Server: fix label opacity when OPACITIES is set

[elpaso]

Fix #48020

Implement multiplyOpacity() for QgsTextFormat and use it to alter labeling opacity during server rendering.

[nyalldawson]

could we use the standard render checker approach here? I'm concerned that these tests will be tricky to debug if they start failing in future (eg due to qt rendering changes)

[elpaso]

I was hoping to skip the mask dance and spare a few hours. This kind of test is much more robust and less prone to false negatives because by checking a pixel in the middle of the rendered object there is no way that aliasing and other small rendering differences will step in the way.

I agree that if that fails it could be harder to visually inspect the differences.

[nyalldawson]

there is no way that aliasing and other small rendering differences will step in the way.

That sounds reasonable in theory, but I've seen numerous times were a Qt update has changed something in how qt handles alpha composition resulting in different overlaid color results 🙃

[nyalldawson]

unique ptr?

[elpaso]

This is tricky, because the settings struct is copied.

Technically the settings never owns the resource, it just keeps it for the duration of its lifetime and transfers it back to the layer when the destructor is called.

Also , the struct is private and it's not exposed to API, given the (non-copyable) complications of turning this into a unique_ptr I'd rather not follow your advice.

[elpaso]

BTW here is an attempt: ae23847

[elpaso]

ping @rldhont @pblottiere can you please review so that I can merge in time for the next release ?

[qgis-bot]

The backport to queued_ltr_backports failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply fb427d17df... Server: fix label opacity when OPACITIES is set
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".

stdout

Auto-merging src/server/services/wms/qgswmsrenderer.cpp
Auto-merging src/server/services/wms/qgswmsrestorer.cpp
Auto-merging tests/src/python/test_qgsserver_wms_getmap.py
CONFLICT (content): Merge conflict in tests/src/python/test_qgsserver_wms_getmap.py

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-53438-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick fb427d17dff9f88e03eda60ce4eaf8a27f7c97ed,a7c803e6c0b8fe30e8fa84917168cbdd532e236a,b9ce813edd027cae7da3d5d7d2a8f19683a6e44d,b1b8fb8e159775c378ce94ab7099a24573cfd3ed,9a81957d3927b1ce899582e22c3afec583f9597c,ae23847c5998208d22343ef89bb0b8dbbaa8fe14,7b89a398f73375cc948962bb34440743814b06b9
# Push it to GitHub
git push --set-upstream origin backport-53438-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-53438-to-queued_ltr_backports.