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From slab texture to position matrix, what's most efficient?

March 7, 2012 | 5:05 pm


I have a patch which does spectrum analysis and writes it to a matrix. That matrix goes to the GPU through and slabs for heavy processing. Then I want to take the output and use it as vertex position data for GL drawing, using right now.

I’m getting a big fps drop from having to write the slab output into a jit.matrix before sending it to (also having to reformat from 4 planes to 3). Rendering stays stuck at 30fps. Which is not very bad but I know it could be lots faster given the relatively simple geometry.

Doing the gl.pix and slab processing on the CPU and thus circumventing readback is slower. Getting around 15fps that way.

Is there a more efficient way of doing this in Max6? Perhaps with Lua?

Tanx, D.

March 8, 2012 | 8:21 pm


March 8, 2012 | 9:05 pm

We’d probably need to see a patch first…

March 8, 2012 | 11:30 pm

I’m afraid it’s not in a state to be posted right now. But hints for efficient GPU texture to GL geometry strategies don’t really need the actual patch, no?

I’ll try to get it in a postable state asap anyway.

March 8, 2012 | 11:34 pm

A custom shader for your should work. You’d send a "blank slate" of vertex points as a jit.matrix to define your default positions (most people use jit.expr for this), and then associate your texture with the mesh. The vertex shader would interpret your texture as a displacement map, which should give you higher performance. I’ve done this for a very similar project and it works well.

Unfortunately does not support multi-texturing, so if you want to apply additional textures for fragment processing you’ll have to figure out a different route.

March 9, 2012 | 12:09 am

actually does now support multi-texturing:

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March 9, 2012 | 12:32 am

Good to know! Can one specify distinct texture coordinates for each texture or do they both inherit the same coordinates?

March 9, 2012 | 5:25 pm

this sounds great, will test asap!

March 9, 2012 | 10:25 pm

Jesse, is this supposed to work? It doesn’t for me. When I send the displacement map the mesh’s color only changes to black. I can ‘t discover the logic behing it.

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March 10, 2012 | 4:28 pm

whoops typo corrected, now it doesn’t turn black anymore but still doesn’t move either:

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March 11, 2012 | 3:00 am

You still have a typo, @planes should be @planecount in jit.expr.

Your example does not work because you are not using a shader to perform the displacement. You have to create a shader that acts on the mesh’s z coordinates by defining a texture for the mesh and then performing displacement in the vertex shader.

It’s possible you could do this in but I have not implemented it there.

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March 11, 2012 | 1:17 pm

weird, planecount is what i corrected, copy/paste mess up i guess…

gonna chew on that shader technique now!

March 11, 2012 | 6:57 pm

Ok i’ve been on it for the whole afternoon but no cigar. I’m probably missing something basic.

uniform sampler2DRect tex0;
varying vec2 texcoord0;

void main()

vec4 position;
vec4 position2;

vec4 delta = texture2DRect(tex0, texcoord0);

// transform matrix
position = gl_ModelViewProjectionMatrix * gl_Vertex;

// offset
position2 = vec4(position.x+delta.x,position.y+delta.y,position.z+delta.z,position.w);

// copy to output position
gl_Position = position2;

// no lighting calculated in this shader
gl_FrontColor = gl_Color;

I thought tex0 should be bound to vp but that returns an error: "setting GLSL param: GL Error: Invalid operation"

What is it I’m missing about this?


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(btw, how do you get the code block formatting in a post, these ` don’t work for me)

March 12, 2012 | 6:17 pm

Your shader crashes Max6 every time for me, and I don’t have time to troubleshoot it. Can you open your patch on your system?

March 12, 2012 | 6:47 pm

here’s an example patch and shader for vertex displacement mapping, based on this tutorial:

@jesse: i don’t believe multiple texture coordinates are currently possible, but we can look into implementing this for a future update. can you provide an example patch showing how you would use this feature?

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  1. vertdisplace.jxs

March 12, 2012 | 7:10 pm

tanx rob, you’re the man!

i was actually looking at that tutorial but didn’t realize i could pretty much paste it in…

March 12, 2012 | 7:34 pm

one thing i don’t fully grasp is why it doesn’t work when i initialize the mesh with jit.expr instead of the gridshape matrixoutput like in my patch above. is it because i’m not sending a texcoord array?

March 12, 2012 | 11:09 pm


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March 13, 2012 | 12:28 pm


June 14, 2012 | 4:20 pm

I’m tring to adapt the Robert Ramirez shader to work with rectangular texture from Andrew Benson shader ab.HSFLOW, but I get a strange behavior, maybe some error in texture coordinates,
could someone have a look at my patch?

June 14, 2012 | 6:02 pm

hi micron. i took a look at your patch and shader.
i’m not entirely sure what you are trying to achieve, and why you changed the way the vertex shader functions.

if you revert back to the original vertex shader, and make the necessary modifications for rectangular texture sampling, it works as expected.

i also needed to provide a normal matrix for the gl.mesh (i’m not sure why the other patch did not need this as well). this is simply a [ jit.matrix 3 float32 320 240 ] connected to the third inlet of gl.mesh, and sent the message, "setall 0 0 1", so that the normal points along the Z axis.

here’s the correct shader code

void main(void)
	vec4 newVertexPos;
	vec4 dv;
	float df;
	texcoord0 = vec2(gl_TextureMatrix[0] * gl_MultiTexCoord0);

	dv = texture2DRect(tex0,texcoord0);
	df = 0.30*dv.x + 0.59*dv.y + 0.11*dv.z;

	newVertexPos = vec4(gl_Normal * df * scale, 0.0) + gl_Vertex;

	gl_Position = gl_ModelViewProjectionMatrix * newVertexPos;
June 14, 2012 | 7:49 pm

thanks helped me a lot, GLSL is not at all easy

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Forums > Jitter