Glimmr Canvas-Based Drawing

version 2/101030 by Erik Temple

  • Home page
  • Beginning
  • Previous
  • Next

  • Section: Bitmaps

    A bitmap element draws a rectangular image composed of individual "bits" that are specified by the author in the form of a list of lists (the "bitmap-array") that defines the status of each bit. For example, a cross might be specified like so:

            { 0, 0, 1, 0, 0 },
    { 0, 0, 1, 0, 0 },
    { 1, 1, 1, 1, 1 },
    { 0, 0, 1, 0, 0 },
    { 0, 0, 1, 0, 0 }

    The ones represent bits that are turned on, while the zeros represent bits that are off. Each row is given as a list of numbers with its own set of braces.

    Remember that the bitmap-array is a list of lists of numbers. We supply the standard set of list braces, and then we supply one list for each row within those braces, each row's list also having its own braces, e.g. here's a simple map with 4 rows:

        { {1, 0, 1}, {0, 1, 0}, {1, 0, 1}, {0, 1, 0} }.

    It is very important that all rows have the same number of entries (i.e., columns).

    When image files such as PNGs or JPEGs are scaled, pixels can be interpolated so that the image file can be presented at virtually any scaling factor. However, the pixels of a bitmap element are set directly, and so the individual pixel is the primary unit of measurement (a bitmap displayed with a bit-size of 2 will be twice as wide and twice as high as the same bitmap displayed with a bit-size of 1). Fractional pixel measurements are not possible, and the bit-size is rounded to the nearest integer. So, if you define a bit-size of 2 pixels, the bit will be 2 pixels from 75% to 100% of full size, but will appear as 1 pixel when scaled to 74% or less. The screen size of the bitmap is determined by the bit-size and the dimensions of its grid: a 10 x 10 bitmap with a scaled bit-size of 2 will occupy 20 x 20 pixels on the screen.

    The "on" bit of a bitmap is rendered using the color provided by the "tint" property (defaults to black). The "off" bit can be implemented in one of two ways. If the "background tint" property is specified, the "off" bit will be rendered using the background color. If the "background tint" property is not set, the "off" bit will do nothing (anything drawn beneath the bitmap will show through the off bits). Both the tint and background tint properties are specified using the glulx color value kind of value (see the Glulx Text Effects extension).

    There are two types of bitmaps:

        Monochrome bitmap - Each bit is either on or off. The "on" bit is specified using 1, the "off" bit using 0.

        Polychrome bitmap - RGB color are provided directly in the bitmap-array. RGB colors must be specified using a decimal version of the hexadecimal representation of the color (see Glulx Text Effects for more information). In a polychrome bitmap, the "off" bit is specified using a negative number (rather than 0 as with a monochrome bitmap).

    Polychrome bitmaps are not as human-readable as monochrome ones (white, for example, will be listed in the bitmap-array as 16777215), but they are a bit more flexible.