include "draw.m";
draw := load Draw Draw->PATH;

setalpha:  fn(rgba: int, alpha: int): int;

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 between 1 and 32 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. Images have one or more channels: colour channels, greyscale channels, colour map indices, and others, as described in colour(6). Each pixel value contains a component of each such channel. All pixels in an image have the same size, or depth, and the same component structure.

When an image is displayed, the value of each pixel determines the colour of the display, according to the interpretation of the image's channels. For instance, on `true colour' displays, the display image might contain red, blue and green colour channels, and each pixel value will have red, blue and green colour components. For displays with only 8 bits per pixel or less, Inferno uses a fixed colour map for each display depth (see colour(6)). Facilities exist in draw-display(2) to convert between (red, green, blue) triplets and colour-mapped pixel values, but the mapping is often done automatically by the graphics operations when images with different channel structures are combined.

Draw uses a standard representation of colour constants in calls to create coloured images or to initialise new images with a given colour. This is referred to as `32-bit RGBA format'. Each constant colour is represented as a 32-bit integer, with 8-bit red, blue and green colour components, and an 8-bit alpha component, in that order from most to least significant byte.

The RGB values in a colour are premultiplied by the alpha value; for example, a 50% red is int 16r7F00007F not int 16rFF00007F. The function Draw->setalpha performs the alpha computation on a given colour rgba in 32-bit RGBA format, ignoring its initial alpha value, and returning the result of multiplying each colour component by the supplied alpha. For example, to make a 50% red color value, one could execute draw->setalpha(Draw->Red, 16r7F).

Terminology

Point

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. The abstract data type, Point defines a coordinate position.

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.

Display

The type Display represents a physical display, corresponding to a single connection to a draw(3) device. 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, see for example draw-image(2).

Screen

The Screen type 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.

Image

The Image type provides basic operations on groups of pixels. Through a few simple operations, most importantly the draw image combination operator (see draw-image(2)), the Image type provides the building blocks for Display, Screen, and Font.

Font

A Font 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 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. See draw-font(2), utf(6), and font(6).

Context

A Context provides an interface to the system graphics and interactive devices. The system creates this context when it starts an application.

Pointer

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

More about Images

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 structure of an Image is stored as Chans value Image.chans; the image's pixel depth in bits is stored as integer Image.depth.

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. See draw-image(2) for examples.

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

Objects of type Display, Font, Screen, and Image must be allocated by the member functions; if such objects are created with a regular Limbo definition, they will not behave properly and may generate run-time errors.

There are no ``free'' routines for graphics objects. Instead Limbo's garbage collection frees them automatically. As is generally so within Limbo, one can eliminate references by assigning nil to reference variables, returning from functions whose local variables hold references, etc.