GPUImage's GPUImageOpacityFilter not behaving as expected, doesn't change alpha channel

问题

I'm trying to do an Overlay Blend of a stock image with the output of the camera feed where the stock image has less than 100% opacity. I figured I could just place a GPUImageOpacityFilter in the filter stack and everything would be fine:

1. GPUImageVideoCamera -> MY_GPUImageOverlayBlendFilter
2. GPUImagePicture -> GPUImageOpacityFilter (Opacity 0.1f) -> MY_GPUImageOverlayBlendFilter
3. MY_GPUImageOverlayBlendFilter -> GPUImageView

But what that resulted in wasn't a 0.1f alpha version of GPUImagePicture blended into the GPUImageVideoCamera, it resulted in kinda softening the colors/contrast of GPUImagePicture and blending that. So I did some searching and on a suggestion tried getting a UIImage out of the GPUImageOpacity filter using imageFromCurrentlyProcessedOutput and sending that into the BlendFilter:

1. GPUImagePicture -> MY_GPUImageOpacityFilter (Opacity 0.1f)
2. [MY_GPUImageOpacityFilter imageFromCurrentlyProcessedOutput]-> MY_alphaedImage
3. GPUImagePicture (MY_alphaedImage) -> MY_GPUImageOverlayBlendFilter
4. GPUImageVideoCamera -> MY_GPUImageOverlayBlendFilter
5. MY_GPUImageOverlayBlendFilter -> GPUImageView

And that worked exactly as I would expect. So, why do I have to imageFromCurrentlyProcessedOutput, shouldn't that just happen in line? Here are the code snippits of the two scenarios above:

first one:

//Create the GPUPicture
UIImage *image = [UIImage imageNamed:@"someFile"];
GPUImagePicture *textureImage = [[[GPUImagePicture alloc] initWithImage:image] autorelease];

//Create the Opacity filter w/0.5 opacity
GPUImageOpacityFilter *opacityFilter = [[[GPUImageOpacityFilter alloc] init] autorelease];
opacityFilter.opacity = 0.5f
[textureImage addTarget:opacityFilter];

//Create the blendFilter
GPUImageFilter *blendFilter = [[[GPUImageOverlayBlendFilter alloc] init] autorelease];

//Point the cameraDevice's output at the blendFilter
[self._videoCameraDevice addTarget:blendFilter];

//Point the opacityFilter's output at the blendFilter
[opacityFilter addTarget:blendFilter];

[textureImage processImage];

//Point the output of the blendFilter at our previewView
GPUImageView *filterView = (GPUImageView *)self.previewImageView;
[blendFilter addTarget:filterView];

second one:

//Create the GPUPicture
UIImage *image = [UIImage imageNamed:@"someFile"];
GPUImagePicture *textureImage = [[[GPUImagePicture alloc] initWithImage:image] autorelease];

//Create the Opacity filter w/0.5 opacity
GPUImageOpacityFilter *opacityFilter = [[[GPUImageOpacityFilter alloc] init] autorelease];
opacityFilter.opacity = 0.5f
[textureImage addTarget:opacityFilter];

//Process the image so we get a UIImage with 0.5 opacity of the original
[textureImage processImage];
UIImage *processedImage = [opacityFilter imageFromCurrentlyProcessedOutput];
GPUImagePicture *processedTextureImage = [[[GPUImagePicture alloc] initWithImage:processedImage] autorelease];

//Create the blendFilter
GPUImageFilter *blendFilter = [[[GPUImageOverlayBlendFilter alloc] init] autorelease];

//Point the cameraDevice's output at the blendFilter
[self._videoCameraDevice addTarget:blendFilter];

//Point the opacityFilter's output at the blendFilter
[processedTextureImage addTarget:blendFilter];

[processedTextureImage processImage];

//Point the output of the blendFilter at our previewView
GPUImageView *filterView = (GPUImageView *)self.previewImageView;
[blendFilter addTarget:filterView];

来源：https://stackoverflow.com/questions/14470786/gpuimages-gpuimageopacityfilter-not-behaving-as-expected-doesnt-change-alpha