I've successfully added a crosshair generator and harris corner detection filter onto a GPUImageStillCamera output, as well as on live video from GPUImageVideoCamera.
I'm now trying to get this working on a photo set on a UIImageView, but continually get a black screen as the output. I have been reading the issues listed on GitHub against Brad Larson's GPUImage project, but they seemed to be more in relation to blend type filters, and following the suggestions there I still face the same problem.
I've tried altering every line of code to follow various examples I have seen, and to follow Brad's example code in the Filter demo projects, but the result is always the same.
My current code is, once I've taken a photo (which I check to make sure it is not just a black photo at this point):
GPUImagePicture *stillImageSource = [[GPUImagePicture alloc] initWithImage:self.photoView.image];
GPUImageHarrisCornerDetectionFilter *cornerFilter1 = [[GPUImageHarrisCornerDetectionFilter alloc] init];
[cornerFilter1 setThreshold:0.1f];
[cornerFilter1 forceProcessingAtSize:self.photoView.frame.size];
GPUImageCrosshairGenerator *crossGen = [[GPUImageCrosshairGenerator alloc] init];
crossGen.crosshairWidth = 15.0;
[crossGen forceProcessingAtSize:self.photoView.frame.size];
[cornerFilter1 setCornersDetectedBlock:^(GLfloat* cornerArray, NSUInteger cornersDetected, CMTime frameTime, BOOL endUpdating)
{
[crossGen renderCrosshairsFromArray:cornerArray count:cornersDetected frameTime:frameTime];
}];
[stillImageSource addTarget:crossGen];
[crossGen addTarget:cornerFilter1];
[crossGen prepareForImageCapture];
[stillImageSource processImage];
UIImage *currentFilteredImage = [crossGen imageFromCurrentlyProcessedOutput];
UIImageWriteToSavedPhotosAlbum(currentFilteredImage, nil, nil, nil);
[self.photoView setImage:currentFilteredImage];
I've tried prepareForImageCapture on both filters, on neither, adding the two targets in the opposite order, calling imageFromCurrentlyProcessedOutput on either filter, I've tried it without the crosshair generator, I've tried using local variables and variables declared in the .h file. I've tried with and without forceProcessingAtSize on each of the filters.
I can't think of anything else that I haven't tried to get the output. The app is running on iPhone 7.0, in Xcode 5.0.1. The standard filters work on the photo, e.g. the simple GPUImageSobelEdgeDetectionFilter included in the SimpleImageFilter test app.
Any suggestions? I am saving the output to the camera roll so I can check it's not just me failing to display it correctly. I suspect it's a stupid mistake somewhere but am at a loss as to what else to try now.
Thanks.
Edited to add: the corner detection is definitely working, as depending on the threshold I set, it returns between 6 and 511 corners.
The problem with the above is that you're not chaining filters in the proper order. The Harris corner detector takes in an input image, finds the corners within it, and provides the callback block to return those corners. The GPUImageCrosshairGenerator takes in those points and creates a visual representation of the corners.
What you have in the above code is image->GPUImageCrosshairGenerator-> GPUImageHarrisCornerDetectionFilter, which won't really do anything.
The code in your answer does go directly from the image to the GPUImageHarrisCornerDetectionFilter, but you don't want to use the image output from that. As you saw, it produces an image where the corners are identified by white dots on a black background. Instead, use the callback block, which processes that and returns an array of normalized corner coordinates for you to use.
If you need these to be visible, you could then take that array of coordinates and feed it into the GPUImageCrosshairGenerator to create visible crosshairs, but that image will need to be blended with your original image to make any sense. This is what I do in the FilterShowcase example.