Using Cairo, I'm placing some text at random positions, and I need to know if they overlap some previously drawn arbitrary shapes. I could clip the path to the previous drawings, and if any clipping occurs it means that there is overlap. However, Cairo doesn't seem to have any functions to tell if clipping did occur or not.
Is there any way to easily accomplish what I want? I guess I just basically want to see if two shapes intersect or not.
I don't see anything - at least, nothing simple - and I wouldn't be surprised if there weren't anything. Cairo is aimed at rasterizing vector drawing operations, not intersection testing.
However, if I were going to see if two pieces of text overlapped, here's what I would do:
It's rather brute force, but it will even cope with anti-aliasing. I've done something like that before (for a different purpose) with GTK on X, which indirectly uses Cairo.
If you don't want to do it that way, and Cairo doesn't provide an API, you might be able to add one. That would probably be difficult; you might want to talk with Carl Worth before doing that.
I solved the problem by using Qt for the rendering instead. It seems to have quite extensive support for different path operations and supports both PDF and SVG output.
Depending on the quality you want to get, you can use cairo_stroke_extents, cairo_fill_extents and cairo_text_extents and work on the boundary boxes.
A better approach would be to compute only the boundary box of the text and check the four corners against the last path with cairo_in_fill or cairo_in_stroke. The maximum error would be the distance from the shape of a single glyph to its bounding box but maybe this is enough for your purpose.
The last option would be to flatten the text and check any single point as in the previous step.