Could someone help clarify this statement in the docs re `ce

Question

Could someone help clarify this statement in the docs re `centroid` <https developer android com reference kotlin androidx compose foundation gestures package summary androidx compose ui input pointer PointerInputScope detectTransformGestures kotlin Boolean kotlin Function4 |https developer android com reference kotlin androidx compose foundation gestures package summary androidx compose ui i olean kotlin Function4 > gt `onGesture` will also provide centroid of all the pointers that are down Are the centroid coordinates relative to the display The window Something else

Chris Fillmore · Answer

Thanks

Kirill Grouchnikov · Answer

There s a code example right there showing how `centroid` is used

Chris Fillmore · Answer

To be more specific I m trying to figure out how to incorporate `centroid` into `transformOrigin`

Chris Fillmore · Answer

which that example doesn t do

Chris Fillmore · Answer

And I m wondering how to interpret the `centroid` coordinates

Chris Fillmore · Answer

I have run that example code and observed the behaviour It s not quite what I m looking for Notably the `graphicsLayer` is placed after `pointerInput` meaning the entire screen accepts input not just the transformed area

Chris Fillmore · Answer

I ll admit I don t quite understand the example It uses `transformOrigin` at 0 0 but rotation occurs about the `centroid`

Chris Fillmore · Answer

Ok it took me a bit of thinking but I reworked the example to do what I want ```var offset by remember mutableStateOf Offset Zero var zoom by remember mutableStateOf 1f var angle by remember mutableStateOf 0f Box Modifier graphicsLayer val rotatedOffset = offset rotateBy angle translationX = rotatedOffset x translationY = rotatedOffset y scaleX = zoom scaleY = zoom rotationZ = angle transformOrigin = TransformOrigin 0f 0f pointerInput Unit detectTransformGestures onGesture = centroid pan gestureZoom gestureRotate gt val oldScale = zoom val newScale = zoom gestureZoom angle += gestureRotate zoom = newScale offset = offset centroid oldScale rotateBy gestureRotate + centroid newScale pan oldScale background Color Yellow fillMaxSize ```

Alex Vanyo · Answer

Nice glad you were able to figure it out Agreed that this type of thing is really hard to visualize with how the different coordinate spaces interact

Chris Fillmore · Answer

One thing that I observe although the behaviour is what I want when using `pointerInput` after `graphicsLayer` the `centroid` doesn t update its position remains fixed to the position it was in when touch slop first occurred Any idea if that is expected behaviour Or should I file a bug

Alex Vanyo · Answer

I m curious how you re using `exampleCentroid` I only see that set not read anywhere Is that related

Chris Fillmore · Answer

Oops sorry that is leftover from some testing it was used to show the centroid in UI

Chris Fillmore · Answer

Forgot to remove it before posting the code

Chris Fillmore · Answer

I ve removed it now

Kirill Grouchnikov · Answer

Chris Fillmore · Answer

But I added logging ```onGesture = centroid pan gestureZoom gestureRotate gt Log d onGesture centroid=$centroid ``` You can see the output ```onGesture D centroid=Offset 612 1 1684 4 onGesture D centroid=Offset 612 0 1684 4 onGesture D centroid=Offset 612 0 1684 4 onGesture D centroid=Offset 612 0 1684 4 onGesture D centroid=Offset 612 0 1684 4 goes on like this until pointers are released ```

Kirill Grouchnikov · Answer

Looks like centroid only looks at down events

Chris Fillmore · Answer

In the original example in the docs `centroid` is updating throughout gesturing

Chris Fillmore · Answer

I tested this

Alex Vanyo · Answer

So I think the tricky part might be describing what the `centroid` is fixed in relation to

Chris Fillmore · Answer

So I should correct what I said In fact I see `centroid` does update but only very tiny increments

Chris Fillmore · Answer

I didn t even notice that in the log I posted above

Chris Fillmore · Answer

It will update a tiny increment then remain the same for a time

Chris Fillmore · Answer

I have filed an issue in case this is in fact a bug <https issuetracker google com issues 233251825> Thanks for your time everyone slightly smiling face

Alex Vanyo · Answer

I think here s what s going on after trying out your version When you re applying the `graphicsLayer` transformation before consuming the the pointer events the pointer events are being transformed as well when you re consuming them in `detectTransformGestures` So in your case the centroid coordinates are in the yellow box s coordinates space as opposed to the screen wide coordinate space Now when you re rotating and zooming in on the yellow box the same part of the box remains underneath the centroid of the gesture which makes sense that makes it natural to interact with because you can rotate the box anchored around a given point So because the same part of the box is remaining underneath the centroid and the centroid is expressed in the box s coordinate space the centroid won t be moving relative to the box except for maybe some minor rounding errors

Chris Fillmore · Answer

Heh you know now that you explain it that seems so obvious

Chris Fillmore · Answer

Thanks Yeah as I play with it on the phone here now I see what you mean

Alex Vanyo · Answer

gt that seems so obvious I don t know about you but the interactions here still hurt my brain laughing

Chris Fillmore · Answer

Oh yeah it s been hurting my brain for a couple days now exploding head

Alex Vanyo · Answer

Well glad you were able to get the behavior you wanted