Question from the Audience – Sharpening a Selection

A Learning Experience

A reader asked me if it were possible to use the grain extract / grain merge sharpening technique on a selection. Taking the question literally and having never tried that before, I immediately opened GIMP and started experimenting. Let's go through the process of sharpening only the face of this African gray parrot and see what we learn.

African gray parrot

After duplicating the original image, I made a selection around the parrot's face using the Free Select tool. I saved the selection to a channel (Select > Save to Channel) in case I needed it again.

Creating and Saving a Selection

Next, I went through the sharpening process I explain in A Non-Destructive Adjustable Way to Sharpen a Photo. Gaussian Blur works on a selection.

Gaussian Blur on a Selection

The layer with Mode Grain extract shows grain only for the blurred area. The rest of the layer is medium gray.

Grain Extract on a Selection

After creating a new layer from what was visible, deleting the blurred layer, changing the layer Mode to Grain merge, and adjusting the Opacity, I now had a sharp-faced parrot with a fuzzy body.

Parrot with Sharpened Face

Cool. So far, so good. However, if you look closely, you'll see that there's a sharp transition between the sharpened face and the rest of the body. OK, not so cool. This effect would look better with a gradual transition from sharp to not sharp. How do we deal with that? A couple of options came to mind instantly.

(Note: Since I saved my selection to a channel, it was easy to get my selection back to test the following options by going to the Channels dialog, right-clicking on Selection Mask copy, and choosing Channel to Selection.)

Option 1: After creating the selection, but before doing anything else, choose Select > Feather and feather the selection by a small amount. Then go through the sharpening process. You might be able to achieve a decent effect using this method, but it's likely you'll have to undo and redo a few times with different feathering amounts until you get what you want. Also, you'll get the same level of transition all the way around the selection, which may or may not work with your image.

Option 2: After creating the selection, but before doing anything else, invert the selection (Ctrl+I), make sure the layer has transparency (Layer > Transparency > Add Alpha Channel), and delete the rest of the image on the layer. Then remove the selection (Ctrl+Shft+A), duplicate the layer (Ctrl+Shft+D), hide the lower layer containing the full image, and go through the sharpening process. Delete all the in-between layers so you end up with only a Visible layer with Mode Grain merge over the layer of the entire image.

Well, that actually looks worse than Option 1 because now there's a visible edge around the selected area.

Visible Edge around Selection Area

But, since the selected area is on a separate layer, you can use the Eraser tool with a fuzzy brush to clean up the edge and make the sharp area blend into the image below.

Cleaning Up the Edge

Hey, that's not too bad, and at least Option 2 is adjustable. But what if you later decide you want to include more of the image in the selection area? Then you have to start over from the beginning.

It eventually occurred to me that perhaps what the reader was really asking was “How do I use the sharpening technique to sharpen only part of an image?” Ah! My instant answer to that question is “Use a mask.” All you have to do then is go through the sharpening technique on the entire image, then create a mask on the final Grain merge layer following the technique I describe in A Non-Destructive, Adjustable Method for Cutting Out an Image. This method gives you much more control over how much of the sharpened area you want to make visible and how you transition the edges, and it's easy to adjust the mask in the future if you decide to make changes.

Masked Sharpened Area

As you can see, you learn the best way to do something through experience, which is what you get while doing things in not the best way. Here's my advice about getting experience with GIMP.

• Always save a copy of your original image so you can start over from the beginning.
• Try to do all your changes and effects on separate layers so that you can make changes easily without redoing a lot of steps.
• If you need to combine some layers, instead of merging them, make the pertinent layers visible, then right-click in the Layers list and choose New from Visible. Hide the original layers so you can go back to them and start from there if you need to.
• Don't be afraid to experiment. Playing around is how you learn new things and discover new techniques.

Please feel free to ask me questions about GIMP. If I don't know the answer, I'll try to figure something out.

Keep Calm and GIMP On