GimpSHOP is Gimp with a great front end that makes it work much like Photoshop which you can find free for downloading here: GimpSHOP.
GimpSHOP, like any good photo editing program supports layers, and layer masks. These features aren’t always simple to get your head around but when you do, you’ll find they’re invaluable tools to use for photo editing tasks. Here, step by step, is how to work with them:
To display the Layers palette press Shift F7 or choose Window, Layers. To add a new layer choose Layer, Duplicate Layer. There are no adjustment layers available in Gimp or GimpSHOP so this process keeps your original photo layer safe in case you need to use it later on.
To adjust the levels in the image to improve its tonal range, choose Image, Adjustments, Levels and drag the sliders on the Input area of the dialog to adjust the contrast. The sliders under the chart should be moved inwards so they appear under the ends of the chart. The middle slider adjusts the midtones. Click Ok to apply this to the layer.
To see how you can apply an effect or adjustment to this layer and blend it with the layer below, first create a duplicate of the top layer by choosing Layer, Duplicate Layer. Apply the edge detect filter by choosing Filter, Edge Detect, Edge and choose the Sobel Algorithm with the default value. This gives a new layer which is predominantly black and which displays the coloured edges in the image.
Now, in the Layers palette, select the Divide blend mode from the dropdown list. Blend modes control how the top layer interacts with the layers below. Drag the opacity slider to the left to reduce the opacity of this layer so that some of the layers below show through thus mitigating some of the effect created by blending the layers together.
To edit the effect the top layer gives to the image, use a Layer mask. Choose Layer, Mask, Add Layer Mask and choose White (full opacity) and click Ok. You should see no change in the image. Now select black or grey as the paint colour and paint the image to show part of the layers below through the image, thus effectively removing the effect of the blended edge filter. Paint with white to reapply the layer effect.
The Layers menu offers tools for viewing a layer mask, applying it permanently to the layer, deleting it and making a selection based on it, etc.. You can also use the menu to configure the size of the thumbnail images by clicking by choosing Preview Size and then choose a view size from the list.