GPUImage的GPUImageOpacityFilter表现不正常，不会更改Alpha通道

Question

我正在尝试使用相机Feed的输出执行Overlay Blend股票图像，其中股票图像的不透明度低于100％。我想我可以在过滤器堆栈中放置GPUImageOpacityFilter，一切都会好的：

1. GPUImageVideoCamera - ＆gt; MY_GPUImageOverlayBlendFilter
2. GPUImagePicture - ＆gt; GPUImageOpacityFilter（Opacity 0.1f） - ＆gt; MY_GPUImageOverlayBlendFilter
3. MY_GPUImageOverlayBlendFilter - ＆gt; GPUImageView

但是导致的不是GPUImagePicture的0.1f alpha版本混合到GPUImageVideoCamera中，它导致有点软化GPUImagePicture的颜色/对比度并混合它。所以我做了一些搜索，并尝试使用imageFromCurrentlyProcessedOutput从GPUImageOpacity过滤器中取出UIImage并将其发送到BlendFilter：

1. GPUImagePicture - ＆gt; MY_GPUImageOpacityFilter（不透明度0.1f）
2. [MY_GPUImageOpacityFilter imageFromCurrentlyProcessedOutput] - ＆gt; MY_alphaedImage
3. GPUImagePicture（MY_alphaedImage） - ＆gt; MY_GPUImageOverlayBlendFilter
4. GPUImageVideoCamera - ＆gt; MY_GPUImageOverlayBlendFilter
5. MY_GPUImageOverlayBlendFilter - ＆gt; GPUImageView

这完全符合我的预期。那么，为什么我必须imageFromCurrentlyProcessedOutput，那不应该只是排队吗？以下是上述两种情况的代码snippits：

第一个：

//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];

第二个：

//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];