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Concepts and Terminology

Since the Draw module provides primitive graphics, its functions and data are at a lower level than is typical of a graphics API. Hence, there are some concepts and terms that would be helpful to introduce first.

The Graphics Context

As discussed in Chapter 3, the graphics Context that is captured by a command line program through an argument to the init function is defined in the Draw module. The Context argument holds the references of the graphics resources.

Two important elements of Context are the display and screen types.

Display

The type display of type ref Draw->Display represents a physical display, corresponding to a single connection to a draw device (/dev/draw). Besides the image of the display itself, the Display type also stores references to off-screen images, fonts, and so on. The contents of such images are stored in the display device, not in the client of the display, which affects how they are allocated and used.

Objects of type Display must be allocated by its member functions. If a Display object is created with a regular Limbo definition, it will not behave properly and may generate run-time errors.

Screen

The type screen of type ref Draw->Screen is used to manage a set of windows on an image, typically but not necessarily that of a display. Screens, and hence windows, may be built recursively upon windows for subwindowing or even on off-screen images.

Objects of type Screen must be allocated by its member functions. If a Screen object is created with a regular Limbo definition, it will not behave properly and may generate run-time errors.

Pixels

Images are defined on a rectangular region of an integer plane with a picture element, or pixel, at each grid point. Pixel values are integers with 1, 2, 4, or 8 bits per pixel, and all pixels in a given image have the same size, or depth. Some operations allow images with different depths to be combined, for example to do masking.

When an image is displayed, the value of each pixel determines the color of the display. For color displays, Inferno uses a fixed color map for each display depth, and the application is responsible for mapping its desired colors to the values available. There are functions to convert from red, green, blue triplets to pixel values.

Point

The type Point defines a coordinate position. The graphics plane is defined on an integer grid, with each (x, y) coordinate identifying the upper left corner of the corresponding pixel. The plane's origin, (0, 0), resides at the upper left corner of the screen; x and y coordinates increase to the right and down.

Rect

The type Rect defines a rectangular region of the plane. It comprises two Points, min and max, and specifies the region defined by pixels with coordinates greater than or equal to min and strictly less than max, in both x and y. This half-open property allows rectangles that share an edge to have equal coordinates on the edge.

Image

The type Image provides basic operations on groups of pixels. Through a few simple operations, the Image type provides the building blocks for Display, Screen, and Font. Objects of type Image must be allocated by its member functions. If an Image object is created with a regular Limbo definition, it will not behave properly and may generate run-time errors.

An image occupies a rectangle, Image.r, of the graphics plane. A second rectangle, Image.clipr, defines a clipping region for the image. Typically, the clipping rectangle is the same as the basic image, but they may differ. For example, the clipping region may be made smaller and centered on the basic image to define a protected border.

The pixel depth of an Image is stored as a logarithm called Image.ldepth. Pixels with 1, 2, 4, and 8 bits correspond to ldepth values 0, 1, 2, and 3.

An image may be marked for replication. When set, the boolean Image.repl causes the image to behave as if replicated across the entire integer plane, thus tiling the destination graphics area with copies of the source image. When replication is turned on, the clipping rectangle limits the extent of the replication and may even usefully be disjoint from Image.r.

The Image member functions provide facilities for drawing text and geometric objects, manipulating windows, and so on.

Font

A Font type defines which character image to draw for each character code value. Although all character drawing operations ultimately use the draw primitive on the underlying images, Fonts provide convenient and efficient management of the display text. Inferno uses the 16-bit Unicode character encoding, so Fonts are managed hierarchically to control their size and to make common subsets such as ASCII or Greek efficient in practice.

Objects of type Font must be allocated by its member functions. If a Font object is created with a regular Limbo definition, it will not behave properly and may generate run-time errors.

Pointer

The Pointer type conveys information for pointing devices, such as mice or trackballs.

Return Values

Most drawing operations operate asynchronously, so they have no error return. Functions that allocate objects return nil for failure. In such cases the system error string may be interrogated (such as by the %r print format) for more information.

Freeing Graphic Objects

There are no functions to free graphics objects. Instead Inferno's garbage collection frees them automatically. In Limbo, references can be eliminated by assigning nil to reference variables, returning from functions whose local variables hold references, and so on.

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