Allow smooth scaling for 100% monitors too

[fedejeanne]

Smooth scaling is used if:

• (in Windows) Monitor-specific scaling is active
• (in Linux) if the zoom level of the monitor is not divisible by 100
• (in Mac) never

In Linux, once the mode changes to smooth, it stays in smooth.

How to test

• One 100% monitor and another in a "fractional" zoom e.g. 150%
• Start the ControlExample with the following VM parameters:

-Dswt.autoScale.updateOnRuntime=true

• (Linux) Drag the application back and forth between monitors

Expected result

In Windows

The autoScaleMethod should always be set to SMOOTH (in org.eclipse.swt.internal.DPIUtil.setDeviceZoom(int))

In Linux

• If the application starts in the monitor with the "non fractional" zoom level (e.g. 100% or 200%) then the mode should be NEAREST
• The autoScaleMethod should be set to SMOOTH when starting at/switching to a monitor with a "fractional" zoom level (e.g. 150%).
• Once in SMOOTH mode, it should never switch back to NEAREST

In Mac

No changes: never SMOOTH.

[github-actions]

Test Results

   502 files  ±0     502 suites  ±0   11m 49s ⏱️ + 3m 54s
 4 334 tests ±0   4 320 ✅ ±0   14 💤 ±0  0 ❌ ±0 
16 575 runs  ±0  16 466 ✅ ±0  109 💤 ±0  0 ❌ ±0 

Results for commit 8dace1d. ± Comparison against base commit df1c412.

♻️ This comment has been updated with latest results.

[HeikoKlare]

The change looks fine to me. If I am not mistaken, it only introduced one change in behavior, which might be considered. It is the following scenario:

• Running on GTK
• Starting with or changing to a deviceZoom that is not a multiple of 100 (like 150), which then enables "smooth" scaling
• Changing to a deviceZoom of 100 (which previously kept "smooth" scaling and now switches back to "nearest" scaling)

On GTK, this is triggered by Display#dpiChanged(). What might be the result? Images are initially created for 150% scale and when then scaled down to 100% after changing deviceZoom to 100, this rescaling happens with "nearest" instead of "smooth", leading to bad results.

Maybe it makes more sense if we say that when auto-scaling was set to "smooth" once, it will stay at "smooth", no matter to what deviceZoom the value is changed?

[fedejeanne]

@HeikoKlare so all in all you propose:

• If it starts in a "non-fractional" monitor (100%, 200%, 400% etc) then use NEAREST
• When switching to (or starting at) a "fractional" monitor, use SMOOTH
• Once in SMOOTH, never go back to NEAREST

All of this shall be done for Windows and Linux (never for Mac) and only when using the "auto" scale mode.

Is that correct?

[fedejeanne]

And of course, in Windows this only applies when activating the monitor-specific scaling

[HeikoKlare]

Exactly 👍 Just that I would apply the logic to Linux only. On Windows I would use "nearest" unless monitor-specific scaling is enabled (then use "smooth" but also when deviceZoom is 100).

[fedejeanne]

Done @HeikoKlare

[fedejeanne]

In a0aebf9 I made sure that the corner case of coming back to a 100% monitor in Linux remains unchanged by this PR i.e.

• Starting with a 100% monitor: NEAREST
• 100% --> 125% --> 100%: SMOOTH (corner case)
• 100% --> 125% --> 200%: NEAREST