Glimmr Canvas-Based Drawing

version 2/101030 by Erik Temple

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  • Chapter: Intermediate and advanced topics

    Section: The default canvas-based drawing rule

    GCBD is constructed around a single, rather complex window-drawing rule. This drawing rule is automatically used for any g-window of the kind "graphics g-window" (defined by the Flexible Windows extension). Here are the basic steps in the processing of this rule, along with what happens at each stage. For more information on details, of course, please see the code.

        1. Calculate the scaling factor for the window: Check the canvas dimensions against the window dimensions and calculate the scaling factor needed to best fit the canvas in the window. This is handled by the "carry out scaling activity".

        2. Calculate window offsets: Unless the aspect ratio of the canvas is exactly the same as the window, the scaled canvas will fall short of the window dimension along one or both axes. In this stage, we calculate the offsets necessary to center the canvas in the window. This is handled by the "offset calculation activity".

        3. Window-framing adjustment: This phase only occurs if we have specified an origin other than {0, 0} for the window. If we have, the offsets are again adjusted to account for the fact that we have changed the window framing (see the "Framing the Window" section below). This is handled by the "window-framing adjustment activity".

        4. Clear the window to prepare for drawing.

        5. Draw the canvas background: If the canvas has a background image, we draw it now. This is handled by the "drawing the canvas background activity".

        6. Draw active graphic elements: Repeat through the display-active elements assigned to the window. For each element, we will first follow the element scaling rules for the element, and then follow the element display rules. This is handled by the "drawing the active elements activity".

    These steps (except for the clearing of the window) are implemented as activities, so we can add before or after rules to any of them, making the whole system very accessible to modification. Some of the topics and examples which follow illustrate ways to "hack" the process.