Hallöchen!
If DT wants to convert a rectilinear image to fisheye, it uses
Lensfun's lf_modifier_apply_subpixel_geometry_distortion function.
It generates an array of float pixel coordinates (0.0, 0.0), (1.0,
0.0) ... (width - 1.0, height - 1.0), and transforms them according
to the mathematics of the projection transform.
Then, DT uses dt_interpolation_compute_sample to make an
interpolated lookup in the original sensor image -- at least I
understand it this way.
Quite often, the transformation leads to invalid values. For
example, things like arcsin(1.5) may occur. Sometimes -- but not
always -- Lensfun catches these cases and returns 1.6e16. (Don't
ask me where *this* value comes from.) DT's interpolation function
casts this into an int. Due to an implementation detail of the GCC,
this becomes a MAXINT -- but according to the C standard, the value
is actually undefined.
By the way, the GCC converts a NaN into 0. But I can cause DT to
segfault with NaNs, so DT doesn't handle the NaN value reliably.
DT's interpolation function simply seems to clip/ignore too large
values, which is correct behaviour. So, the whole thing mostly
works because:
1. Mostly, LF returns 1.6e16 for invalid values.
2. DT converts this unchecked to int, which the GCC luckily(!)
converts into MAXINT.
3. DT clips such pixel coordinates.
I want to make all this
inf-nan-zero-1.6e16-clip-implemenation-detail mess well-defined.
How do I do this? Possibilities may be:
* Check everything in LF and return a very large (but not too large)
float value if invalid.
* Do the maths in LF completely unchecked and let the caller (aka
DT) check.
Note that *somewhere* we *have* to check. So some sort of
performance impact is unavoidable AFAICS.
Tschö,
Torsten.
--
Torsten Bronger Jabber ID: torsten.bron...@jabber.rwth-aachen.de
or http://bronger-jmp.appspot.com
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