GPUImage 的 GPUImageOpacityFilter 行为不符合预期，不会更改 Alpha 通道

我正在尝试做一个Overlay Blend具有相机源输出的库存图像，其中库存图像的不透明度小于 100%。我想我可以放一个GPUImageOpacityFilter在过滤器堆栈中，一切都会好起来的：

1. GPUImageVideoCamera -> MY_GPUImageOverlayBlendFilter
2. GPUImagePicture -> GPUImageOpacityFilter (不透明度 0.1f) -> MY_GPUImageOverlayBlendFilter
3. MY_GPUImageOverlayBlendFilter -> GPUImageView

但结果并不是将 GPUImagePicture 的 0.1f alpha 版本混合到 GPUImageVideoCamera 中，而是软化了 GPUImagePicture 的颜色/对比度并将其混合。所以我做了一些搜索，并根据建议尝试使用 GPUImageOpacity 过滤器获取 UIImageimageFromCurrentlyProcessedOutput并将其发送到 BlendFilter 中：

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

这完全符合我的预期。那么，为什么我必须imageFromCurrentlyProcessedOutput，这不应该发生在队列中吗？以下是上述两种场景的代码片段：

第一：

//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];