image_composite "reflects" image position when offset moves composite_image out of area of image

[chroetz]

library(magick)
## Linking to ImageMagick 6.9.12.93
## Enabled features: cairo, freetype, fftw, ghostscript, heic, lcms, pango, raw, rsvg, webp
## Disabled features: fontconfig, x11
image_blank(100, 100, color="pink") |> 
  image_composite(image_blank(20, 50, "yellow"), gravity="center") |> 
  image_composite(image_blank(20, 40, "green"), gravity="center", offset = "+52+0") |> 
  image_composite(image_blank(20, 30, "red"), gravity="center", offset = "-35+0") |> 
  image_composite(image_blank(20, 20, "orange"), gravity="center", offset = "-40+0") |> 
  image_composite(image_blank(20, 10, "blue"), gravity="center", offset = "-50+0")

The blue rectangle should be moved further to the left so that part of it is outside the background image. Instead, the "negative x position" is turned into a positive one. This does not happen on the right border (green rectangle).

Maybe this is connected to #274 ?

sessioninfo::session_info() |>  
    details::details(summary = 'current session info')

current session info

─ Session info ─────────────────────────────────────────────────────────────────────────────────
 setting  value
 version  R version 4.2.3 (2023-03-15 ucrt)
 os       Windows 10 x64 (build 19044)
 system   x86_64, mingw32
 ui       RStudio
 language (EN)
 collate  English_Germany.utf8
 ctype    English_Germany.utf8
 tz       Europe/Berlin
 date     2023-08-27
 rstudio  2023.06.1+524 Mountain Hydrangea (desktop)
 pandoc   NA

─ Packages ─────────────────────────────────────────────────────────────────────────────────────
 package     * version date (UTC) lib source
 cli           3.6.0   2023-01-09 [1] CRAN (R 4.2.2)
 clipr         0.8.0   2022-02-22 [1] CRAN (R 4.2.0)
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 Rcpp          1.0.10  2023-01-22 [1] CRAN (R 4.2.2)
 rlang         1.1.0   2023-03-14 [1] CRAN (R 4.2.2)
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 [1] C:/Users/cschoetz/AppData/Local/R/win-library/4.2
 [2] C:/Program Files/R/R-4.2.3/library

────────────────────────────────────────────────────────────────────────────────────────────────

[jeroen]

I see the same on MacOS. It looks not like the negative x is turned into a positive one, but the value wraps around when outside the image?

If I do image_composite(image_blank(20, 10, "blue"), gravity="center", offset = "-40+0") then the blue square is all the way at the left, but further increasing the negative offset makes it bounce back again...

[chroetz]

Right! I think we mean the same thing. Assume the left of the background image is $x_{\mathsf{background}}=0$ and the left of the blue rectangle at its intended location is $x_{\mathsf{blue,intent}} = x_{\mathsf{blue,start}} + x_{\mathsf{blue,offset}}$, where $x_{\mathsf{blue,start}}$ is essentially determined by gravity and $x_{\mathsf{blue,offset}}$ is the offset given in the offset parameter. But it seems to be placed at $x_{\mathsf{blue,is}} = |x_{\mathsf{blue,start}} + x_{\mathsf{blue,offset}}|$.