Glance & transparent background quirk

In my glance view, I typically use the following pattern in onUpdate:

onUpdate( dc as Dc ) as Void {
    dc.setColor( Graphics.COLOR_WHITE, Graphics.COLOR_TRANSPARENT );
    dc.clear();
   // draw some stuff
}

This clears the previous frame using a transparent background and redraws the content. Using a transparent background allows the underlying gradients or graphics that Garmin applies to glance backgrounds on some devices to remain visible.

However, I've run into an issue when handling drawing errors.

If an error occurs during rendering, I’d like to clear the canvas again and display a warning instead. The logic looks like this:

onUpdate( dc as Dc ) as Void {
    dc.setColor( Graphics.COLOR_WHITE, Graphics.COLOR_TRANSPARENT );
    dc.clear();
    // draw some stuff
    dc.setColor( Graphics.COLOR_RED, Graphics.COLOR_TRANSPARENT );
    dc.clear();
    // draw some other stuff
}

Here’s the problem: while the first dc.clear() works as expected and clears the previous frame, the second dc.clear()—which is meant to clear the failed rendering—doesn’t fully clear the display. It seems that using Graphics.COLOR_TRANSPARENT a second time causes the canvas to behave as if it’s layering transparency, letting prior content “bleed through.”. Or, bluntly put, the second dc.clear() does nothing at all.

The only workaround I’ve found is to use Graphics.COLOR_BLACK instead of transparent for the second clear. Unfortunately, this removes any background gradients or images, which breaks the intended visual design.

What do you guys think, bug or feature?

0 _psx_ 8 days ago

Which colour do you want use to paint?

According to doc you use transparent-

clear() as Void

Erase the screen using the background color.

Note:

Starting form version 3.1.0, COLOR_TRANSPARENT will also be honored as background color, which will cause the value of pixels in the clip region to be replaced by COLOR_TRANSPARENT. For example, this can be used to clear the transparent overlay layer so animation background is visible.

0 flocsy🤠 7 days ago

to me it sounds like "layering" is not the problem but the solution. Instead of drawing to one canvas, draw each part to a different layer. The layer still can have transparent background, and when you detect the problem, then instead of dc.clear(), you can remove the "bad" layer.

0 the-ninth 7 days ago in reply to _psx_

_psx_ said:
Starting form version 3.1.0, COLOR_TRANSPARENT will also be honored as background color, which will cause the value of pixels in the clip region to be replaced by COLOR_TRANSPARENT. For example, this can be used to clear the transparent overlay layer so animation background is visible.

I think that part is not working in my example. The value of pixels is NOT replaced in my second call, instead the pixels are left as they are.

0 the-ninth 7 days ago in reply to flocsy🤠

flocsy🤠 said:
to me it sounds like "layering" is not the problem but the solution. Instead of drawing to one canvas, draw each part to a different layer. The layer still can have transparent background, and when you detect the problem, then instead of dc.clear(), you can remove the "bad" layer.

Yes, that would probably work, as would drawing the content on a BufferedBitmap. But that are relatively memory-intense solutions to a rather simple problem.

0 the-ninth 7 days ago in reply to the-ninth

I created a bug report for this:

forums.garmin.com/.../dc-clear-with-color_transparent-does-not-work-as-documented

0 pstoppani 6 days ago

As side note, glances should not be cleared. On some watches it looks bad because Garmin uses non-solid backgrounds for the Glances list.

0 the-ninth 6 days ago in reply to pstoppani

Yes, that is the whole point of clearing it with transparent color as background, which would let that non-solid background show through.

0 flocsy🤠 6 days ago in reply to the-ninth

But if I understand correctly from this thread then clearing with transparent doesn't clear garbage. Not sure if that's intended (probably not) but it means it makes no sense clearing before you draw anything (won't clear previous garbage) or after you draw anything....

0 the-ninth 6 days ago in reply to flocsy🤠

Yes, or in other words, in the context of the Dc passed into GlanceView.onUpdate, Dc.clear with transparent background has no point at all. I did not test it in other contexts, like with Dc of other types of views, layers or buffered bitmaps.

0 pstoppani 6 days ago in reply to the-ninth

Sorry, I skimmed too lightly :)