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Sprite Plotting under RISC OS 3

From: jbye@acorn.co.uk (James)
Subject: Re: sprite rotation
Date: 15 Jun 92 10:06:01 GMT

Under RISC OS 3 there are a couple of new SWI's that allow sprites to be
transformed and subsequently painted.  The entry points for the SWI are as
follows :-

Transformed sprite SWIs

        SWI OS_SpriteOp
        In:     R0 = 55 (SpriteReason_PlotMaskTransformed) (+ 0, 256 or 512)
                R0 = 56 (SpriteReason_PutSpriteTransformed) (+ 0, 256 or 512)
                R1 -> sprite area (if R0 > 255)
                R2 -> sprite name or header (depends on R0)
                R3 = flag word:
                        bit 0 set => R6 -> destination coords, else matrix
                        bit 1 set => R4 -> source rectangle inside sprite
                        bits 2..31 reserved (must be 0)
                R4 -> source rectangle coordinate block (if R3 bit 1 set):
                        R4!0,4 = x0, y0 one corner in sprite (pixels)
                        R4!8,12 = x1,y1 second corner in sprite (pixels)
                R5 = GCOL action (for PutSpriteTransformed)
                          +8 if mask is to be used
                R6 -> matrix (if R3 bit 0 clear):
                        R6!0,4,8,12,16,20 = matrix (as for Draw module)
                R6 -> destination coordinate block (if R3 bit 0 set):
                        R6!0,4 = X0,Y0 on screen (1/256th OS unit)
                        R6!8,12 = X1,Y1 on screen (1/256th OS unit)
                        R6!16,20 = X2,Y2 on screen (1/256th OS unit)
                        R6!24,28 = X3,Y3 on screen (1/256th OS unit)
                R7 -> translation table ( <= 0 => none)

The source coordinates are inclusive at the bottom-left, and exclusive at the

When specifying a destination parallelogram, the source rectangle is mapped
onto the destination as follows:

        x0,y0   ->      X0,Y0
        x1,y0   ->      X1,Y1
        x1,y1   ->      X2,Y2
        x0,y1   ->      X3,Y3

In future it may be possible to set the destination to an arbitrary
quadrilateral, rather than a parallelogram.  In order to reserve this
possibility, the current version returns a user-intelligible (-ish) error if
the destination is not a parallelogram.

For PutSpriteTransformed, the sprite is plotted through its mask only
if (a) it has one, and (b) R5 bit 3 is set.  R5 is ignored for

The SWI returns an error if any of R3 bits 2..31 are set, to ensure
that these are left clear by software developers.

The SWI covers exactly those pixels on the screen that a call to
Draw_Fill would produce for a rectangle of the same size with the same
transformation matrix, where it is filling to half-way through the boundary.
When plotting using a destination parallelogram, the source rectangle must
be entirely within the sprite. For plotting with a matrix, the source rectangle 
will be clipped to the sprite boundaries prior to transformation.

If the source rectangle (after clipping, if using a matrix) has no area, i.e.
x0=x1 OR y0=y1 then an error will be generated, as it is not possible to choose
a colour in which to fill the destination.
Note that the SWI does allow x0>x1 or y0>y1 or both. When plotting with a matrix
there is no difference between x0 and x1 swapped, or y0 and y1 swapped, but when
specifying a destination parallelogram the image will be reflected.

Due to the mechanism of the routine the accuracy is not absolute. The SWI
will always cover the same area as a Draw filled path, but not necessarily
with the right source pixel data from the sprite. The worst possible error
(in a fraction of a source pixel) at one end of the plotted area is given by

The table beloe gives more information on the maximum errors attainable:    

   Destination size                     Worst possible error in source pixels
         5                                              0.0000763
         10                                             0.0001526
         50                                             0.0007629
        100                                             0.0015259
        500                                             0.0076294
       1000                                             0.0152588
       5000                                             0.0762939
      10000                                             0.1525879
[ The largest output possible is 32767 pixels ]
i.e. When plotting a sprite to a destination width of 5000 pixels, the worst
error possible in the position in the source rectangle of the final pixel
plotted is about 1/13 of a source pixel.

Note that if these errors (usually too small to notice) must be avoided then
the sprite should be plotted in parts - perhaps by dividing the plotting
into four areas.

        Attempt to set reserved flags
                Bits 2 to 31 of R0 must be zero in the current specification
        Source rectangle area zero
                The area of the source rectangle must be non-zero, so the
                sprite routines will have some valid colour(s) to plot the
                output rectangle in. If the area is zero,
                there would be no valid colour(s) with which to plot the output.
        Source rectangle not inside sprite
                The source rectangle must be totally inside the sprite
        SpriteExtend can only do linear transformations
                The current version of the transformation routines can only
                perform linear transformations, and not any arbitrary

Hope this is of some help.