[PD] per-pixel time based(with memory) effects, is it possible with fragment shaders?

vade doktorp at mac.com
Mon Jan 7 07:11:43 CET 2008


You can completely do what you want on the GPU with a shader or  
shaders. In fact, the very shader you describe exists from cycling74  
and is included with jitter.

As for dealing with color planes, the RGBA values, its VERY easy to do  
with shaders. You simply sample a texture with:

vec4 mytexture = texture2DRect(textureSampler, textureCoordinate);

and then you can address you individual vector components with


each as a floating point value.


On Jan 7, 2008, at 12:28 AM, Batuhan Bozkurt wrote:

> This is a very elegant solution for a delay effect I should say.  
> I've never known something called "3d texture" exists but it  
> completely makes sense. I've searched through documentation and the  
> list for gem support for such thing but no luck yet. But thanks for  
> the shader and the idea anyways!
> The problem here is that, I actually need a more generalised  
> framework of working with time and past frames(or pixels) when  
> dealing with time based effects. This vertex shader approach with 3D  
> texture and a 2D map probably would not help let's say, if I wanted  
> to use an averaging lowpass filter(depending on past pixel values)  
> to the R G and B values of each pixel seperately.
> I'd like to know if there is any way to hold values back in  
> buffers(like arrays) on shader level and make operations on that  
> data by shader programming. So maybe I could say "get current value  
> and average it with the previous value and set it as the new current  
> value" or something like that. If there is a way, than I'd like to  
> dig more deeply into it, but if this question looks very stupid than  
> I'm probably getting the concept of graphics pipeline and purpose of  
> shading language completely wrong so maybe I should stop wasting  
> time on thinking like this and try to find other approaches.
> BB
> vade wrote:
>> This is entirely possible, however you would want to use a 3D  
>> texture, something on the order of dimensions 320 by 240 by x  
>> (where x is how many frames of time 'back' you want to go). This  
>> will be relatively heavy video memory wise I would imagine.  
>> However, I have a shader for you that does this.
>> Absolutely no idea what so over if GEM can deal with 3D textures.  
>> Id assume since it has low level openGl support it should be able  
>> to, but how to get 2D video frames into that 3D texture via pix_xxx  
>> I have no idea.
>> Here is the shader, written by Andrew Benson from Cycling74 iirc.
>> This is sans vertex shader, but the vertex shader is basically a  
>> passthrough, so its very simple.
>> texa is a 2d lookup table - greyscale, where the luminance of the  
>> pixel at the point determines how far back in time to go. This  
>> shader assumes a 512x512 map, but you could make that dynamic by  
>> passing a varying variable from the vertex shader that looks at the  
>> dims of the 3D texture.
>> HTH.
>> varying vec2 texcoord0;
>> varying vec2 texcoord1;
>> uniform float slice;
>> uniform sampler3D texo;
>> uniform sampler2DRect texa;
>> const vec4 coeff = vec4(0.299, .587, 0.114, 0.);
>> void main( void )
>> {
>>   float v1 = dot(texture2DRect(texa,texcoord0*vec2(512.)),coeff);
>>   //assumes 512  x 512 slice map.  Pretty arbitrary...
>>   vec4 v0 = texture3D(texo, vec3(texcoord0.xy,v1));
>>   gl_FragColor = v0;
>> }
>> On Jan 6, 2008, at 4:21 AM, Batuhan Bozkurt wrote:
>>> Hello,
>>> Let me start off by saying that I don't have much experience with
>>> computer graphics, so my knowledge is very limited on this area, I'm
>>> more of an audio guy but I have some ideas in mind that would  
>>> apply to
>>> graphics well. I just want to experiment.
>>> I want to give a simple example to show my question. Suppose I  
>>> have a
>>> video running and I want to delay each pixel between 0 and a maximum
>>> time seperately. This is the most basic time based effect I can  
>>> think
>>> of. So for a 320x200 video for example, I would need 64000 delaying
>>> units running seperately and to be able to do this in realtime, I  
>>> think
>>> I'd need to use GPU for computation.
>>> So I'm thinking of using pixel shaders. But I'm not really sure if  
>>> this
>>> is possible or not with it. I grabbed the orange book to have an  
>>> idea
>>> about the process, I did not have much time tolook in detail but I  
>>> could
>>> not find any references to such an operation that made me think  
>>> that I'm
>>> on the wrong track. So before going any further learning GLSL, I'd  
>>> like
>>> to have your ideas on this.
>>> Is GLSL along with GEM is a nice way to do such an operation? Is  
>>> it even
>>> possible to do it in realtime?
>>> If GPU powered realtime operation is not possible, is there any tool
>>> that you know that is capable of doing such things(realtime or  
>>> offline)?
>>> I have many ideas on processes that modifies pixels which are  
>>> dependant
>>> on the states of pixels before them(i.e. with memory) and I'm  
>>> trying to
>>> find a way to implement them. Any help is appreciated.
>>> Thanks
>>> BB
>>> _______________________________________________
>>> PD-list at iem.at mailing list
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