For what is worth, the blending with different premult is in: Preferences->Compositing->Blend Mode
Sadly, I am no longer updating or supporting mrViewer. There's now a successor called "mrv2" (OpenGL based) and "vmrv2" (Vulkan based), which has a whole set of new features and a new UI. You can find the main project web page at: https://mrv2.sourceforge.io
Added Fl_PDF_File_Surface and fixed Fl_Device.
Changed use of contrib/ directory for plug-in widgets, modified setup.py file and added documentation.
Added glob.glob("./contrib/*.cpp") to easily compile all contributed widgets and extend pyFLTK easily.
Added get/set methods for Fl:: static variables.
Made fl_measure use INOUT parameters for x and y sizes.
Added Fl_Image.data() to return a python binary object.