# Schizophrenic renderer when in fullscreen mode?

**URL:** <https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590>\
**Category:** SDL Development\
**Created:** [January 10, 2019, 11:33pm UTC](https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590 "2019-01-10T23:33:29Z")\
**Posts on this page:** 10\
**Page:** 1

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**Author:** ![Feldheld](https://discourse.libsdl.org/letter_avatar_proxy/v4/letter/f/4bbf92/32.png) [@Feldheld](https://discourse.libsdl.org/u/Feldheld)\
**Post date:** [January 10, 2019, 11:33pm UTC](https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590/1 "2019-01-10T23:33:29Z")

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When trying to render stuff fullscreen using desktop resolution (1920x1080) I encountered a weird issue. When I create the window fullscreen, every 2nd time SDK\_renderPresent is called it swaps to a content buffer that has already been overridden but seems for some weird reason still be stored in the renderer. Needless to say Im using only one renderer, and in windowed mode the program shows no such issue.

Im using a loop like this  
while(isRunning){  
updateScreen();  
handleEvents();  
}

In handleEvents() mouse and keyboard clicks are processed from the queue to allow me to draw stuff on the screen, in updateScreen() only SDK\_renderPresent is called. So when I first tried out fullscreen the screen was flipping madly between the black screen that was created on start by SDK\_renderClean and the initial content I created with my own code using only SDK\_setRenderDrawColor and SDK\_renderDrawPoint.

Im now using a hasChanged flag in order to only call renderPresent when stuff on the screen has actually changed, so at least as long as Im not drawing stuff the picture remains stable and fine. But as soon as I start drawing stuff and renderPresent is called repeatedly to update the screen in short order to get smooth changes, the picture permanently flips between the actual content and an old content. When I draw a rectangle by pressing the mouse button and dragging the opposite corner over the screen I can very clearly see that renderer alternates between two completely different pictures and even my drawing affects both buffers alternatingly so that half the rectangles appear on the new stuff and the other half on the old buffer.

Is there anything important I need to know about how the internal buffer works when in fullscreen mode? Is there some trick I can use to avoid this issue?

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**Author:** ![Feldheld](https://discourse.libsdl.org/letter_avatar_proxy/v4/letter/f/4bbf92/32.png) [@Feldheld](https://discourse.libsdl.org/u/Feldheld)\
**Post date:** [January 11, 2019, 7:45am UTC](https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590/2 "2019-01-11T07:45:20Z")

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I just found that this issue does not occur when I just create the window in desktop resolution without activating the fullscreen flag.

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**Author:** ![rtrussell](https://discourse.libsdl.org/user_avatar/discourse.libsdl.org/rtrussell/32/4768_2.png) [@rtrussell](https://discourse.libsdl.org/u/rtrussell)\
**Post date:** [January 11, 2019, 12:20pm UTC](https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590/3 "2019-01-11T12:20:46Z")

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> [@Feldheld](#):
>
> as soon as I start drawing stuff and renderPresent is called repeatedly to update the screen in short order to get smooth changes, the picture permanently flips between the actual content and an old content

I wonder if you are taking this into account: “_The backbuffer should be considered invalidated after each present; do not assume that previous contents will exist between frames. You are strongly encouraged to call [SDL\_RenderClear](https://wiki.libsdl.org/SDL_RenderClear)() to initialize the backbuffer before starting each new frame’s drawing, even if you plan to overwrite every pixel_”.

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**Author:** ![Feldheld](https://discourse.libsdl.org/letter_avatar_proxy/v4/letter/f/4bbf92/32.png) [@Feldheld](https://discourse.libsdl.org/u/Feldheld)\
**Post date:** [January 11, 2019, 12:45pm UTC](https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590/4 "2019-01-11T12:45:29Z")

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Ah. So I have to do a renderClear() and recreate the backbuffer completely for every frame, even if just one pixel has changed? I already considered trying this but it seemed such a wasteful way of doing it and I would have expected a major performance drop.

Thanks for the hint, this is exactly what I needed to know.

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**Author:** ![rtrussell](https://discourse.libsdl.org/user_avatar/discourse.libsdl.org/rtrussell/32/4768_2.png) [@rtrussell](https://discourse.libsdl.org/u/rtrussell)\
**Post date:** [January 11, 2019, 1:47pm UTC](https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590/5 "2019-01-11T13:47:49Z")

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> [@Feldheld](#):
>
> So I have to do a renderClear() and recreate the backbuffer completely for every frame, even if just one pixel has changed?

You can create a ‘target texture’ (separate from the backbuffer) to which all your rendering is done. Then, once a frame, you copy that texture to the backbuffer. So your `updateScreen` function becomes:

```
	SDL_SetRenderTarget(renderer, NULL);
	SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
	SDL_RenderClear(renderer);
	SDL_RenderCopy(renderer, texture, NULL, NULL);
	SDL_RenderPresent(renderer);
	SDL_SetRenderTarget(renderer, texture);

```

Now you can change just one pixel (or none) each frame.

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**Author:** ![Ed\_Phillips](https://discourse.libsdl.org/letter_avatar_proxy/v4/letter/e/57b2e6/32.png) [@Ed\_Phillips](https://discourse.libsdl.org/u/Ed_Phillips)\
**Post date:** [January 11, 2019, 3:30pm UTC](https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590/6 "2019-01-11T15:30:31Z")

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If you are using a “target texture”, can’t you skip the “clear” each frame, assuming the texture is the size of the screen/window? It seems wasteful to do a “clear” and then do a “blit” of the fullscreen texture copy too…

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**Author:** ![rtrussell](https://discourse.libsdl.org/user_avatar/discourse.libsdl.org/rtrussell/32/4768_2.png) [@rtrussell](https://discourse.libsdl.org/u/rtrussell)\
**Post date:** [January 11, 2019, 3:58pm UTC](https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590/7 "2019-01-11T15:58:17Z")

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> [@Ed\_Phillips](#):
>
> can’t you skip the “clear” each frame, assuming the texture is the size of the screen/window?

I would refer you to the extract from the [SDL docs](https://wiki.libsdl.org/SDL_RenderPresent) that I quoted earlier: "_You are strongly encouraged to call SDL\_RenderClear)() … even if you plan to overwrite every pixel_\*. I don’t like the phrase “_strongly encouraged to_” in formal documentation, but I assume it means “_must_”.

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<div class="post-metadata">

**Author:** ![Feldheld](https://discourse.libsdl.org/letter_avatar_proxy/v4/letter/f/4bbf92/32.png) [@Feldheld](https://discourse.libsdl.org/u/Feldheld)\
**Post date:** [January 11, 2019, 8:35pm UTC](https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590/8 "2019-01-11T20:35:40Z")

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About SDL\_renderCopy() the doc says “Use this function to copy a portion of the texture to the current rendering target.” I guess it means “copy a portion of the texture to the backbuffer”, right?

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**Author:** ![rtrussell](https://discourse.libsdl.org/user_avatar/discourse.libsdl.org/rtrussell/32/4768_2.png) [@rtrussell](https://discourse.libsdl.org/u/rtrussell)\
**Post date:** [January 12, 2019, 12:21pm UTC](https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590/9 "2019-01-12T12:21:27Z")

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> [@Feldheld](#):
>
> I guess it means “copy a portion of the texture to the backbuffer”, right?

No, if you have set a target texture it copies to there, not to the backbuffer. To copy to the backbuffer you must ensure that it is selected as the target:

```
SDL_SetRenderTarget(renderer, NULL); // select backbuffer as target

```

If `SDL_RenderCopy()` always copied to the backbuffer you couldn’t use it to copy from one texture to another:

```
SDL_SetRenderTarget(renderer, destination_texture);
SDL_RenderCopy(renderer, source_texture, NULL, NULL);

```

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<div class="post-metadata">

**Author:** ![Feldheld](https://discourse.libsdl.org/letter_avatar_proxy/v4/letter/f/4bbf92/32.png) [@Feldheld](https://discourse.libsdl.org/u/Feldheld)\
**Post date:** [January 12, 2019, 6:01pm UTC](https://discourse.libsdl.org/t/schizophrenic-renderer-when-in-fullscreen-mode/25590/10 "2019-01-12T18:01:49Z")

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Got it 🙂 Many thanks!