Glimmr Canvas-Based Drawing

version 2/101030 by Erik Temple

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  • Section: Activate elements for display

    Each time a graphics window is redrawn, Glimmr iterates through the elements that are associated with that window's current canvas. If the element is marked for display, the element is drawn to the screen according to the parameters we have specified. If it is not marked for display, it is ignored. We set the display status to either g-active (marked for display) or g-inactive to initialize it.

    All elements are marked for display by default, so if we say nothing about the display status of an element when we define it, it will display as soon as the appropriate graphics window is opened.

    Very often, however, we do not want all of our elements on all of the time; a more dynamic approach is desired. For example, If we have designed a map in which each room is represented by a sprite, we want only the rooms that a player has visited to be displayed. We might begin by defining all of these as of the sub-kind room-sprite, and then mark all of them as initially not for display:

        A room-sprite is a kind of a sprite. The associated canvas of a room-sprite is the map-canvas. A room-sprite has a room called the associated room. The display status of a room-sprite is g-inactive.

    Now, none of our rooms will show up when the map window is first opened. If we add a rule to the rules for the looking action, which are carried out each time we enter a room, we can cause rooms to display as we enter them (note that we have added a property here to room-sprites, the "associated room," which represents the room that the sprite depicts):

        Carry out looking:
            Repeat with current-element running through room-sprites:
                if the associated room of the current-element is the location:
                    if current-element is display-inactive:
                        activate the current-element.

    "Activate <a g-element>" is a shortened way of writing "change the display status of <g-element> to g-active." (We can also use "mark <g-element> for display".) To mark an element inactive, we use "deactivate <g-element>" or "unmark <g-element> for display". Finally, we can test whether an element is currently active or inactive by using one of the following phrases:

        if <g-element> is display-active
        if <g-element> is display-inactive

    There are many appropriate ways to manage the display of elements. The examples below will provide more ideas.

    And those are the basic steps in using Glimmr Canvas-Based Drawing. The remainder of this documentation explores the element types and other options in more detail, and also explores intermediate and advanced usage of the extension. Finally, the examples illustrate basic usage as well as a number of techniques that are likely to be of interest.