Do Buttons dirty the canvas?

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Original Question: I'd like to know if a button that uses Sprite Swap, will mark the canvas as dirty when the user hovers the mouse over the button?

*Edit for clarity: When certain parameters change in a canvas, it causes Unity to "rebuild" the mesh via Canvas.buildBatch. This is cpu intensive and can cause spikes/hiccups.

This article mentions only Transform changes as a cause of a rebuild: https://support.unity3d.com/hc/en-us/articles/115000355466-Split-canvas-for-dynamic-objects

The article goes so far as saying that changing text does NOT involve a batch build. My question seeks certainty regarding other types of changes such as a button sprite-swap on hover or the tint effect on hover. (Assume there are NO animations)

Answer 1

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Answer by LilGames · Mar 08 at 06:06 PM

I keep asking myself the same initial question and when I Google, I find my own question as a top result, so here's what resources I've found to help me through the whole Static vs Dynamic Canvas decisions:

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A simple tutorial about correctly setting up the UI hierarchy of mixed Static and Dynamic parts of your UI: https://learn.unity.com/tutorial/nested-canvas-optimization-2019-3#

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A deeper tutorial explaining a 2-canvas setup and using the profiler to see the impact: https://learn.unity.com/tutorial/working-with-static-and-dynamic-canvases#5ffbd711edbc2a002099bd00

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Answer 2

Answer by xxmariofer · Apr 22, 2019 at 11:15 AM

anything that changes the canvas will force a canvas repaint, not only images, scroll bars buttons animations, anything. thats why you should separate canvas depending on when their objects get dirty, if you have a button that is animated, it should go in a separate canvas from the static buttons.

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LilGames · Apr 22, 2019 at 08:50 PM 0

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I guess this is where I am looking for more specifics. I assume that animation, scaling, rotation cause a canvas rebuild, but would a simple alpha or tint change? What about sprite-swap method if the sprites are all from the same atlas?

Thank you for your time.

xxmariofer LilGames · Apr 23, 2019 at 07:53 AM 0

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it changes always, even if is such a tiny thing as a tint change, unity doesnt "check" what has changed in the canvas, just as an example, if you have an animation of a button that doesnt do anything the first 5 seconds and the last 5 seconds it applies a tint chaange, the canvas will be redrawn all 10 seconds (all the time, since it doesnt know if something has changed so you would repaint the exact same canvas during 5 seconds) here are some performance tips for canvas uses Link

LilGames xxmariofer · Apr 23, 2019 at 02:06 PM 0

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Redraw/repaint is fast compared to rebuilding the mesh: https://support.unity3d.com/hc/en-us/articles/115000355466-Split-canvas-for-dynamic-objects

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Answer 3

Answer by Bip901 · Mar 28, 2020 at 07:57 PM

According to Unity 2017 Game Optimization by Chris Dickinson, the only action which doesn't dirty the canvas is changing the Color property of a UI element.

Hence I assume a color tint transition won't rebuild the canvas, but a sprite swap will.

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Do Buttons dirty the canvas?

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