RGBA and InexactError about colors.jl

[timholy]

In Julia, "white" = 1, not 255.

https://github.com/JuliaGraphics/ColorTypes.jl#rgb-plus-bgr-rgb1-rgb4-and-rgb24-the-abstractrgb-group

[cormullion]

Oh. I was trying:

julia> RGBA(Colors.color_names["red"])

because

julia> Colors.color_names["red"]
(255,0,0)

julia> Colors.color_names["white"]
(255,255,255)

[timholy]

That's just for internal use. Did you come across a recommendation to use it somewhere, or did you find it by poking through the source code?

Try this instead:

julia> using Colors

julia> colorant"red"
RGB{U8}(1.0,0.0,0.0)

[cormullion]

No problem. I like using the named colors: I was trying to find the name matching a specific color. I thought 0-255 values were supported, and was confused with the "Inexact error".

color_to_be_matched = (0.835, 0.388, 0.361)
for (colname, colvalue) in Colors.color_names
  r, g, b = colvalue
  x = colordiff(RGB(color_to_be_matched...), RGB(r/256, g/256, b/256))
  if x < 5
    println(colname, "\t", x)
  end
end

I didn't think the 0-255 values weren't supported.

[timholy]

It's a natural mistake. The reason why we don't support integers in the 0-255 range is that they lead to some troublesome ambiguities, esp. now that 10, 14, and 16-bit images are common. For a 16-bit image, 255 barely registers as brighter than "black"---and note that you're typing 255 as a 64-bit integer, which if taken literally is even worse!

This is what allows Julia to avoid needing stuff like this: http://scikit-image.org/docs/stable/user_guide/data_types.html#image-data-types-and-what-they-mean and to be able to write algorithms that take image values at face value, rather than having separate implementation for UInt8, UInt16, etc. (which is found all over the place in OpenCV's codebase).

In terms of making your life easier: you can use 255 as 255uf8 if you prefer that (need using FixedPointNumbers, I think). It also would be nice to throw a more informative error, but there could be performance consequences. We're fine with people specifying RGB(1,0,0) for example, so we can't make the error happen purely dependent on input types (unless we make RGB(1,0,0) illegal). If you have any thoughts, I'm all ears---you're not the first to be confused by this.

[timholy]

Closed by JuliaGraphics/ColorTypes.jl#51. The new error is

julia> RGBA(56, 34, 23)
ERROR: ArgumentError: (56,34,23,1) are integers in the range 0-255, but integer inputs are encoded with the UFixed8
  type, an 8-bit type representing 256 discrete values between 0 and 1.
  Consider dividing your input values by 255, for example: RGBA{UFixed8}(56/255,34/255,23/255,1/255)
  See the READMEs for FixedPointNumbers and ColorTypes for more information.
 in throw_colorerror(::Type{FixedPointNumbers.UFixed{UInt8,8}}, ::Tuple{Int64,Int64,Int64,Int64}) at /home/tim/.julia/v0.5/ColorTypes/src/types.jl:615
 in throw_colorerror(::Type{FixedPointNumbers.UFixed{UInt8,8}}, ::Int64, ::Int64, ::Int64, ::Int64) at /home/tim/.julia/v0.5/ColorTypes/src/types.jl:585
 in checkval at /home/tim/.julia/v0.5/ColorTypes/src/types.jl:575 [inlined]
 in ColorTypes.RGBA{FixedPointNumbers.UFixed{UInt8,8}}(::Int64, ::Int64, ::Int64, ::Int64) at /home/tim/.julia/v0.5/ColorTypes/src/types.jl:453
 in ColorTypes.RGBA{T<:Union{AbstractFloat,FixedPointNumbers.FixedPoint}}(::Int64, ::Int64, ::Int64) at /home/tim/.julia/v0.5/ColorTypes/src/types.jl:478

which hopefully qualifies as user-friendly.

[cormullion]

@timholy This newbie thanks you! I'll continue my quest of making Julia the easiest to use and most powerful language around... :)

[timholy]

You're no newbie 😄. But you're very good at discovering things that would trip up a newbie, and that's invaluable. Keep 'em coming.