Update one filter in GPUImageFilterGroup without redraw all filters

Question

I use GPUImageFilterGroup to apply some filters to a image. All filters stable (all parameters is constant), but last filter is variable (some parameter is changed).

I need redraw image after last filter was changed.

Now I call processImage on source GPUImagePicture, but this invocation redraw all filters and speed is too slow.

How can I redraw only last filter in the group?

I think, I should save copy of a frame buffer before last filter will draw, and when I've change some parameter in the last filter, I should use saved frame buffer to redraw last filter. But I can't find how I can save copy of the frame buffer.

Answer 1

I've solved this, by subclassing GPUImageFilter and GPUImageFilterGroup. In GPUImageFilter I've overloaded method

- (void)newFrameReadyAtTime:(CMTime)frameTime atIndex:(NSInteger)textureIndex
<...>

[self renderToTextureWithVertices:imageVertices textureCoordinates:[[self class] textureCoordinatesForRotation:inputRotation]];
_bufferCallback(self);
[self informTargetsAboutNewFrameAtTime:frameTime];
<...>

In GPUImageFilterGroup I've overloaded methods:

- (void)addFilter:(GPUImageOutput<GPUImageInput> *)newFilter
{
    NSParameterAssert([newFilter isKindOfClass:    [FAEShiftFilterWithBackOutputBuffer class]]);
    if ([newFilter isKindOfClass:[FAEShiftFilterWithBackOutputBuffer class]])
    {
        __weak typeof(self) selfWeak = self;
        [(FAEShiftFilterWithBackOutputBuffer*)newFilter setOutputBufferCallback:^(FAEShiftFilterWithBackOutputBuffer *sender) {
        __strong typeof(selfWeak) selfStrong = selfWeak;
        if (selfStrong)
        {
            if (!selfStrong.lastFramebuffer)
            {
                if ([selfStrong isPreLastFilter:sender])
                {
                    selfStrong.lastFramebuffer = [sender framebufferForOutput];
                    [selfStrong.lastFramebuffer lock];
                }
            }
        }
    }];
}
[super addFilter:newFilter];
}

This method stores outputFrameBuffer from preLast filter. And method:

- (void)newFrameReadyAtTime:(CMTime)frameTime atIndex:(NSInteger)textureIndex
{
    if (self.filterCount > 1)
   {
    if (self.lastFramebuffer)
    {
        GPUImageFilter* lastFilter = (GPUImageFilter*)self.terminalFilter;
        [lastFilter setInputFramebuffer:self.lastFramebuffer atIndex:0];
        [lastFilter newFrameReadyAtTime:frameTime atIndex:textureIndex];
    }
    else
    {
        [super newFrameReadyAtTime:frameTime atIndex:textureIndex];
    }
}
else
{
    [super newFrameReadyAtTime:frameTime atIndex:textureIndex];
}
}

Also I reset saved framebuffer in dealloc and forceProcessingAtSize and forceProcessingAtSizeRespectingAspectRatio methods.

- (void)_clearLastFrameBuffer
{
 if (_lastFramebuffer)
 {
     [_lastFramebuffer unlock];
     _lastFramebuffer = nil;
 }
}