sdl2-compat SDL_RenderSetScale not compatible with SDL2

[LordOfTrident]

In my game, I draw a menu background, and then a menu overlay with a scale up/down animation using SDL_RenderSetScale:

	Rect viewport = Rect_new(CANVAS_X, CANVAS_Y, CANVAS_W, CANVAS_H);
	setViewport(viewport);

	SDL_SetRenderDrawColor(core.ren, 0, 0, 0, state.darken);
	SDL_RenderFillRect(core.ren, NULL);

	setViewport(Rect_lerpSize(viewport, state.scale));
	SDL_RenderSetScale(core.ren, state.scale, state.scale);
        ...

setViewport directly calls SDL_RenderSetViewport with the global renderer.

On original SDL2, this code worked fine. The menu background got rendered with regular scale and then the overlay with a scaling animation. With sdl2-compat, it seems to apply the SDL_RenderSetScale call to everything rendered in that frame, instead of only the subsequent render calls.

Expected outcome (with original SDL2):

demo.mp4

What I get with sdl2-compat:

2025-03-23.16-08-47.mp4

[icculus]

This is significantly better with the latest in revision control, but it's not perfect yet.

You can see that the menu looks like it's sliding in/out to the top/left. In SDL2, it scales from the center. Maybe the viewport is getting reset unexpectedly.

Screencast.from.2025-03-28.21-51-43.webm

Most of the fix was SDL_SetRenderTarget() not resetting viewport/scaling/etc, like SDL2 does. SDL3 lets each texture have its own state, and it isn't reset when setting a new render target.

I'll poke at the last problem some more.

[icculus]

Okay, in SDL2, SDL_RenderSetViewport() sets a viewport modulated by the current scale, and SDL_RenderSetScale() just sets a new scale for later drawing. In SDL3, SDL_SetRenderViewport() sets a new viewport for later drawing, and SDL_SetRenderScale adjusts the current viewport for the new scale.

Noike's intro_scene.c's render() does this:

	setViewport(Rect_lerpSize(viewport, state.scale));
	SDL_RenderSetScale(core.ren, state.scale, state.scale);

So in SDL2, this sets a new viewport times the current scale (whatever it might be), then changes the current scale. In sdl2-compat, this sets a new viewport, then changes the current viewport and scale.

I haven't done the math to see if this is actually where it's going wrong, but it's a likely culprit.

[icculus]

Kinda feeling like we're going to run into a million more of these little rendering quirks unless we copy the SDL2 scale/viewport/logicalpresent logic wholesale into sdl2-compat, and never set any of this stuff at the SDL3 level, but that might be a serious nuclear option.

[icculus]

copy the SDL2 scale/viewport/logicalpresent logic wholesale into sdl2-compat

I think I'm going to take a run at this, but let's ship the milestone before I do, since it's going to be a bigger change, and Noike is now playable with the latest in revision control.