ERROR: MethodError: no method matching length(::Nothing)

[denfc]

Here's the full code:

using AstroImages

AstroImages.set_cmap!(nothing)
antred = AstroImage(download("http://www.astro.uvic.ca/~wthompson/astroimages/fits/antenae/red.fits"))
antgreen = AstroImage(download("http://www.astro.uvic.ca/~wthompson/astroimages/fits/antenae/green.fits"))
rgb2 = composecolors(
    [antred, antgreen]
    )

Running Julia Version 1.11.3 in VSCode through WSL2 (same error when excuting directly in WSL2 without using VSCode) , the separate images can be viewed when after the error, just antred, for example, is typed into the REPL. Here's the full error message:

ERROR: MethodError: no method matching length(::Nothing)
The function `length` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  length(::FITSIO.FITSHeader)
   @ FITSIO ~/.julia/packages/FITSIO/VqV3x/src/header.jl:351
  length(::DimensionalData.Dimensions.Lookups.NoMetadata)
   @ DimensionalData ~/.julia/packages/DimensionalData/VxDQB/src/Lookups/metadata.jl:84
  length(::Core.SimpleVector)
   @ Base essentials.jl:933
  ...

Stacktrace:
 [1] composecolors(images::Vector{…}, cmap::Nothing; clims::Percent, stretch::Function, contrast::Float64, bias::Float64, multiplier::Float64)
   @ AstroImages ~/.julia/packages/AstroImages/eSsEw/src/ccd2rgb.jl:59
 [2] composecolors
   @ ~/.julia/packages/AstroImages/eSsEw/src/ccd2rgb.jl:41 [inlined]
 [3] composecolors(images::Vector{AstroImageMat{Float32, Tuple{X{…}, Y{…}}, Tuple{}, Matrix{Float32}, Tuple{X{…}, Y{…}}}})
   @ AstroImages ~/.julia/packages/AstroImages/eSsEw/src/ccd2rgb.jl:41
 [4] top-level scope
   @ ~/Gitted/Fractals/tests/Untitled-1.jl:6
Some type information was truncated. Use `show(err)` to see complete types.

What do you think is happening? Is a work around possible on my end? Many thanks!

-- dfc

[icweaver]

Hi Denis, thanks for reporting this! I think three channels (R, G, B) are expected by default, so specifying just the two that you want to use should do the trick:

using AstroImages

# Using R and B here because G seems to be a different size on the ESA site
antred = load(download("https://esahubble.org/static/projects/fits_liberator/datasets/antennae/red.fits"))
antblue = load(download("https://esahubble.org/static/projects/fits_liberator/datasets/antennae/blue.fits"))

rb = composecolors([antred, antblue], ["red", "blue"])

A more informative error message could maybe be thrown for this. Happy to open a PR if so

• @sefffal, unrelated, but should the image urls used in the docs be updated from your previous institution? I guess it's been a couple years already, but congrats on graduating and on the new position!!

  • Edit: opened a PR (Update urls for some data downloads #62) if that works for you. No worries though if you had something else in mind

[denfc]

Thanks, Ian! Might also be worth adding the case of two to the Readme to make it explicit that adding the colors is necessary when not using the default of three.

Thanks again.

-- Denis

[icweaver]

Hi all, I've opened #63 to address the error messages.

Ah, I see what you mean Denis. I see here that we say it in words about the non-default case, but we don't show an explicit code example like we do in the docs:

or in the Examples section of the docstring for composecolors:

I think that would be a great first PR to make to this project if you are keen!

[denfc]

But Ian, I just pointed it out --- you presented the solution and did the work that describes the environment well. Plus, I should have looked at the docs more carefully. If I were to do a PR --- which I thought I did, but I don't see it now --- I would simply suggest modifying "... for example merging two bands according to color schemes from ColorSchemes.jl. See the docs for more information." to "... for example merging two bands according to color schemes from ColorSchemes.jl. Note that when the number of bands does not equal the default value, 3, colors must be inserted explicitly, e.g., composecolors([antred, antblue], ["red", "blue"]). See the docs for more information." What do you think? Feel free to improve or steal!

…

-- Denis

On Fri, Apr 4, 2025 at 1:27 AM Ian Weaver ***@***.***> wrote: Hi all, I've opened #63 <#63> to address the error messages. Ah, I see what you mean Denis. I see here <https://github.com/JuliaAstro/AstroImages.jl/blob/553232999c670dc03de0341f75abb26b2f6f4102/README.md#forming-color-composite-images> that we say it in words about the non-default case, but we don't show an explicit code example like we do in the docs <https://juliaastro.org/AstroImages.jl/stable/manual/converting-to-rgb/#Using-composecolors> : image.png (view on web) <https://github.com/user-attachments/assets/bde920ca-a890-4389-8fc7-fd8972ef885e> or in the Examples section of the docstring for composecolors <https://juliaastro.org/AstroImages.jl/stable/api/#AstroImages.composecolors> : image.png (view on web) <https://github.com/user-attachments/assets/10bd0c07-a9e5-4435-977c-66719c38aa82> I think that would be a great first PR to make to this project if you are keen! — Reply to this email directly, view it on GitHub <#53 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AE2LUY6RFOUP2ONPZ444VID2XYJ3XAVCNFSM6AAAAABWPYPZDKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDONZXGU4TCNJTGU> . You are receiving this because you modified the open/close state.Message ID: ***@***.***> [image: icweaver]*icweaver* left a comment (JuliaAstro/AstroImages.jl#53) <#53 (comment)> Hi all, I've opened #63 <#63> to address the error messages. Ah, I see what you mean Denis. I see here <https://github.com/JuliaAstro/AstroImages.jl/blob/553232999c670dc03de0341f75abb26b2f6f4102/README.md#forming-color-composite-images> that we say it in words about the non-default case, but we don't show an explicit code example like we do in the docs <https://juliaastro.org/AstroImages.jl/stable/manual/converting-to-rgb/#Using-composecolors> : image.png (view on web) <https://github.com/user-attachments/assets/bde920ca-a890-4389-8fc7-fd8972ef885e> or in the Examples section of the docstring for composecolors <https://juliaastro.org/AstroImages.jl/stable/api/#AstroImages.composecolors> : image.png (view on web) <https://github.com/user-attachments/assets/10bd0c07-a9e5-4435-977c-66719c38aa82> I think that would be a great first PR to make to this project if you are keen! — Reply to this email directly, view it on GitHub <#53 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AE2LUY6RFOUP2ONPZ444VID2XYJ3XAVCNFSM6AAAAABWPYPZDKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDONZXGU4TCNJTGU> . You are receiving this because you modified the open/close state.Message ID: ***@***.***>

[icweaver]

I think that sounds great! How about adding that code example you mentioned directly into the README?

AstroImages.jl/README.md

Lines 117 to 118 in 5532329

	`composecolors` also supports more complex mappings, for example merging two bands according to color schemes from
	ColorSchemes.jl.

By the way, this is an open invitation for you to make this addition yourself. I'd be happy to help review any PR and merge this into the repo. We need more folks like you in the JuliaAstro community =]