Draw Clipping to PathDocument version 0.10, January 2003 PrinciplesThe Draw module in RISC OS 5.00 has been extended to support path clipping via two new SWIs. It can use an arbitrary shape clipping path - it need not be rectangular.Draw works by flattening a path from curves into lots of short lines. This new path is then scanned for horizontal lines which can be plotted to the screen. These are placed in an array and plotted using the hline routine (from OS_ReadVduVariables) - i.e. no code actually writes to the screen in Draw. For clipped paths, the clipping path is passed to the processor and scan lines that would be plotted are 'remembered'. The main path is then passed to the processor and only those scan lines which fall within the entries in the array are actually plotted. This is very inefficient. Some of the optimisations that could be applied to this, such as sorting the list are not fully implemented. APITo make clipping a number of regions simple, and because of the lack of registers passed into the SWI call, the clipping path is described in a description block:
SWI Draw_FillClipped
Fill the path specified, clipping it against the clip description block (3 word block describing the clipping area). SWI Draw_StrokeClipped
Stroke (draw the outline) of the specified, clipping it against the clip description block (3 word block describing the clipping area). There are a number of further optimisations that still could take place, so speed may not yet be optimised.
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