Help - Comments - Graphics - External - ExternalScreen
ExternalScreen represents a DisplayScreen that is not part of the Squeak universe. Subclasses must implement the appropriate primitives for creating, destroying and allocating the appropriate external objects.
Note: It is assumed that all external display surfaces are accessible by FXBlt, meaning that any support code must register the surfaces with the surface plugin. This requires that the support code will have a way of accessing the bits of the surface. Although this can be terribly expensive (such as on X where a roundtrip to the server might be required or for an OpenGL display where glReadPixels usually is slow as hell) the appropriate methods should be implemented. This allows for a gradual transition to less expensive model (such as implementing an X11Canvas supporting the drawing primitives of X) and is therefore the preferred solution.
In the eventual case that it's known that BitBlt/FXBlt will *never* be used in conjunction with a particular drawing surface, the support code should return a handle that is a) not a SmallInteger (these are used by the surface plugin) and b) not of the 'bitsSize' of a Form. One possible representation for such a handle would be a ByteArray of a non-integral word size (e.g., a ByteArray of size 5,6, or 7). In this case, all attempts to use FXBlt with the drawing surface will simply fail.