`Fm::IconInfo`, scale factor and symbolic icons: not a bug but details

[tsujan]

Fm::IconInfo correctly scales icons with scale factors > 1; actually, it lets Qt do it. It's responsible for icons inside pcmanfm-qt's (as well as LXQt file dialog's) view and side-pane. Other icons are controlled by Qt, whether directly or through an icon engine (like that of LXQt).

But, for example, with a scale factor of 2, LXQt's icon engine will choose symbolic icons for the sizes 16×2, 22×2 and 24×2 if the icon theme supports them, while Fm::IconInfo shows ordinary icons. The end result is sharp icons in both cases but Fm::IconInfo doesn't use symbolic icons with a scale factors of 2.

You could check that by launching QT_SCALE_FACTOR=2 pcmanfm-qt (after stopping pcmanfm-qt from LXQt session settings). If your icon theme supports symbolic icons, the toolbar icons will be symbolic but the side-pane and view icons won't.

Since Fm::IconInfo takes the icon name from GIcon and uses QIconEngine to find the icon, there doesn't seem to be a place in it for dealing with scale factors directly. So, I'm not sure this could be changed in future. However, IMO, it's better to have an open issue for it.

EDIT: See https://github.com/lxqt/libfm-qt/issues/788#issuecomment-1057998958 → 4, for why you might get confused by the Breeze icon set when testing this.

[tsujan]

Also, see @palinek's comment on why we need an icon engine: https://github.com/lxqt/libfm-qt/pull/789#issuecomment-1056556932

[palinek]

Can you, please, exactly describe icon names, themes etc. for me to replicate it. Maybe we can find a solution/workaround.

[tsujan]

Any icon set with SVG symbolic icons, like Breeze.

[palinek]

What does "symbolic" mean here?

[tsujan]

Let me show you with an image:

The symbolic SVG icons change color, depending on their backgrounds (our icon engine started to support that years ago, with your and my codes).

[tsujan]

What does "symbolic" mean here?

They have simple forms and are usually monochrome. For coders, I should have said, "with stylesheets".

[tsujan]

Finally, and for the sake of completeness, this screenshot is taken with a scale factor of 2:

The toolbar icons are still symbolic, but the side-pane and view icons aren't because libfm-qt's icon engine interferes with that of LXQt.

[palinek]

but Fm::IconInfo doesn't use symbolic icons with a scale factors of 2.

It is using what is provided by the icon theme. E.g. the desktop folder -> icon user-desktop, this is what is provided by breeze:

$ find /usr/share/icons/breeze -name user-desktop\* -exec ls -l {} \; | sort -k8,8
-rw-r--r-- 1 root root 452 2022-01-01 13:10 /usr/share/icons/breeze/places/16/user-desktop.svg
-rw-r--r-- 1 root root 427 2022-01-01 13:10 /usr/share/icons/breeze/places/22/user-desktop.svg
-rw-r--r-- 1 root root 6069 2022-01-01 13:10 /usr/share/icons/breeze/places/32/user-desktop.svg
-rw-r--r-- 1 root root 3733 2022-01-01 13:10 /usr/share/icons/breeze/places/48/user-desktop.svg
-rw-r--r-- 1 root root 8415 2022-01-01 13:10 /usr/share/icons/breeze/places/64/user-desktop.svg
-rw-r--r-- 1 root root 7003 2022-01-01 13:10 /usr/share/icons/breeze/places/96/user-desktop.svg
lrwxrwxrwx 1 root root 26 2022-02-11 23:46 /usr/share/icons/breeze/places/symbolic/user-desktop-symbolic.svg -> ../16/folder-documents.svg

So the theme icon engine just follows the decision made by icon theme. (with the scale factor > 1 there is never requested icon smaller than 32)

[tsujan]

No! Breeze also has 16@2x, 22@2x and 24@2x folders. They are for being used with scaling. If you remove Fm::IconEngine (→ https://github.com/lxqt/libfm-qt/pull/789), LXQt icon engine will show them correctly.

[palinek]

Oh sorry... missed that. Then the cause is is in IconEngine::virtual_hook... I see it now.

I have something in my mind, need to test if it can solve this problem.

[palinek]

Please, what icon theme are you using ~and which showed icons are you considering to be shown wrongly~?

[tsujan]

Please, what icon theme are you using

It makes no difference; the "problem" appears with all icon themes that have symbolic icons. I use https://github.com/tsujan/Plataro but, IMO, Breeze is better for testing

This is a screenshot of LXQt file dialog with a scale factor of 2 and Breeze:

Like in my other screenshot, no view/side-pane icon is symbolic, but all would be symbolic if Fm::IconEngine was removed.

[palinek]

Yes, but it depends what sizes you configured

• size of small icons 22px
• size of small icons 28px

...and based on some tests, icons in left pane have different size for pcmanfm-qt & lxqt file dialog, I'll check this further.

[tsujan]

• Side-pane icons are 24-pix.
• Fusion isn't self-consistent with 22-pix (and the Breeze widget style seems to inherit that). Either use 24-pix view icons to see the problem in the view or forget about the view and focus on the side-pane. (Kvantum is self-consistent.)
• There's no 28-pix symbolic icon in Breeze (and in most icon sets).
• In a nutshell, where you see symbolic icons without a scale factor, you should also see symbolic icons with a scale factor of 2, provided that you use the Breeze set.

[palinek]

Ok... here is some debug output (with the lxqt/libqtxdg#275, but it probably doesn't make difference) how the icon is searched (user-desktop):

• 24px (scale=1) - best match (distance = (4 - 0) / 2 = 2) -> places/22/user-desktop.svg

entryForSize enter 0x7fc8e4005f80 "user-desktop" QSize(24, 24) 1
entryForSize 0x7fc8e4005f80 QSize(24, 24) 1 distance 144 "/usr/share/icons/breeze/places/96/user-desktop.svg"
entryForSize 0x7fc8e4005f80 better match
entryForSize 0x7fc8e4005f80 QSize(24, 24) 1 distance 80 "/usr/share/icons/breeze/places/64/user-desktop.svg"
entryForSize 0x7fc8e4005f80 better match
entryForSize 0x7fc8e4005f80 QSize(24, 24) 1 distance 48 "/usr/share/icons/breeze/places/48/user-desktop.svg"
entryForSize 0x7fc8e4005f80 better match
entryForSize 0x7fc8e4005f80 QSize(24, 24) 1 distance 16 "/usr/share/icons/breeze/places/32/user-desktop.svg"
entryForSize 0x7fc8e4005f80 better match
entryForSize 0x7fc8e4005f80 QSize(24, 24) 1 distance 85 "/usr/share/icons/breeze/places/22@3x/user-desktop.svg"
entryForSize 0x7fc8e4005f80 QSize(24, 24) 1 distance 41 "/usr/share/icons/breeze/places/22@2x/user-desktop.svg"
entryForSize 0x7fc8e4005f80 QSize(24, 24) 1 distance 4 "/usr/share/icons/breeze/places/22/user-desktop.svg"
entryForSize 0x7fc8e4005f80 better match
entryForSize 0x7fc8e4005f80 QSize(24, 24) 1 distance 49 "/usr/share/icons/breeze/places/16@3x/user-desktop.svg"
entryForSize 0x7fc8e4005f80 QSize(24, 24) 1 distance 17 "/usr/share/icons/breeze/places/16@2x/user-desktop.svg"
entryForSize 0x7fc8e4005f80 QSize(24, 24) 1 distance 16 "/usr/share/icons/breeze/places/16/user-desktop.svg"

• 48px (scale=2) - best match (distance = (1 - 1) / 2 = 0) -> places/48/user-desktop.svg, places/16@3x/user-desktop.svg

entryForSize enter 0x55c0b31419d0 "user-desktop" QSize(48, 48) 2
entryForSize 0x55c0b31419d0 QSize(48, 48) 2 distance 97 "/usr/share/icons/breeze/places/96/user-desktop.svg"
entryForSize 0x55c0b31419d0 better match
entryForSize 0x55c0b31419d0 QSize(48, 48) 2 distance 33 "/usr/share/icons/breeze/places/64/user-desktop.svg"
entryForSize 0x55c0b31419d0 better match
entryForSize 0x55c0b31419d0 QSize(48, 48) 2 distance 1 "/usr/share/icons/breeze/places/48/user-desktop.svg"
entryForSize 0x55c0b31419d0 better match
entryForSize 0x55c0b31419d0 QSize(48, 48) 2 distance 33 "/usr/share/icons/breeze/places/32/user-desktop.svg"
entryForSize 0x55c0b31419d0 QSize(48, 48) 2 distance 37 "/usr/share/icons/breeze/places/22@3x/user-desktop.svg"
entryForSize 0x55c0b31419d0 QSize(48, 48) 2 distance 8 "/usr/share/icons/breeze/places/22@2x/user-desktop.svg"
entryForSize 0x55c0b31419d0 QSize(48, 48) 2 distance 53 "/usr/share/icons/breeze/places/22/user-desktop.svg"
entryForSize 0x55c0b31419d0 QSize(48, 48) 2 distance 1 "/usr/share/icons/breeze/places/16@3x/user-desktop.svg"
entryForSize 0x55c0b31419d0 QSize(48, 48) 2 distance 32 "/usr/share/icons/breeze/places/16@2x/user-desktop.svg"
entryForSize 0x55c0b31419d0 QSize(48, 48) 2 distance 65 "/usr/share/icons/breeze/places/16/user-desktop.svg"

So for this example, how/why should we determine the 16@3x better match than the 48 ? (I don't know how the QSettings allKeys work, but I would determine the order of traversing the entries in theme rather unspecified)

[tsujan]

So for this example, how/why should we determine the 16@3x better match than the 48 ?

I feel we may be talking of different things, but 16@... icons should match only 16-pix icons, not 32/48-pix ones. With a scale factor of 2, the pixmap of a 16-pix icon has a device pixel ratio of 2.

[tsujan]

This screenshot is taken with the Breeze set and a scale factor of 3 and, again, shows that LXQt icon engine has no problem:

The toolbar icons are 16-pix (in Kvantum), their pixmaps have a device pixel ration of 3, and all of them are symbolic (except for the rightmost one, which doesn't have a symbolic icon at all).

[palinek]

Can you have a look in #792?

[tsujan]

Can you have a look in https://github.com/lxqt/libfm-qt/pull/792?

I did. Please also read my comment about the Breeze set there. Problem inside problem...

[tsujan]

Let's summarize it again, for the sake of removing confusions (if you don't agree with what follows, please tell me):

1. With the current internal icon engine of libfm-qt (namely, Fm::IconEngine), scaled symbolic icons aren't picked up by the side-pane or view.
2. If we remove Fm::IconEngine, scaled symbolic icons reappear (because LXQt's icon engine works fine).
3. With something like https://github.com/lxqt/libfm-qt/pull/792, scaled symbolic icons reappear too.
4. The Breeze icon set can be very confusing in this regard because it doesn't include places/24@2x in its index.theme. If you want to test what's explained in the first comment of this report, use an icon set that really has scaled, symbolic, places icons.