print composer pdf 10x filesize when opacity is less than 100

[Jeroen-GroeneBij]

What is the bug or the crash?

When exporting a map through the print composer - export as PDF the pdf file increases about 10x in filesize when the symbology opacity is set to anything less than 100.

Steps to reproduce the issue

on windows 10, with QGIS 3.36.1

take any vector polygon layer (I have not tested this with lines or points)
make sure the opacity of a simple fill is set to 100%
create a layout a printcomposer
Export as PDF (with default settings), note the file size

set the opacity of the simple fill to anything less than 100
create a layout a printcomposer
Export as PDF (with default settings), note the file size

Versions

<style type="text/css"> p, li { white-space: pre-wrap; } </style>

QGIS version	3.36.1-Maidenhead	QGIS code revision	3e58945
Qt version	5.15.3
Python version	3.9.18
GDAL/OGR version	3.8.4
PROJ version	9.3.1
EPSG Registry database version	v10.098 (2023-11-24)
GEOS version	3.12.1-CAPI-1.18.1
SQLite version	3.41.1
PDAL version	2.6.0
PostgreSQL client version	16.2
SpatiaLite version	5.1.0
QWT version	6.1.6
QScintilla2 version	2.13.4
OS version	Windows 10 Version 2009
Active Python plugins
db_manager	0.1.20
grassprovider	2.12.99
MetaSearch	0.3.6
processing	2.12.99

QGIS version 3.36.1-Maidenhead QGIS code revision [3e58945](https://github.com/qgis/QGIS/commit/3e589453) Qt version 5.15.3 Python version 3.9.18 GDAL/OGR version 3.8.4 PROJ version 9.3.1 EPSG Registry database version v10.098 (2023-11-24) GEOS version 3.12.1-CAPI-1.18.1 SQLite version 3.41.1 PDAL version 2.6.0 PostgreSQL client version 16.2 SpatiaLite version 5.1.0 QWT version 6.1.6 QScintilla2 version 2.13.4 OS version Windows 10 Version 2009

Active Python plugins
db_manager
0.1.20
grassprovider
2.12.99
MetaSearch
0.3.6
processing
2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

Maybe related to #50543

[Jeroen-GroeneBij]

I did test this now with 3.22.16 and the same issue is there as well

[nyalldawson]

This is expected. When you set a layer wide opacity (for a vector layer at least) it forces a rasterised rendering of the layer during pdf exports. Depending on the size of the layer, this may be a considerable increase in file size.