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I have an RGB image with one layer, and I want to apply Round Corners filter to it. However, the filter is disabled (look at the image).

What am I doing wrong???

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If alpha channel is added, you can not use Round Corner Filter.

To remove alpha channel, use Layer -> Transparency -> Remove Alpha Channel

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Why would it matter if there is an alpha channel? – cdmckay Jan 13 '14 at 19:29
It seems like after round corners is applied, if you don't have "add background" checked, then you end up with an alpha channel anyhow!? – GreenAsJade Jan 15 '15 at 22:25
up vote 1 down vote accepted

I solved this. If you meet the problem, check the file extension. It seems that round filter does not work with PNG images even if there is no transparency at all. When I loaded JPG, it worked well.

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For me, it worked fine with PNGs after I removed the alpha channel. Maybe your PNG was not in RGB or grayscale mode? – Thomas Jan 4 '15 at 16:17
as far as i know, JPGs don't have alpha channels, so eventually it goes down to the same solution as zelon's one – WayneEra Nov 23 '15 at 7:59

Some filters do not work on pallet based images. Make sure your image has 24-bit colour depth rather than something like 8-bit (be aware that some formats, like PNG, can support 1-, 2-, 4, or 8-bit pallet based images as well as 24-bit RGB ones).

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