Provide hints about how to handle InexactErrors during saving

[timholy]

Currently we have this:

julia> A = [-1.0 0.0; 0.5 1.0]
2x2 Array{Float64,2}:
 -1.0  0.0
  0.5  1.0

julia> using FileIO

julia> save("/tmp/test.png", A)
WARNING: InexactError()
 in trunc at float.jl:357
 in _map_a! at /home/tim/.julia/v0.4/Images/src/map.jl:489
 in image2wand at /home/tim/.julia/v0.4/ImageMagick/src/ImageMagick.jl:151
 in save_ at /home/tim/.julia/v0.4/ImageMagick/src/ImageMagick.jl:146
 in save at /home/tim/.julia/v0.4/ImageMagick/src/ImageMagick.jl:64
 in save at /home/tim/.julia/v0.4/FileIO/src/loadsave.jl:92
 in save at /home/tim/.julia/v0.4/FileIO/src/loadsave.jl:51

which is pretty cryptic (see JuliaImages/Images.jl#422).

This adds a helpful hint:

WARNING: Mapping to the storage type failed; perhaps your data had out-of-range values?
Try `map(Images.Clamp01NaN(img), img)` to clamp values to a valid range.

It's a little verbose to include the module name, but if people are working with plain arrays then I was concerned that Clamp01NaN might not be in-scope.

What do you think, @xanderdunn?

[SimonDanisch]

That's a lot better! :) Maybe we should start introducing InvalidData exceptions, which indicate that it's expected that no loader will save correctly.
I think the exception handler in FileIO needs a revamp anyways. I find the Warning --> Exception(same_warning) pretty annoying.
Also, has there been a discussion already, if we can have more modes to clamp by default? I run into this when doing some arithmetic with colors.
It's really annoying, that you have to clamp colors yourself all the time ;) I basically ended up defining -,+,* which do clamping after the arithmetic. This was arithmetic on U8, though, which is probably not that common...

[timholy]

@xanderdunn, I'm going to assume that you'd find this helpful.

[timholy]

Also, has there been a discussion already, if we can have more modes to clamp by default? I run into this when doing some arithmetic with colors.
It's really annoying, that you have to clamp colors yourself all the time ;) I basically ended up defining -,+,* which do clamping after the arithmetic.

The problem with the latter strategy is that if you define a clamping +, then
c = (a+b)/2 may saturate, and then go to half-saturation, for no good reason. AFAICT the only way to get sensible behavior is for the programmer to specify when clamping should occur. See also JuliaGraphics/Colors.jl#7

I'm not exactly sure what you mean by "more modes"? Do you mean something besides clamping to the interval [0,1]?

[SimonDanisch]

I'm not exactly sure what you mean by "more modes"?

I really didn't express myself very well, since it wasn't that thought out whatsoever!
I basically mean modes, to make this easier:

AFAICT the only way to get sensible behavior is for the programmer to specify when clamping should occur

like a series of functions, e.g. clamped_plus, clamped_minus. This wouldn't be very elegant, though.
clamp(a+b) seems nicer in comparision. So maybe everything is fine already!