES 3.2 Specification; 16.1.2 ReadPixels; page 406

An INVALID_OPERATION error is generated if the value of READ_FRAMEBUFFER_BINDING (see section 9) is non-zero, the read framebuffer is framebuffer complete, and the effective value of SAMPLE_BUFFERS for the read framebuffer is one.

....

If the read framebuffer is multisampled (its effective value of SAMPLE_BUFFERS is one) ...

另见 OpenGL-Refpages; OpenGL ES 3.0; glReadPixels:

GL_INVALID_OPERATION is generated if GL_READ_FRAMEBUFFER_BINDING is non-zero, the read framebuffer is complete, and the value of GL_SAMPLE_BUFFERS for the read framebuffer is greater than zero.

这意味着，您不能使用 glReadPixels 多采样帧缓冲区。您将获得 GL_INVALID_OPERATION 操作错误。在glReadPixels之后使用glGetError获取错误信息。

要解决这个问题，您必须绕过传统的帧缓冲区。执行以下步骤：

创建第二个帧缓冲区对象，这是常规的（非多重采样）。
绑定多采样帧缓冲区以供读取 (GL_READ_FRAMEBUFFER_APPLE)。
为绘图绑定传统的帧缓冲区(GL_DRAW_FRAMEBUFFER_APPLE)。
使用 glBlitFramebuffer 复制将像素从多重采样（读取）帧缓冲区复制到常规（绘制）帧缓冲区。
复制完成后绑定常规framebuffer读取(GL_READ_FRAMEBUFFER_APPLE)。
最后使用glReadPixels从常规帧缓冲区读取。

作为替代方案，您当然可以直接从默认帧缓冲区读取像素。

glReadPixels returns null 多重采样

glReadPixels returns null with multi-sampling

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