Ebiten: Add a mode to skip clearing the screen at the beginning of a frame

[hajimehoshi]

EDIT: The title was: skip sending vertices when they are out of the region

Inspired by https://github.com/hajimehoshi/ebiten/issues/1130. In theory, we can skip sending the vertices to GPU if they are out of the region. I'm not sure how efficiently we can do it.

[hajimehoshi]

Idea: Calculate a circumscribed circle of the region rectangle and a circumscribed circle of a triangle, and see whether those circles are excluded or not.

[hajimehoshi]

Before optimization, let's check why pixel can work efficiently first:

Pixel: https://github.com/jrcichra/gollercoaster/blob/62570d8957c033fe121c111bc883e7df919eb24a/game/game.go#L162-L176

Ebiten: https://github.com/jrcichra/gollercoaster/blob/4766cc2cb7e2e36915f656285263117f6341a008/game/game.go#L129-L151

CC @jrcichra

[hajimehoshi]

Ah, in the Pixel version, rendering onto the screen happens only when necessary, and that's why.

https://github.com/jrcichra/gollercoaster/blob/62570d8957c033fe121c111bc883e7df919eb24a/game/game.go#L111-L127

[jrcichra]

Yeah and camera movement in pixel, it was easier for me to move around without redrawing.

[jrcichra]

Because of this line not taking any time: https://github.com/jrcichra/gollercoaster/blob/62570d8957c033fe121c111bc883e7df919eb24a/game/game.go#L129

[hajimehoshi]

Thanks!

Yeah and camera movement in pixel, it was easier for me to move around without redrawing.

Ebiten doesn't have a mode not to clear the screen at the beginning of a frame. I'll consider this.

For the camera movement, Ebiten can do a similar thing by (*GeoM).Concat. I think this is a more suitable solution for Ebiten than introducing a global matrix.

[beoran]

Actually I didn't realize Ebiten does this, but this is normally not needed. If you use a tile map, then this will cover the background completely. Clearing the screen at the beginning of the frame should only happen if the back ground is set to a solid color or similar.

[tomlister]

@hajimehoshi
I have a commit that adds isClearingSkipped and SetClearingSkipped. Would you like me to PR?

[hajimehoshi]

Yes, I appreciate it!

I prefer the name IsClearingScreenSkipped and SetClearingScreenSkipped.

[tomlister]

At the moment this is how it's implemented.

if game, ok := c.game.(interface{ Draw(*Image) }); ok {
    if !IsClearingSkipped() {
        c.offscreen.Clear()
    }
    game.Draw(c.offscreen)
}

Are there any other clear instructions that should be skipped? I noticed that there were a couple more in uicontext.

[hajimehoshi]

There is one more c.offscreen.Clear() call for Update.