title: Cannot use include_graphics to insert png in Rmarkdown (error: file is not in PNG format)
body: <p>This error is very strange. When I knitr the doc, I got the following error:</p>
  
  <pre class="lang-r prettyprint-override"><code>Quitting from lines 42-43 (sigminer-doc.Rmd) 
  Error in png::readPNG(path, native = TRUE, info = TRUE) : 
    file is not in PNG format
  </code></pre>
  
  <p>However, I can read the png by <code>png::readPNG</code> and it also works when I run the chunk in RStudio.</p>
  
  <p><a href="https://i.stack.imgur.com/tyxCM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/tyxCM.png" alt="enter image description here"></a></p>
  
  <p>Use the knitr button in RStudio for <code>index.Rmd</code> cannot work.</p>
  
  <p><a href="https://i.stack.imgur.com/5EuVa.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5EuVa.png" alt="enter image description here"></a></p>
  
  <p>I also reinstall the <code>png</code>, <code>knitr</code> package, the idea cannot work.</p>
  
  <hr>
  
  <p><strong>UPDATE</strong>:</p>
  
  <p>I tried to debug the working directory and add the following code to the header.</p>
  
  <pre class="lang-r prettyprint-override"><code>knitr::opts_knit$set(root.dir = rprojroot::find_rstudio_root_file())
  knitr::knit_hooks$set(debug = function(before, options, envir) {
    if (!before) {
      envir = as.list(envir)
      message("Objects: ", paste(names(envir), collapse = " "))
      for (i in names(envir)) {
        if (!startsWith(i, ".")) {
          message(
            i, " = ", envir[[i]]
          )
        }
      }
    }
  })
  </code></pre>
  
  <p>The working directory is as expected.</p>
  
  <pre class="lang-r prettyprint-override"><code>Objects: workdir root.dir
  workdir = /Users/wsx/Documents/GitHub/sigminer-doc
  root.dir = /Users/wsx/Documents/GitHub/sigminer-doc
    |.....                                                                                                                                 |   4%
    ordinary text without R code
  
    |......                                                                                                                                |   5%
  label: unnamed-chunk-3 (with options) 
  List of 2
   $ echo   : logi FALSE
   $ fig.cap: chr "The illustration of SBS signature, fig source: https://www.nature.com/articles/nrg3729"
  
  Quitting from lines 64-65 (sigminer-doc.Rmd) 
  Error in png::readPNG(path, native = TRUE, info = TRUE) : 
    file is not in PNG format
  </code></pre>
  
  <p>The knitr also did not stop in code chunk</p>
  
  <pre class="lang-r prettyprint-override"><code>stopifnot(file.exists("fig/sbs_signature_overview_nat_review.png"))
  </code></pre>
  
  <p>I found that this issue comes from the figures.</p>
  
  <pre class="lang-r prettyprint-override"><code>&gt; png::readPNG("fig/sbs_signature_overview.png")
   [ reached getOption("max.print") -- omitted 3 matrix slice(s) ]
  Warning message:
  In png::readPNG("fig/sbs_signature_overview.png") :
    libpng warning: iCCP: known incorrect sRGB profile
  &gt; png::readPNG("fig/sbs_signature_overview_nat_review.png")
  Error in png::readPNG("fig/sbs_signature_overview_nat_review.png") : 
    file is not in PNG format
  </code></pre>
  
  <p>However, I can correctly knit this document before...</p>
  
  <p>My session:</p>
  
  <pre class="lang-r prettyprint-override"><code>&gt; devtools::session_info()
  ─ Session info ───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
   setting  value                       
   version  R version 4.0.0 (2020-04-24)
   os       macOS High Sierra 10.13.6   
   system   x86_64, darwin17.0          
   ui       RStudio                     
   language (EN)                        
   collate  zh_CN.UTF-8                 
   ctype    zh_CN.UTF-8                 
   tz       Asia/Shanghai               
   date     2020-05-11                  
  
  ─ Packages ───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
   package      * version date       lib source        
   assertthat     0.2.1   2019-03-21 [1] CRAN (R 4.0.0)
   backports      1.1.6   2020-04-05 [1] CRAN (R 4.0.0)
   bibtex         0.4.2.2 2020-01-02 [1] CRAN (R 4.0.0)
   Biobase      * 2.48.0  2020-04-27 [1] Bioconductor  
   BiocGenerics * 0.34.0  2020-04-27 [1] Bioconductor  
   bookdown       0.18    2020-03-05 [1] CRAN (R 4.0.0)
   callr          3.4.3   2020-03-28 [1] CRAN (R 4.0.0)
   cli            2.0.2   2020-02-28 [1] CRAN (R 4.0.0)
   cluster      * 2.1.0   2019-06-19 [2] CRAN (R 4.0.0)
   codetools      0.2-16  2018-12-24 [2] CRAN (R 4.0.0)
   colorspace     1.4-1   2019-03-18 [1] CRAN (R 4.0.0)
   crayon         1.3.4   2017-09-16 [1] CRAN (R 4.0.0)
   data.table     1.12.8  2019-12-09 [1] CRAN (R 4.0.0)
   desc           1.2.0   2018-05-01 [1] CRAN (R 4.0.0)
   devtools       2.3.0   2020-04-10 [1] CRAN (R 4.0.0)
   digest         0.6.25  2020-02-23 [1] CRAN (R 4.0.0)
   doParallel     1.0.15  2019-08-02 [1] CRAN (R 4.0.0)
   dplyr          0.8.5   2020-03-07 [1] CRAN (R 4.0.0)
   ellipsis       0.3.0   2019-09-20 [1] CRAN (R 4.0.0)
   evaluate       0.14    2019-05-28 [1] CRAN (R 4.0.0)
   fansi          0.4.1   2020-01-08 [1] CRAN (R 4.0.0)
   foreach        1.5.0   2020-03-30 [1] CRAN (R 4.0.0)
   fs             1.4.1   2020-04-04 [1] CRAN (R 4.0.0)
   furrr          0.1.0   2018-05-16 [1] CRAN (R 4.0.0)
   future         1.17.0  2020-04-18 [1] CRAN (R 4.0.0)
   ggplot2        3.3.0   2020-03-05 [1] CRAN (R 4.0.0)
   globals        0.12.5  2019-12-07 [1] CRAN (R 4.0.0)
   glue           1.4.0   2020-04-03 [1] CRAN (R 4.0.0)
   gridBase       0.4-7   2014-02-24 [1] CRAN (R 4.0.0)
   gtable         0.3.0   2019-03-25 [1] CRAN (R 4.0.0)
   htmltools      0.4.0   2019-10-04 [1] CRAN (R 4.0.0)
   iterators      1.0.12  2019-07-26 [1] CRAN (R 4.0.0)
   knitr          1.28    2020-02-06 [1] CRAN (R 4.0.0)
   lifecycle      0.2.0   2020-03-06 [1] CRAN (R 4.0.0)
   listenv        0.8.0   2019-12-05 [1] CRAN (R 4.0.0)
   magrittr       1.5     2014-11-22 [1] CRAN (R 4.0.0)
   memoise        1.1.0   2017-04-21 [1] CRAN (R 4.0.0)
   munsell        0.5.0   2018-06-12 [1] CRAN (R 4.0.0)
   NMF          * 0.22.0  2020-02-12 [1] CRAN (R 4.0.0)
   pacman       * 0.5.1   2019-03-11 [1] CRAN (R 4.0.0)
   pheatmap       1.0.12  2019-01-04 [1] CRAN (R 4.0.0)
   pillar         1.4.4   2020-05-05 [1] CRAN (R 4.0.0)
   pkgbuild       1.0.8   2020-05-07 [1] CRAN (R 4.0.0)
   pkgconfig      2.0.3   2019-09-22 [1] CRAN (R 4.0.0)
   pkgload        1.0.2   2018-10-29 [1] CRAN (R 4.0.0)
   pkgmaker     * 0.31.1  2020-03-19 [1] CRAN (R 4.0.0)
   plyr           1.8.6   2020-03-03 [1] CRAN (R 4.0.0)
   png            0.1-7   2013-12-03 [1] CRAN (R 4.0.0)
   prettyunits    1.1.1   2020-01-24 [1] CRAN (R 4.0.0)
   processx       3.4.2   2020-02-09 [1] CRAN (R 4.0.0)
   ps             1.3.2   2020-02-13 [1] CRAN (R 4.0.0)
   purrr          0.3.4   2020-04-17 [1] CRAN (R 4.0.0)
   R.cache        0.14.0  2019-12-06 [1] CRAN (R 4.0.0)
   R.methodsS3    1.8.0   2020-02-14 [1] CRAN (R 4.0.0)
   R.oo           1.23.0  2019-11-03 [1] CRAN (R 4.0.0)
   R.utils        2.9.2   2019-12-08 [1] CRAN (R 4.0.0)
   R6             2.4.1   2019-11-12 [1] CRAN (R 4.0.0)
   RColorBrewer   1.1-2   2014-12-07 [1] CRAN (R 4.0.0)
   Rcpp           1.0.4.6 2020-04-09 [1] CRAN (R 4.0.0)
   registry     * 0.5-1   2019-03-05 [1] CRAN (R 4.0.0)
   remotes        2.1.1   2020-02-15 [1] CRAN (R 4.0.0)
   reshape2       1.4.4   2020-04-09 [1] CRAN (R 4.0.0)
   rlang          0.4.6   2020-05-02 [1] CRAN (R 4.0.0)
   rmarkdown      2.1     2020-01-20 [1] CRAN (R 4.0.0)
   rngtools     * 1.5     2020-01-23 [1] CRAN (R 4.0.0)
   rprojroot      1.3-2   2018-01-03 [1] CRAN (R 4.0.0)
   rstudioapi     0.11    2020-02-07 [1] CRAN (R 4.0.0)
   scales         1.1.0   2019-11-18 [1] CRAN (R 4.0.0)
   sessioninfo    1.1.1   2018-11-05 [1] CRAN (R 4.0.0)
   sigminer     * 1.0.5   2020-05-09 [1] local         
   stringi        1.4.6   2020-02-17 [1] CRAN (R 4.0.0)
   stringr        1.4.0   2019-02-10 [1] CRAN (R 4.0.0)
   styler         1.3.2   2020-02-23 [1] CRAN (R 4.0.0)
   testthat       2.3.2   2020-03-02 [1] CRAN (R 4.0.0)
   tibble         3.0.1   2020-04-20 [1] CRAN (R 4.0.0)
   tidyselect     1.0.0   2020-01-27 [1] CRAN (R 4.0.0)
   usethis        1.6.1   2020-04-29 [1] CRAN (R 4.0.0)
   vctrs          0.2.4   2020-03-10 [1] CRAN (R 4.0.0)
   withr          2.2.0   2020-04-20 [1] CRAN (R 4.0.0)
   xfun           0.13    2020-04-13 [1] CRAN (R 4.0.0)
   xtable         1.8-4   2019-04-21 [1] CRAN (R 4.0.0)
   yaml           2.2.1   2020-02-01 [1] CRAN (R 4.0.0)
  
  [1] /Users/wsx/R_library
  [2] /Library/Frameworks/R.framework/Versions/4.0/Resources/library
  </code></pre>