Capturing UIImage with GPUImageSketchFilter produces black and white image about gpuimage HOT 6 CLOSED

Good catch. For a two-pass filter like the edge detection and sketch filters, I was writing the first pass to disk, not the end result. That should now be fixed in the repository.

Thanks for pointing this out.

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Thanks Brad. Pulled in the changes and looks great.

Thanks again for putting together this library. My experience in image processing/filtering is non-existent and this has been a great experiment in learning.

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Hi!
Thanks for that great piece of software ;)!
I find it very helpful, expecially for me: I don't know anything about elaborating images and I was asked to add some effects to photo sharing app.

I've some problem and I think it can be something similar to the one discussed in this thread.

I have a problem with Sketch effects (and TiltShift too): when I apply Sketch effect on preview image it works really well, but when I use it on full size image (and save image data to file) it seems like just half effects was applied.

For example with Sketch effetc I got image without too much contrast (eg. the edge aren't very "dark").

Do you have some insights?

Thanks!

William

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That's really a different issue. The sketch filter is based on edge detection routines. These are calculated based on neighboring pixels, so when you have a preview at a lower resolution the effect will be performed over a larger effective area in the image than when you capture a high-res still photo. You can control this to some degree by setting the texelWidth and texelHeight parameters of the filter to be 1/width and 1/height of the image size you start with. This will keep the edge detection calculations relatively constant.

The tilt shift filter is a different case, in that it uses a Gaussian blur. That blur is calculated based on local pixels, and again if you increase the image resolution it is now blurring over a smaller relative radius. You could try increasing the blurSize scaling value, but you will quickly see banding in that. I don't really have a good way of maintaining a constant image processing quality for this filter between images of various resolutions.

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Has anyone come up with a solution to the Tilt Shift preview and final output not look the same?
I get a nice effect on the preview but the final captured image has a lot less effect to it.

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@pednaa - This comment doesn't really have anything to do with this issue.

The reason for the difference in preview and photo appearance is that the Gaussian blur used for the out-of-focus section of the image is calculated based on neighboring pixels. The blur samples over a 9-pixel radius around each pixel, so for lower-resolution previews this blurs a larger region than in the high-resolution final image. You need to apply the blur over a larger number of pixels to achieve the same effect in the higher-resolution image.

Unfortunately, my blur filters are currently hardcoded to sample from a set number of pixels for performance reasons, and increasing the pixel spacing will introduce image artifacts. Multiple blur passes or a custom Gaussian blur that works over a larger radius would be needed here to achieve the same size of blur on a higher-resolution image.

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