There is some complex behavior here to be careful of -- IIRC it has to do with a bitmap being in motion or not. In lower SWF version a one-frame movie would result in the smoothing behavior, while a multiple frame movie would be unsmoothed unless quality would be set to BEST.

I'll try to remember the exact behavior and get back to you!

Oh, this sounds similar to #5340!

I see this PR is only changing swf, and indeed those comments are incorrect! So this is good to add. We do need to make sure we emulate the above behavior at some point, though.

Actually thinking that this might be a little dangerous to merge as-is as this will incorrectly cause a lot of classic SWFs to be smoothed. We should either properly implement the _quality logic, or at least force-disable smoothing on the core side for SWF<v8 for now.

Okay, I wasn't sure of it either, without any corresponding changes in the renderer - hence my wish for relevant real-world content.
I meant this mostly to gather our collective thoughts about how to handle this.

No worries, I'll try to post some minimal example SWFs.

You can try Wasted Sky from the demo page -- renders as unsmoothed in FP but smoothed with this PR.

Here's some minimal examples:
bitmap-smooth-swf7.zip
The singleframe SWF renders smoothed with default _quality = "HIGH";, but the multiframe one doesn't. I think the criteria may be as simple as checking swf.num_frames > 1.

To implement, perhaps we add a RenderBackend::set_allow_bitmap_smoothing accessor that will be set appropriately by core::display_object::Stage, or add a smoothing parameter to RenderBackend::set_quality.

Some more examples:
bitmap-smooth-flag.zip

2 frame SWF with 2 bitmaps, one flagged smooth and one unsmoothed,

• bitmap-smooth-swf7-flag.swf: SWFv7, "HIGH" quality. Neither bitmap smoothed.
• bitmap-smooth-swf7-flag-best.swf: SWFv7, "BEST" quality. One bitmap smoothed.
• bitmap-smooth-swf8-flag.swf: SWFv8, "HIGH" quality. One bitmap smoothed.

The linked z0r SWF is 1 frame, so it honors the smoothing setting. The docs above indicate that the smoothing should take place "if the movie clip is static", which sounds like it should smooth if the timeline is stopped or something -- but so far my tests only shows it being affected by the # of frames on the timeline, regardless if it is stopped or not.

Another point of confusion is the Flash IDE will always export the bitmap smoothing flag when SWF version is set to 7 and below, regardless of what is set in the UI, so I had to manually edit these SWFs.