title page background image opacity not working

[espinielli]

Description

My understanding of the documentation for All possible titlepage-theme values is that you can set the background image opacity.
In the example below my settings for opacity are not enacted...I checked the code and it also seems to me that wallpaper.sty which is used for the background image, does not really (unfortunately) support that.

Here is the content of _quarto.yml:

project:
  type: book

book:
  title: A Sample Title
  author:
    - name: Jane Doe
  chapters:
    - index.qmd
  output-file: book

format: 
  titlepage-pdf:
    keep-tex: true
    coverpage: false
    documentclass: scrbook
    classoption: ["oneside", "open=any"]
    titlepage: "formal"
    titlepage-bg-image: "sceau-a-quadri.png"
    titlepage-theme:
      bg-image-size: "0.25\\paperwidth"
      bg-image-location: "Center"
      bg-image-fading: "top"
      bg-image-opacity: 0.1
      keep-tex: true

My final aim is about being able to positioning at discretion a background image with opacity in the title page.
The wallpaper.sty manual mentions the possibility of offestting from ULCorner/.../Center via \wpXoffset or \wpYoffset but i haven't been able to getting it done...

Steps to Reproduce

See repo, for reproducibility:
https://github.com/espinielli/bug_quarto-titlepages
Simply quarto render
I attached the rendered PDF book.pdf

Setup

• quarto --version: 1.7.30
• operating system (sw_ver command):

  ProductName: macOS
  ProductVersion: 15.4.1
  BuildVersion: 24E263

• the bug relate to PDF output
• I am rendering Quarto docs via RStudio
• TeX version (tex --version command output):

  TeX 3.141592653 (TeX Live 2025)
  kpathsea version 6.4.1
  Copyright 2025 D.E. Knuth.
  There is NO warranty. Redistribution of this software is
  covered by the terms of both the TeX copyright and
  the Lesser GNU General Public License.
  For more information about these matters, see the file
  named COPYING and the TeX source.
  Primary author of TeX: D.E. Knuth.