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     DRAW-DISPLAY(2)                                   DRAW-DISPLAY(2)

          Display - connection to draw device

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

          Display: adt
              image:       ref Image;
              ones:        ref Image;
              zeros:       ref Image;

              allocate:    fn(dev: string):
                           ref Display;
              startrefresh:fn(d: self ref Display);
              publicscreen:fn(d: self ref Display, id: int):
                           ref Screen;
              newimage:    fn(d: self ref Display,
                           r: Rect, ldepth, repl, color: int):
                           ref Image;
              color:       fn(d: self ref Display, color: int):
                           ref Image;
              rgb:         fn(d: self ref Display, red, green, blue: int):
                           ref Image;
              open:        fn(d: self ref Display, name: string):
                           ref Image;
              readimage:   fn(d: self ref Display, fd: ref Sys->FD):
                           ref Image;
              writeimage:  fn(d: self ref Display, fd: ref Sys->FD,
                           i: ref Image): int;
              rgb2cmap:    fn(d: self ref Display, red, green, blue: int):
              cmap2rgb:    fn(d: self ref Display, color: int):
                           (int, int, int);
              cursor:      fn(d: self ref Display, i: ref Image, p: ref Point):
              cursorset:   fn(d: self ref Display, p : Point);

          The Display type represents a connection to a draw(3)
          device.  This device is the external representation of a
          physical display, such as a CRT, and its associated memory.
          It contains the storage for all images, even invisible ones,
          so all Image objects must be allocated through Display mem-
          ber functions.  Graphics operations that use multiple Image
          objects may not mix images from different Displays.

          image     The visible contents of the display; draw on image

     DRAW-DISPLAY(2)                                   DRAW-DISPLAY(2)

                    to change the display.

          ones, zeros
                    Replicated images of a single pixel, either all
                    ones (true, black) or all zeros (false, white).
                    Useful as masks for basic graphical operations.

                    Attach to a new display, represented by the
                    draw(3) device mounted in the specified dev direc-
                    tory.  If dev is the empty string, /dev is used.
                    The return value is nil if the allocation fails.

                    After allocating a Display object, the application
                    should spawn a process to call startrefresh; this
                    thread will receive and process window refresh
                    events from the device.

                    Create a locally addressable pointer to a public
                    Screen; see display-screen(2).

          d.newimage(r, ldepth, repl, color)
                    Allocate an off-screen Image.  The arguments sup-
                    ply values for the Image's r, ldepth, and repl,
                    and an initial color map index, used to paint the
                    image when created.  The image's clipr is initial-
                    ized to r.

                    Creates a single-pixel, replicated off-screen
                    image of the specified colour.  The Draw module
                    defines constants for several common colours:
                    Draw->Black, Draw->Blue, Draw->Green, Draw->Red,
                    Draw->White, and Draw->Yellow.

          d.rgb(red, green, blue)
                    Uses the values (from 0 (off) through 255 (full
                    on)) of red, green, and blue to find the closest
                    matching colour map entry, and create a single-
                    pixel replicated image of that colour.  Equivalent
                    to d.color(d.rgb2cmap( r, g, b)).

                    Read an image description from the named file and
                    return an Image holding the picture.  See image(6)
                    for more information about image files.

                    Analogous to open, but from an open file descrip-
                    tor rather than a named file.

     DRAW-DISPLAY(2)                                   DRAW-DISPLAY(2)

          d.writeimage(fd, i)
                    Complement of readimage: write an image file rep-
                    resenting i to the open file descriptor.

          d.rgb2cmap(red, green, blue)
                    Return the colour map index of the colour that
                    best matches the colour triple.  The values of the
                    components range from 0 (no colour) to 255 (satu-

                    Decompose the colour into a colour triple and
                    return the result.

          d.cursor(i, p)
                    Set the current cursor.  If i is the image of the
                    current display, then the graphics cursor will be
                    set to its default value, otherwise i must be an
                    image with ldepth 0 and the following rules apply:
                    the size of the cursor will be half the horizontal
                    height of i (subject to system-dependent restric-
                    tions on cursor size). The top half and the bottom
                    half of the image are treated as two independent
                    masks. When the cursor is drawn, pixels congruent
                    with non-zero bits in the top half are cleared and
                    then pixels congruent with non-zero bits in the
                    bottom half are set.  P gives the offset added to
                    the mouse position when drawing the cursor image.

                    Set the position of the mouse cursor to p.

          The interface to cursor does not allow the use of colour
          mouse cursors, even on systems that allow them. The inter-
          face is likely to change in this respect.