#225 Going through the slides the first time, the background flashes on every slide transition

[Sven Köhler]

I just tested Xournal on the computers in our lecture halls. We are using WACOM tablets. Using Xournal was a pleasure! It just works great! Unlike PDF XChange Viewer, the lines are not altered and handwritten text remains readable. I'm using Xournal to present slides made with Latex (so they are PDFs).

There is one big drawback however: when switch from slide to slide, the audience experiences a visible "flash" every time I move from slide to slide. This happens only when I visit a slide for the first time.

My best guess is the following: When I visit a slide for the first time, Xournl first renders a gray background image. Then, the slide is rendered and copied to the screen. So for a short moment, the audience can see the gray background. So in full screen mode, the whole screen goes from a largely white slide to the gray background and back to a mostly white slide again (hence the "flash"-effect). This is particularly annoying, if you have "animations" on your Latex slides. "Animation" here means, that subsequent slide are mostly identical beside some minor changes.

A workaround seems to be to go through the whole PDF file first. Then, Xournal seems to have a rendered image ready and the "fashing" is not present.

[Denis Auroux]

To avoid this issue, you should simply disable Options -> Progressive
Backgrounds (first you may need to uncheck the option to shorten menus
since this option is normally hidden),then save preferences -- or edit
.xournal/config to change the line "progressive_bg=true" to
"progressive_bg=false".

When this option is disabled, xournal will generate renderings of all
PDF pages immediately, not just when you first look at the page.
There is obviously a performance and memory consumption cost --
high-resolution renderings of long documents take a while and use a fair
bit of RAM (though you can start working while the remaining page
backgrounds are still processing).

Note that each time you zoom in or out, pages need to be re-generated at
the appropriate resolution for optimal display. (To save memory, we
don't save old copies of the rendered pages at previous zoom levels).
If this is too annoying during the presentation, you can always
re-enable "progressive backgrounds" once the document is loaded -- this
way, zoom-related refreshes will only occur on the visible pages. (A
symptom is then that when switching to a new page it may temporarily
appear blurry if the cached copy is at the wrong zoom level, but this is
less annoying than flashing). I imagine, though, that if your system is
reasonably powerful then you'll choose to just keep the option disabled.

Hope this helps.
Denis

[Sven Köhler]

Disabling progressive backgrounds is an excellent workaround. Do you plan on implementing a "proper fix"? When transitioning from slide X to Y, a way to avoid the flashing is to continue to display slide X until rendering slide Y has finished.