Zoomed in pdf is blurry

[maxdexh]

The rendering to fit-screen format causes the page to be blurry when zooming in past fill-screen, since we are zooming in on a rendered image, not the original.

This limitation also makes it so we cannot implement zooming out past fit-screen without falling victim to severe rounding errors.

See #121 (comment)

[maxdexh]

The way this is handled by firefox and zathura is that it zooms into the existing render, which is blurry, but asynchronously requests a new (local) higher-resolution render. The zoomed-in render is shown once it is done. Zooming in further or moving the view also initially zooms in on that, as long as the view stays in bounds of the local render. Leaving the local render, e.g. by zooming out, discards it, replacing it with the (potentially blurry) full render again.

[itsjunetime]

Yeah, I think this would be good to do - I think we probably want the re-renders to be somewhat granular, e.g. every 5 zoom 'levels' or smth requests a re-render (instead of anytime the document zoom level is increased at all).

Do we want to keep around the original fit-screen render for when they zoom back out, or just re-render once they zoom out? Could save a bit of memory to not hold onto it?

[maxdexh]

Do we want to keep around the original fit-screen render for when they zoom back out, or just re-render once they zoom out? Could save a bit of memory to not hold onto it?

Assuming we can get mupdf to render only a section of the page, I'd say definitely keep the original around so we can show something at all times. Showing a black screen for any amount of time is bad imo. Though then the question becomes what region should be rendered, since zooming in is almost always followed by panning.

If we're forced to render the whole page, then it doesn't really matter what we do with the original. Only concern then becomes that a 4K RGBA render at 10x zoom (max zoom on firefox) takes up ~3.3GB of space.

[maxdexh]

We can use PdfPage::set_crop_box to change the region of the pdf to be rendered (Looking at the PdfPage::try_from(Page) impl and the mupdf docs, the conversion should always succeed for tdf).

[itsjunetime]

hmmm, yeah we could start a debounced job to re-render whatever subsection of the page they're currently viewing, then if they happen to stay zoomed in to that subsection for a bit, they'll see the benefits of a clear picture and low memory usage. But if they move away from it, the debouncer does its job and we don't bother rendering for it. I'm gonna take a poke at this.