How can I use a LaTeX3 variable as a path in includegraphics (solving the expansion problem)?

Question

This problem is driving me nuts. I'd like to change the includegraphics path depending on some conditions. It seems simple, right?

I already read the answers to this question: Passing image path to \includegraphics using a macro, but I was unable to solve my issue. I learned that for \includegraphics the following must be true:

• The path must be expandable (requires \edef or cannot be protected therefore no xparse unless \DeclareExpandableDocumentCommand is relevant?) https://tex.stackexchange.com/a/120972/13552
• includegraphics only supports one level of expansion. https://tex.stackexchange.com/a/120975/13552

Thanks David Carlisle -> Incorrect Statement: From my understanding, my code only requires one level of expansion, but is still not working.

Is this a rare time when I need to use \DeclareExpandableDocumentCommand for \qikpicpath?

Code

A prerequisite is having a jpg file on directory above the tex file called test_pic.jpg. I commended out the problematic code.

\documentclass{article}
\usepackage{fontspec}
\newcommand{\qikrootdirectory}{../}
\usepackage{xparse}
\usepackage{xstring}
\usepackage{tikz}

\ExplSyntaxOn % https://tex.stackexchange.com/a/275552/13552
\tl_new:N \g_qik_pic_class_tl % allocate a (global) variable
\tl_gset:Nn \g_qik_class_tl { I } % initialize the value
\NewDocumentCommand{\qikpicclass}{} % define a user level command for printing the value
    {
    \tl_use:N \g_qik_class_tl
    }
\NewDocumentCommand{\qiksetpicclass}{m} % define a user level command for changing the value
    {
    \tl_gset:Nn \g_qik_class_tl { #1 }
    }
\NewDocumentCommand{\qikpicpath}{}
    {
    \tl_if_eq:VnTF \g_qik_class_tl { I }
        { \qikrootdirectory~test_pic.jpg } % true
        { \qikrootdirectory~test_pic.jpg } % false
    }
\cs_generate_variant:Nn \tl_if_eq:nnTF { V }
\ExplSyntaxOff

\begin{document}
\qikpicpath % Visually inspect expanded text.

\includegraphics[width=\textwidth]{\qikrootdirectory test_pic.jpg}% Works

%\includegraphics[width=\textwidth]{\qikpicpath}% Expansion Problem

\tikz \node (pic) {\includegraphics[width=\textwidth]{../test_pic.jpg}};% Works

%\tikz \node (pic) {\includegraphics[width=\textwidth]{\qikpicpath}};% Expansion Problem Related to Above

\end{document}

Answer 1

This could be a case where \DeclareExpandableDocumentCommand turns out to be useful, but it's simpler to use a different approach.

Let's say you have the same images in color and b/w versions, in two different folders, say ./colorimages and ./bwimages. Your purpose is to load pictures from one folder or another in case you compile your PDF for viewing or printing.

Let's also say that the images have consistent names like color-image1.jpg and bw-image1.jpg, so you want to do

\mmincludegraphics[width=.5\textwidth]{image1}

and this will load the right image, either color or b/w, just with one switch to be set in the preamble.

\ExplSyntaxOn

\NewDocumentCommand{\mmincludegraphics}{O{}m}
 {
  \includegraphics[#1]{
    \g_macmadness_folder_tl/ % the folder
    \g_macmadness_prefix_tl % the prefix
    #2
  }
 }

\NewDocumentCommand{\setimagefolder}{m}
 {
  \str_case:nn { #1 }
   {
    {color}
     {
      \tl_gset:Nn \g_macmadness_folder_tl { ./colorimages }
      \tl_gset:Nn \g_macmadness_prefix_tl { color- }
     }
    {bw}
     {
      \tl_gset:Nn \g_macmadness_folder_tl { ./bwimages }
      \tl_gset:Nn \g_macmadness_prefix_tl { bw- }
     }
   }
 }

\tl_new:N \g_macmadness_folder_tl
\tl_new:N \g_macmadness_prefix_tl

\ExplSyntaxOff

Now you can state in the preamble

\setimagefolder{color}

or

\setimagefolder{bw}

and call, in the document, \mmincludegraphics as stated before.

Answer 2

If you add

{\tracingmacros1
\setbox0\hbox{\qikpicpath}
}

Then TeX traces the expansion steps of this macro:

\qikpicpath ->\tl_if_eq:VnTF \g_qik_class_tl {I}{\qikrootdirectory test_pic.jpg
}{\qikrootdirectory test_pic.jpg}

\tl_if_eq:VnTF ->\exp_args:NV \tl_if_eq:nnTF 

\exp_args:NV #1#2->\exp_after:wN #1\exp_after:wN {\exp:w \__exp_eval_register:N
 #2}
#1<-\tl_if_eq:nnTF 
#2<-\g_qik_class_tl 

\__exp_eval_register:N #1->\exp_after:wN \if_meaning:w \exp_not:N #1#1\if_meani
ng:w \scan_stop: #1\__exp_eval_error_msg:w \fi: \else: \exp_after:wN \use_i_ii:
nnn \fi: \exp_after:wN \exp_end: \tex_the:D #1
#1<-\g_qik_class_tl 

\use_i_ii:nnn #1#2#3->#1#2
#1<-\exp_after:wN 
#2<-\exp_end: 
#3<-\tex_the:D 

\g_qik_class_tl ->I

\tl_if_eq:nnTF #1#2->\group_begin: \tl_set:Nn \l__tl_internal_a_tl {#1}\tl_set:
Nn \l__tl_internal_b_tl {#2}\if_meaning:w \l__tl_internal_a_tl \l__tl_internal_
b_tl \group_end: \prg_return_true: \else: \group_end: \prg_return_false: \fi: \
exp_end: 
#1<-I
#2<-I

\tl_set:Nn #1#2->\cs_set_nopar:Npx #1{\exp_not:n {#2}}
#1<-\l__tl_internal_a_tl 
#2<-I

\tl_set:Nn #1#2->\cs_set_nopar:Npx #1{\exp_not:n {#2}}
#1<-\l__tl_internal_b_tl 
#2<-I

\prg_return_true: ->\exp_after:wN \use_i:nn \exp:w 

\use_i:nn #1#2->#1
#1<-\qikrootdirectory test_pic.jpg
#2<-\qikrootdirectory test_pic.jpg

\qikrootdirectory ->../

Counting -> shows that it takes twelve expansions to get ../ in this case.

It would still work with includegraphics if you use the type and read keys to tell it that it is a jpg file, but usually this is the wrong approach.