Bug#906060: Coordinate overflow when rendering

[Simon Richter]

Package: kicad,libwxgtk3.0-gtk3-0v5,libcairo2,libpixman-1-0
Severity: important

Hi,

this bug is difficult to pin down to a specific package.

KiCad uses the graphics context from wxWidgets for rendering in the
schematic editor (which somewhat works) and the PCB editor (which fails
utterly).

When wxWidgets is linked against GTK 3, it uses a Cairo context for
rendering, as is proper for GTK 3.

Cairo is given appropriate coordinates (as doubles) for a path, into a
zoomed(!) canvas. The logical coordinates used are dependent on the current
zoom level, and the scaling factor between device and logical coordinates
is somewhere between 1:50 and 1:300000.

Depending on zoom level, different failure modes can be observed, but there
is no zoom level at which the rendering is correct. At some levels,
something resembling the correct output can be seen, but as if the
coordinates were wrapped modulo a certain number.

Short video is at http://psi5.com/~geier/overflow.ogv

This can be reproduced best by compiling KiCad with libwxgtk3.0-gtk3-dev
installed, running "pcbnew" and switching to "legacy" canvas.

Quick debugging has shown that the coordinates given to Cairo still make
sense, even if the zoom level makes them numerically large. As I'd need
significant time to debug into optimized drawing routines, I'd like to pass
this on. I suspect that this is mostly an interaction between Cairo and
Pixman, with an overflow happening somewhere in a conversion from double to
an integer type.

This is a major showstopper for linking KiCad 5 against GTK3, so this
requires us to keep GTK2 around longer.

   Simon

-- System Information:
Debian Release: 9.5
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