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GIMP

GIMP Notes


GIMP is a powerful tool but I found a number of the more advanced features complex until I understood the underlying principles. You may be different but when I was learning to drive a manual car I didn't "get it" until I understood how the clutch actually worked, then it was easy.

So in case you are struggling with GIMP maybe this will help you.

CAVEAT: I'm only documenting the bits I've needed so this is NOT a tutorial but it does explain some concepts and should include a few howtos. I don't promise it is either correct or complete. YMMV.

Concepts


Gimp is primarily a pixel painting tool not a vector drawing tool. Having said that the path concept within GIMP allows a reasonably sophisticated degree of vector manipulation when needed.

  • A canvas is the overall container in which you draw. It has a width, height and various other properies.
  • A layer is what you actually draw on. Think of layers just like the original cell animation, clear sheets of plastic with paint on them. There would be a background layer, then layers for various characters and finally a foreground layer. The whole stack would then be photographed. The clear ;-) benefit of layers is that they introduce a form of object oriented drawing - parts that don't change as often in a scene (eg background) can be reused in image after image. Layers add a third dimension to images. Layers can be reordered (layer tab), the topmost layer will be drawn last and any paint in this layer will obscure paint in the layers below. Because of this non-explicitly painted parts of all layers other than the bottom-most are often transparent. A layer is always rectangular but may be smaller than the canvas.
  • Selection region (there is only one) is a used to mask many operations. That means the operation only takes effect within the selection region. Normally the selection region is set to the current layer boundary. A region as large as the canvas can be selected, however all operations are masked not only by the selected region but also by the current layer. So in practice operations apply to the intersection of the selected region and the current layer boundary. Many operations affect the selection region so you should always be aware of what it is at any point in time. Use the Selection tab to check it.
  • Paths are a non-drawn, infinitely thin mathematical constructs (the basis of vector drawings). To be visible a path has to be used to paint something. Paths have their own dialog tab.
  • Text is worthy of special mention as underneath it normally consists of automatically drawn paths. Modern computer fonts are defined in terms of paths (same concept as paths used in GIMP, but implemented by the computer operating system not by GIMP). The use of paths allows fonts to be scaled up/down and always appear smooth because the mathematical, resolution-independent paths are scaled and _then_ the pixels along the paths are painted. The side effect of all this is that text can always be edited/resized and the painted characters magically update without further effort on the user's part. Understanding this may help when dealing with more advanced features like text-along-a-path. In order to update text on an canvas without losing/damaging the rest of the image, each piece of text is by default placed in its own layer. Text can be placed on a layer with other drawn items but then one loses the ability to change the text without affecting the rest of the drawing on that layer.
  • Channels Channels are related to colour and transparency. TO BE DOCUMENTED.

Painted pixels (which includes imported images etc) are the only things visible in a GIMP image. I shall use the general term paint to refer to visible image contents. Images may also have transparent parts to them.

There are a number of ways to add paint to an image. The simplest ways include freehand lines, area fill (paint bucket), gradient fill, airbrush etc.
All paint is added to a layer, specifically the current layer (the current layer is highlighted in the layer tab).
An important implication is that painting is always limited to the size/location of the current layer - ie you cannot paint outside the bounds of the current layer.

As a result of all these concepts any complex image constructed in GIMP (apart from the "trivial" all-in-one traditional painting) will end up with multiple layers and probably a collection of paths. To produce a normal image file format (png, jpeg etc) most of which have no concept of layers, the GIMP canvas has to be flattened into a normal 2D image.

Having learned the GIMP drawing engine concepts you now know enough to do (and more importantly to understand how to do) fairly complex items like drawing text along a curved path.



  • + : A leading plus sign indicates that this word must be present in every object returned.
  • - : A leading minus sign indicates that this word must not be present in any row returned.
  • By default (when neither plus nor minus is specified) the word is optional, but the object that contain it will be rated higher.
  • < > : These two operators are used to change a word's contribution to the relevance value that is assigned to a row.
  • ( ) : Parentheses are used to group words into subexpressions.
  • ~ : A leading tilde acts as a negation operator, causing the word's contribution to the object relevance to be negative. It's useful for marking noise words. An object that contains such a word will be rated lower than others, but will not be excluded altogether, as it would be with the - operator.
  • * : An asterisk is the truncation operator. Unlike the other operators, it should be appended to the word, not prepended.
  • " : The phrase, that is enclosed in double quotes ", matches only objects that contain this phrase literally, as it was typed.

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