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Grokking The Gimp
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9.1.2 GIMP Layers Can Behave Like Animation Frames

Animations are sequences of images that can be played according to timing information attached to each frame. The GIMP has built-in support for animations because each layer in an image can be treated as an animation frame by the GIMP. The GIMP supports this equivalence by allowing timing information to be attached to layers, by providing a tool for playing layered images as animations, and by supporting the conversion   of layered images to Indexed format and output as GIF files.

Figure  9.1

Figure 9.1: Example of Some GIMP Animation Features
Figure 9.1

illustrates some of the GIMP's built in animation features. Figure  9.1(a) shows an image that consists of five layers. The organization of the layers is seen in the Layers dialog shown in Figure  9.1(b). This image is constructed by creating an image with a white background, and then adding four new layers, each one containing, in sequence, one of the four letters G, I, M, and P. The letters were colored using the Bucket Fill tool. The animation sequence is illustrated here:


(Note: The animation can be turned off by clicking on the Stop button of your browser.) Notice that each layer is named and that the names contain timing information  used for the frame animation. The timing information is indicated by the text in the format (XXXXms), where XXXX is a 1 to 4 digit number, and ms indicates that the units of the number are in milliseconds. The parentheses are a required part of the format. Thus, for this example, the Background layer appears empty for 1 second (1000ms) followed by the four letters appearing at intervals of 100ms, 400ms, 600ms, and 800ms. Name and timing information is added to each layer by double-clicking the layer title area to the right of the thumbnail as described in Section  2.1.1.

The animation in Figure  9.1(a) can be viewed in the GIMP using the Animation Playback tool found in the Image:Filters/Animation menu. As you can see in Figure  9.1(c), the tool consists of a Play/Stop button, a Step button, which allows the animation to be stepped along a frame at a time, and a Rewind button, which can be used to set the animation back to the first frame. This last button is particularly useful if the animation consists of a very large number of frames. The Animation Playback  tool plays the animation associated to the image using the timing information as specified in the name of each layer. Note that there is also a progress bar that shows the animation's current frame number.

By default, GIMP animations sequentially present frames using the Combine mode.  This means that as each new frame is displayed, it is stacked on the previous one. Thus, if a new frame is partially transparent the previous frame can be seen through its transparent parts. This is not the traditional movie paradigm for animation, which, instead of combining frames, replaces each frame with a new one. Consequently, although the frames in Figure  9.1 all consist of single letters on a transparent field, the animation spells out the word GIMP a letter at a time on the white background of the first layer.

Figure  9.2

Figure 9.2: Example of Replace Mode
Figure 9.2

illustrates the same example as in Figure  9.1 except now each layer uses Replace  instead of Combine mode in the animation sequence. The Layers dialog in Figure  9.2(b) shows that the Replace mode is specified by typing the text (replace) in the title field of each layer. The Animation Playback tool shown in Figure  9.2(c) shows the animation at the same point in the sequence as before. However, now, due to Replace mode being used, the white background and the red letter G are not visible. There is only the green letter I on a transparent background. This is the traditional movie paradigm for animation. The new animation sequence is shown here:


(Note: The animation can be turned off by clicking on the Stop button of your browser.) The explicit use of Combine mode can be made by typing the text (combine) instead of (replace) in a layer title. The two modes can be used together in an animation with some frames replacing and others combining.

Grokking The Gimp
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