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Specular Difference

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This plugin is for isolating specular light in a scene. It uses two photographs of the scene, one with specular light filtered by a polariser and the other allowing it through. The former is subtracted from the latter to leave mostly specular light.

The plugin does the subtraction using layers, which can be done onto a separate layer if needed. Both images need to be aligned to overlap accurately and their layers can be moved to do this if necessary. Some noise is likely and there's a controller to reduce this, it defaults to 100% and is best left at that. To keep colour distortion to a minimum the photographs should be exposed in the mid to high range, avoiding exposure near white. A brightness control is included to compensate for this.

Getting good results can be tricky, I've put a blog post together which outlines some gotchas and can be found here:

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Tahnk you. Interesting plugin. I easily built a Windows version. I just got a little trouble trying it: I copied two images from your blog, then Ctrl-V them as layers into Gimp, but I did not notice that the top layer had an alpha channel so that nothing happened before I removed this stupid alpha channel;) This is not a bug, but a tip for those who will try your plugin.

Thanks a lot for checking out the plugin and pointing this out. I was dragging and dropping images into GIMP on my Ubuntu box while writing it, so I wouldn't have been aware of this if you didn't bring it up! I agree, it's not a bug, but it is a bit inconvenient. I've added an error message for when this happens, so hopefully that'll save some confusion.

The full sized unfiltered and filtered images can be found here and here.


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