[Octave-bug-tracker] [bug #43505] build gm with quantum-depth=16 in mxe octave

[Carnë Draug]

Follow-up Comment #10, bug #43505 (project octave):

The failing test is real, I have seen it reported a few times (I don't
remember if ever happened to me though). The reason is the following:

* imwrite and imread are tested by creating random images. These are always
uint8 because if the user does have GM installed, all builds will support
this.

* the whole reason why quantum depth is a build option is because then GM will
ONLY handle images of that bitdepth. What this means is that when it reads an
uint8 and is built with quantum depth 16, it will read them scaled in the
16bit range. Example, if the file has uint8 values but GM was built with
quantum depth 16, then GM will give the value 65535. The same happens for
writing, i.e., you must give a value of 65535 when you actually want to write
a 255 uint8.

* this means that rounding errors can creep but I don't know where (and I
can't replicate). I'll guess that some autoconf macros would be required to
modify how the rounding is done depending on the system where it is built.

Alternatives are: 1) live with the error, 2) ditch GM.

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