This is on how I've been doing shadowing of an object within GIMP. Now bare in mind, I've not learned "Script-FU" or Python-FU"...so if you're working at that level, I'd love to see a tutorial from YOU! PLEASE share in the comments section. I'm thinking these are GIMP's version of "macros"?
When I made the move to digital scrapbooking, I wanted to get a realistic 3D look to this 2D format. That's been my style even in the painting and sketching that I've done in the "real world"...so this is also my aim in the virtual world.
To that end, my art background leads me to want a slight shadow all the way around an object and a deeper shadow opposite the light source.
If you are mathematically inclined you might want to also catch my post covering the difference between GIMP Shadowing and Mathematical graphing, because they are NOT the same. (I'll be linking to this post a little later, so check back, if you have a mathematical background and have found it a little confusing in GIMP. It may clear some things up for you.)
Before I walk you through how GIMP does it's shadowing, take a look at what I mean by a slight shadow all around an object. I picked this banner shape from Janet Scott's One Stop Bunting Shop - Shape Mask Flags Template Kit over at Pixel Scrapper Digital Scrapbooking. With this flag, we can see several different angles as we do our shadowing.
While sitting on the object's layer, select your shadowing
Filters> Light and Shadow> Drop Shadow....
Gimp let's you control these items...
Offset of X
Offset of Y - Offset X & Y is how far in pixels, GIMP should the move the shadow from the object.
Opacity of the shadowing
ALLOW RESIZING - if needed GIMP can allow a layer or image re-sizing if needed. (Be Careful with this because you don't want your image size to go over the 12" x 12" or whatever size you are using...ie if the object and/or the shadow moves "off" the edge of your paper size, if this is checked GIMP will increase the image size to accommodate the shadow. In most scrapbooking cases, you DO NOT want to do this. But if you are making an "asset" that is on a transparency, like the case of a PNG file, then you need to make sure this is checked or you'll lose part of your shadow as it gets larger than the objects layer and get "cut-off" in areas.)
By putting a slight shadow all the way around the object, this separates the item from what would be underneath or behind the object. Something that our eyes automatically does with even "paper thin" objects in the real world. But to do it in a 2D environment, our eyes need a little bit of help.
The shadow, that I usually place in the lower right of my objects, give the effect of a light coming from the top left of your desk. This is the typical place for a light to be placed, at least by those that are right handed, which let's face it, is the majority of people...even though I'm ambidextrous, and often do things as a lefty would, I have stuck to this format.
This light format is also the format that GIMP uses, so they have made it easy to place the shadow in the lower right of the objects by using positive numbers for X & Y offsets. (NOTE: This is where GIMP and Mathematical quadrants deviate.)
Since our deep shadow is a paper item, we will move X and Y offsets both 7 places or pixels. So enter seven for both X and Y offsets. In the Photo below I've entered 7, and used a blur radius of 21.
In the photo below you can see the deep shadow in the object.
Now we will enter a smaller shadow all the way around. So let's make this one offset by 3 pixels. To place the next shadow, we will do it in the quadrant opposite the deep shadow, by using BOTH negative numbers for the offset of X & Y. Also we will make the shadow significantly lighter by changing the opacity to only 30%. This way, there is just a hint of shadow and where they overlap will not be stronger than the deep shadow. GIMP does allow for you to be able to adjust the opacity at a later time in case you need to, but the other variables are set within that Drop Shadow layer. You'll also see in the screen shot below, that I've renamed the layer for the first Drop Shadow, adding the variables that I used. I do this not only so that I can remember what still needs to be done, but also so that I have a reference point to look at, when trying to decide how strong the shadows for other items in the layout need to be. Depending on what the item is, how thick it should be, I might adjust and change my numbers.
When working with the blur radius, what we are doing is like air-brushing or using a spray can. How tight is the shadow density...how many of the surrounding pixels are going to be blurred? As a general rule of thumb, I usually make this slightly larger than the offset number. So let's drop the blur rate on these lighter side shadows to 7 pixels.
Now only negative offset in the X, and everything else stays the same.
One more time with the Negative in only the Y offset. After using the Drop Shadow once, there are also some shortcut key strokes you can use. Shift-Ctrl-F controls the pop-up of the last filter. Use this if you need to change the variables to make the shadow. Ctrl-F repeats the last filter used, in this case the Drop Shadow. I use this short-cut to speed up the amount of time I spend shadowing assets. IE I'll do the deep shadow on several items that are about the same thickness, then I'll use the Shift-Ctrl-F and go in and change the settings for the next shadow layer.
To rename the layers, I double click on the layer name, to gain access to naming it. There are other ways to rename layers, but this is quick and convenient. Just remember to get yourself back to the object layer, or you'll be making a shadow of a shadow! lol.
You can right click on a top layer, and "Merge Down" to combine all the shadows, or the object AND the shadow. Just remember this "Merge Down" is the same as "flattening the two layers...because you loose the ability to work with the layers individually.
I hope this helps you get a firm grasp on using the Filter Drop Shadow in GIMP.
Thanks for stopping by, and if you have any questions, or found this helpful, leave a comment, I'll try to answer or do some research if I don't know how to answer your question.