# Question: Blur filters on SDL\_Textures?

**URL:** <https://discourse.libsdl.org/t/question-blur-filters-on-sdl-textures/21145>\
**Category:** SDL Development\
**Created:** [December 31, 2014, 9:02pm UTC](https://discourse.libsdl.org/t/question-blur-filters-on-sdl-textures/21145 "2014-12-31T21:02:06Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![BadManiac](https://discourse.libsdl.org/user_avatar/discourse.libsdl.org/badmaniac/32/2722_2.png) [@BadManiac](https://discourse.libsdl.org/u/BadManiac)\
**Post date:** [December 31, 2014, 9:02pm UTC](https://discourse.libsdl.org/t/question-blur-filters-on-sdl-textures/21145/1 "2014-12-31T21:02:06Z")

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The situation is I have a raycasted shadow map for a 2D tile based environment, but it casts hard edged shadows. I want to run a simple blur filter on this texture before rendering it, to soften the edges and make the effect more visually appealing.

So far I’ve come up with two plausible methods, and neither really work well.  
Method one that I’ve tested is to scale the texture down to a quarter the size or even less, and scaling it up with linear filtering enabled (SDL\_HINT\_RENDER\_SCALE\_QUALITY) , and while this works, the blur isn’t quite strong enough for what I want to achieve. It also can’t really be improved because scaling down the texture further makes it extremely blocky when scaled up. I also tried scaling it down and the adding “jitter” to it by additively rendering it four times offset by a pixel each time, at 25% opacity. This improved the result, but again can’t really be taken any further. It can also create quite ugly aliasing artifacts when there are small details on the texture. And it eats performance at higher resolutions…

Method two would be to render the shadow map in software, giving full access to manually write a blur filter, but this requires sending data across the bus every frame, which is hardly optimal, it also seems like a backwards move in todays world of stupendously powerful GPU’s everywhere.

Suggestions? Ideas? Is there a way to blur an SDL texture in hardware? Is there some way to run a GLSL shader on a SDL\_Texture? Are there any other techniques that work at run time?

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**Author:** ![Sik\_the\_hedgehog](https://discourse.libsdl.org/letter_avatar_proxy/v4/letter/s/db5fbb/32.png) [@Sik\_the\_hedgehog](https://discourse.libsdl.org/u/Sik_the_hedgehog)\
**Post date:** [January 1, 2015, 3:41am UTC](https://discourse.libsdl.org/t/question-blur-filters-on-sdl-textures/21145/2 "2015-01-01T03:41:49Z")

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2014-12-31 18:02 GMT-03:00, BadManiac \<martin\_maniac at [hotmail.com](http://hotmail.com)\>:

> Method two would be to render the shadow map in software, giving full access  
> to manually write a blur filter, but this requires sending data across the  
> bus every frame, which is hardly optimal, it also seems like a backwards  
> move in todays world of stupendously powerful GPU’s everywhere.

Sending a single framebuffer once per frame isn’t as bad as it sounds,  
especially when you consider the shadow map doesn’t have to have the  
same resolution as the screen (you could impose a cap on its size, for  
example, or even make it a setting, which can help for people with old  
hardware).

That said, as you mention it this is something that would be better  
off as a GPU shader, and that’s beyond the renderer API’s  
capabilities. You would need to redo everything in pure OpenGL instead  
(maybe SDL\_gpu can handle this, but even in that case you’ll need to  
rewrite the code for the same reason).

If you can’t do that the best you can get is just faking it. (also for  
the record, this is an issue even in modern GPUs because you’re  
essentially trying to fake antialiasing out of an image without  
antialiasing so to get anything even remotely optimal you’ll need to  
apply some heuristics)

PS: I’d rather have hard shadows over ugly looking soft shadows, so if  
you have issues coming up with something good take that into  
account…

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**Author:** ![JonnyD](https://discourse.libsdl.org/user_avatar/discourse.libsdl.org/jonnyd/32/1604_2.png) [@JonnyD](https://discourse.libsdl.org/u/JonnyD)\
**Post date:** [January 1, 2015, 2:56pm UTC](https://discourse.libsdl.org/t/question-blur-filters-on-sdl-textures/21145/3 "2015-01-01T14:56:31Z")

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Yeah, SDL\_gpu can load shaders and send arbitrary uniform/attribute data to  
them. Rewriting code to use the SDL\_gpu renderer is not bad at all, except  
that the documentation isn’t awesome yet. The shader-demo in the source  
repository might be a good place to look and I’m always ready to answer  
questions.

Jonny DOn Wed, Dec 31, 2014 at 10:41 PM, Sik the hedgehog \< sik.the.hedgehog at [gmail.com](http://gmail.com)\> wrote:

> 2014-12-31 18:02 GMT-03:00, BadManiac \<martin\_maniac at [hotmail.com](http://hotmail.com)\>:
> 
> > Method two would be to render the shadow map in software, giving full  
> > access  
> > to manually write a blur filter, but this requires sending data across  
> > the  
> > bus every frame, which is hardly optimal, it also seems like a backwards  
> > move in todays world of stupendously powerful GPU’s everywhere.
> 
> Sending a single framebuffer once per frame isn’t as bad as it sounds,  
> especially when you consider the shadow map doesn’t have to have the  
> same resolution as the screen (you could impose a cap on its size, for  
> example, or even make it a setting, which can help for people with old  
> hardware).
> 
> That said, as you mention it this is something that would be better  
> off as a GPU shader, and that’s beyond the renderer API’s  
> capabilities. You would need to redo everything in pure OpenGL instead  
> (maybe SDL\_gpu can handle this, but even in that case you’ll need to  
> rewrite the code for the same reason).
> 
> If you can’t do that the best you can get is just faking it. (also for  
> the record, this is an issue even in modern GPUs because you’re  
> essentially trying to fake antialiasing out of an image without  
> antialiasing so to get anything even remotely optimal you’ll need to  
> apply some heuristics)
> 
> PS: I’d rather have hard shadows over ugly looking soft shadows, so if  
> you have issues coming up with something good take that into  
> account…
> 
> * * *
> 
> SDL mailing list  
> SDL at [lists.libsdl.org](http://lists.libsdl.org)  
> [http://lists.libsdl.org/listinfo.cgi/sdl-libsdl.org](http://lists.libsdl.org/listinfo.cgi/sdl-libsdl.org)

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<div class="post-metadata">

**Author:** ![Sik\_the\_hedgehog](https://discourse.libsdl.org/letter_avatar_proxy/v4/letter/s/db5fbb/32.png) [@Sik\_the\_hedgehog](https://discourse.libsdl.org/u/Sik_the_hedgehog)\
**Post date:** [January 1, 2015, 4:18pm UTC](https://discourse.libsdl.org/t/question-blur-filters-on-sdl-textures/21145/4 "2015-01-01T16:18:40Z")

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2015-01-01 11:56 GMT-03:00, Jonathan Dearborn :

> Rewriting code to use the SDL\_gpu renderer is not bad at all, except  
> that the documentation isn’t awesome yet.

The problem is if there’s a lot of code that needs rewriting, nothing  
will help you there 😛

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**Author:** ![BadManiac](https://discourse.libsdl.org/user_avatar/discourse.libsdl.org/badmaniac/32/2722_2.png) [@BadManiac](https://discourse.libsdl.org/u/BadManiac)\
**Post date:** [January 2, 2015, 10:12am UTC](https://discourse.libsdl.org/t/question-blur-filters-on-sdl-textures/21145/5 "2015-01-02T10:12:45Z")

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Jonny D wrote:

> Yeah, SDL\_gpu can load shaders and send arbitrary uniform/attribute data to them.?? Rewriting code to use the SDL\_gpu renderer is not bad at all, except that the documentation isn’t awesome yet.?? The shader-demo in the source repository might be a good place to look and I’m always ready to answer questions.  
> Jonny D

Thanks for all your replies, it sounds pretty hopeful. I’ve contacted you about this Jonny, hopefully we can get it figured out. It’s not that I have that much code to rewrite, I’m just gathering techniques BEFORE I start the major rewrite. This is all stuff I was able to do in my old 16bpp software library, but I’m trying to update my tools and knowledge a century or so 🙂