Copyright (C) 1992, Digital Equipment Corporation                         
 All rights reserved.                                                      
 See the file COPYRIGHT for a full description.                            
 by Steve Glassman, Mark Manasse and Greg Nelson           
 Last modified on Mon Feb 24 13:57:54 PST 1992 by muller   
      modified on Thu Dec 12  1:43:15 PST 1991 by gnelson  
      modified on Fri Aug  3 16:59:33 PDT 1990 by steveg   
A ScreenType.T represents a class of screens that have a common pixel depth, a common set of operations on the pixels, and common repositories for cursors, pixmaps, and fonts.

When the screentype of a VBT changes, any screen-dependent resources for the old screentype become useless. The application must use the new screentype's {\it oracles} to look up resources that are valid for the new screentype. This is all handled automatically if you use screen-independent resources that are predefined or defined by somebody else. But you will need to use this interface if you are implementing your own screen-independent resources.


IMPORT ScrnCursor, VBT, ScrnColorMap, ScrnFont,
  ScrnPaintOp, ScrnPixmap;

TYPE T = VBT.ScreenType;

REVEAL VBT.ScreenType <: Public;

  Public = VBT.ScreenTypePublic OBJECT (*CONST*)
    bg, fg: ScrnPaintOp.Pixel;
    bits: T;
    op: ScrnPaintOp.Oracle;
    cursor: ScrnCursor.Oracle;
    pixmap: ScrnPixmap.Oracle;
    font: ScrnFont.Oracle;
    cmap: ScrnColorMap.Oracle;
For a screentype st, the values st.bg and st.fg are the pixel values that represent the user's default background and foreground colors on st. If the screen is color-mapped, these are appropriate for the default colormap. For applications doing simple painting, bg is logical white and fg is logical black. Depending on the screen and user preferences, the actual colors that the user sees might be different.

The screentype st.bits is the screentype for 1-bit deep pixmap sources for painting on screens of type st. It is guaranteed that st.bits.bits=st.bits, st.bits.fg=1, and st.bits.bg=0.

The oracles st.op, st.font, st.cursor, and st.pixmap contains methods that provide screen-dependent resources appropriate for st---for example, st.font has a method that will look up fonts by name.

If st.cmap # NIL, st is a color-mapped screen, which means that the color of a pixel is determined by looking up its value in a table. The color map can be either readonly or writable.

END ScreenType.