[cairo] Rotating patterns (sprites)?

Ian Britten britten at caris.com
Tue Sep 8 08:56:36 PDT 2009

Hi all,
Continuing on with the pattern (sprite) drawing questions I was
asking last week, I have a couple more questions, mostly focused
on rotation.  [ NB - Cairo v1.8.8 ]

- Draw my patterns/sprites to an output surface that has a
   rotation angle set in it.
- Get my patterns to be rotated the same amount.
- Draw my patterns at specific user positions.

Attached is an updated sample program.  When drawn with no rotation
angle, it draws a diagonal line through two user positions (Sanity
check), then draws a grid of 16 sprites, which point downwards.

Note that for now, I'm just drawing the corner of the pattern at
the desired position, but ideally/eventually, it'll be the centre
of the sprite at the position.

- When I set a rotation on the output surface, the sprites don't
   actually rotate - They still point downwards.  Shouldn't
   painting a pattern onto a rotated surface result in a rotated
   If not, how should I best proceed?  Rotate the sprite when
   drawing my original data into it?  Or muck about with the
   matrix of the [surface?|sprite?] when trying to paint it?
- When the rotation is set, most of my sprites disappear.  I
   assume this is due to my translate/set_matrix() calculations,
   and not accounting for the rotation of the output somehow.
   Is this something I should really have to do though?  I don't
   have to calculate/rotate the points for lines.  Can't I just
   (somehow) specify the user position I want the pattern at, and
   let Cairo handle the transformations, the way it does for lines?
   Doesn't it seem kindof non-symmetrical that I can draw lines in
   user units, but not patterns?
- How the heck do most people debug this sort of problem?  When
   the data simply doesn't appear, how do you know where it is?
- Finally, can I achieve the effect I need fairly easily?  Or is
   this going to require a bunch of matrix stuff?  :P

Many thanks for any help!
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