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I'm not particularly experienced in graphic design, so maybe there's an easy way to do this that I just haven't noticed, but...

I have a layered PNG image in Gimp. It's a complex shape (a photo of a real-world object) with transparency throughout the image (having carefully and painstakingly erased background parts from around and within the shape). It works great on a website I'm designing.

However, now there's an element on the website which would, in a natural setting, cast a bit of a shadow over part of this image. So the brightness of the image no longer looks right when viewed in the context of the page.

I tried reducing the brightness of the whole image, but the shading still looks unnatural. There would only be a bit of a shadow over part of the image. Is there a way to adjust the brightness along a gradient? Or maybe have a kind of brightness "brush" that I could nudge along until it looks right?

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    In Photoshop there is a solution, but in Gimp I'm stumped for an answer, sorry – ueberkim Jan 27 '12 at 19:09
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Looks like I found something that will help my particular situation. The Dodge/Burn tool. I set it to Burn and Midtones and used a fuzzy circle brush. It feels very much like painting a shadow on the object. The only catch is that it's cumulative so one has to use as few strokes of the brush as possible or it'll get too burned too fast.

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You can use the dodge/burn brushes but you can also set a layer style to dodge/burn, allowing you to make a shape layer over the object you want to alter rather than having to worry about the cumulative effect of the dodge/burn brush tools.

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Try reducing Opacity of Gradient, which will help in reducing its intensity and in-turn helping out the problem.

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Basically involves making your selection, creating a channel, applying gradient to your selection in the channel and then applying it back to your image.

User ofnuts gives a detailed answer on http://gimpforums.com/thread-change-brightness-contrast-for-part-of-image

Method 1: create a selection with a gradient:

  1. Create the rectangle selection on your layer

  2. Open the Channels list (Windows/Dockable dialogs/Channels)

  3. Click the "New channel" icon at bottom, and initialize to 100% opacity

  4. Your canvas fills with black, with the "marching ants" still marking your selection

  5. Fill the selection with a black to white gradient (white is replaced by transparency, so the parts of the image that appear are those that will be selected in the end, in proportion to their visibility)

  6. Right-click the channel, and "Channel to selection"

  7. Click the visibility icon of your channel (makes is invisible, so you should now see you full image)

  8. The selection looks too small (half the height): this is normal, the ants are really traveling the 50% selection line so they only show the part where pixels are more than 50% selected.

  9. In the Layers list, select your Layer (if you omit this, you'll be editing you channel)

  10. Play with Brightness/Contrast

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