[cairo] Blitting surfaces onto each other with coordinate translation

[Johannes Bauer]

Hi list,

I really have no clue what's going on anymore and really any input could
help me. I've maneuvered myself into a really crappy corner.

What I'm doing: Using pycairo to blit a hardware source with real
measurements. The given external coordinate system has (0, 0) in its
bottom left corner and, for convenience, I calculate everything in
inches in my "hardware" coordinate system, but translate to pixels when
going to my cairo surface.

There's multiple of these surfaces I create, with different offsets to
each other. So one might describe a corner from (0, 0) to (1, 1) and the
next might describe (0.5, 2) to (4, 5) inches. In the end, I want to
compose them all onto each other.

When I create the individual layer surfaces, I translate the Y
coordinates appropriately:

matrix = cairo.Matrix()
matrix.scale(1, -1)
matrix.translate(0, -height)
matrix.translate(-offset.x, -offset.y)

This allows me to convert from hardware to pixels by just dpi
multiplication and then just draw on the surface.

When I want to create the final surface that all others are merged onto,
I originally had the same setup (i.e., also the final surface would have
an inverted Y axis). But then, for some weird reason, when trying to
blit the sub-surfaces on the destination:

final_cctx.set_source_surface(sub.ctx, sub.offset.x, sub.offset.y)
final_cctx.cctx.paint()

Then the Y-axis would again be inverted. I'm guessing because it's
inverted again through the projection matrix of the final destination
surface.

Now in my testcases I cannot reproduce any of this, there the double
inversion does not happen and, for the life of me, I cannot figure out
what is going on or even how I would start debugging this. I also start
to question if I should also have done the inch/pixel conversion within
the Cairo translation matrix and let everything be handled by Cario
instead of doing a part on my own.

Overall, I'm questioning my sanity.
Can someone help me out on how to debug or move forward? Any help is
greatly appreciated.

All the best,
Johannes