In GIMP, is there a way to control a gradient brightness and/or contrast of a selection? The application is this: I'd like to control the brightness/contrast of one end of my selection to match a darker area of a particular image, while also controlling the brightness/contrast of the other end to match a lighter area of the same image while obtaining a smooth transition between the two ends. Thank you, in advance.
You can paint on a transparent layer and then apply brightness contrast.
But there are possibly better ways:
You can sample the colors on both points to set the foreground and background color, and the use the
FG to BG (RGB) gradient.
- Start the Blend too and click and drag to create the gradient
- You can then click on any extremities to make it editable in the small on-canvas dialog
- In that canvas you can edit the color in a color selector (click the color button), and this changes the displayed gradient.
So you can very easily match the colors of the extremities
- Cherry on the cake: if you click on the gradient line near the middle, you can grab the gradient mid-point and then move it to adjust the gradient curve.
- 2nd cherry on the cake, if you click somewhere else on the gradient line, you create a gradient "stop" and change the colors on both sides.
So in practice you can fully edit the gradient that you are creating.
You can make to separate layers two copies A and B of the image that should have sliding brightness and contrast. Adjust A and B to match with the different edges of your reference image.
Let A be ok in the left and let B be ok in the right.
Drag A to the top and insert a layer mask to it. Let B be the next layer downwards.
Insert a left to right gradient from white to black to the layer mask of A. You may have to experiment a little with the stops of the gradient. Alternatively you can apply curves to the layer mask. Before clicking OK tweak the curve for best match.
I guess you have a good reason to avoid Photoshop, where one could simply use a curves adjustment layer with a gradient mask and "next layer only"-switch=ON. But that functionality can be found also in freeware. It's in web application Photopea and it can be found in Krita.