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## Scaling instance data individually in vertex shader

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5 replies to this topic

### #1carkey  Members

Posted 16 September 2013 - 05:54 AM

Hi,

I'm trying to scale instance data individually in the vertex shader with directX and hlsl. have an input layout that takes a bool and if this is true I want to scale the instance.

The bool goes across to the shader fine but the scaling seems to be wrong.

Basically, I have a model matrix that exists in the shader but before I do:

position = mul(input.position, model);
position = mul(input.position, view);
position = mul(input.position, projection);


I first ask if this bool is true, if so I do:

model = mul(model, scaleMatrix);


scaleMatrix is just a simple scale matrix where the scale values are 0.5.

My problem is that it doesn't seem to be scaling the instance at it's own origin (which is what I want), it is scaling it at some other origin but I don't know why this is the case.

Any ideas?

I don't have much experience with instancing so any help would be greatly appreciated.

Thanks.

### #2kauna  Members

Posted 16 September 2013 - 11:08 AM

If you scale your model matrix with the scale matrix you explained, you'll be scaling according to the world origin probably.

To implement per-object scaling in shader you'll need a bit more information. That is, around which point your want to scale your object and then you'll need to construct a scale-transform matrix accordingly.

or

You could apply your scaling beforehand to the model matrix instead in the shader

or

If you want to scale each mesh around it's origin you can use: position = mul(input.position, scaleMatrix) before other transforms. Should be same as multiplying the model matrix in reverse order.

Cheers!

Edited by kauna, 16 September 2013 - 11:09 AM.

### #3carkey  Members

Posted 16 September 2013 - 01:33 PM

If you scale your model matrix with the scale matrix you explained, you'll be scaling according to the world origin probably.

To implement per-object scaling in shader you'll need a bit more information. That is, around which point your want to scale your object and then you'll need to construct a scale-transform matrix accordingly.

or

You could apply your scaling beforehand to the model matrix instead in the shader

or

If you want to scale each mesh around it's origin you can use: position = mul(input.position, scaleMatrix) before other transforms. Should be same as multiplying the model matrix in reverse order.

Cheers!

Hi,

Thanks for the reply, I tried this and as you say, each instance does now scale around its own origin. The problem is that its origin is in its bottom left corner, not in its center.

I tried translating to the center, scaling and translating back but this didn't seem to make any difference, I don't quite understand what I'm doing wrong:

float4x4 scale = float4x4(
0.5f, 0,    0,    0,
0,    0.5f, 0,    0,
0,    0,    0.5f, 0,
0,    0,    0,    1);

float4x4 translate = float4x4(
1, 0, 0, 3,
0, 1, 0, 3,
0, 0, 1, 3,
0, 0, 0, 1);

float4x4 translateInverse = float4x4(
1, 0, 0, -3,
0, 1, 0, -3,
0, 0, 1, -3,
0, 0, 0, 0 );

float4 pos = (input.pos, 1.0f);
pos = mul(pos, translate);
pos = mul(pos, scale);
pos = mul(pos, translateInverse);


(I know my mesh is 6x6x6 therefore I'm trying to translate to the center at 3x3x3 from 0,0,0).

Thanks for the help so far!

Edited by carkey, 16 September 2013 - 01:34 PM.

### #4kauna  Members

Posted 16 September 2013 - 06:38 PM

Is your shader expecting row or column major matrices?

Placing translation to the wrong place will produce incorrect results.

Cheers!

### #5carkey  Members

Posted 17 September 2013 - 03:25 AM

Is your shader expecting row or column major matrices?

Placing translation to the wrong place will produce incorrect results.

Cheers!

I believe it is column major matrices but I'm not really sure, it is currently a very basic instance vertex shader that just takes the position, adds on the instance position and then draws it.

Should the process of translating to the center of the mesh, scaling and then translating back out work in theory as I detail above?

It seems to be strange to me that it has no effect, as in doing the scale and the translate,scale,translate both produce the same result.

Thanks.

### #6kauna  Members

Posted 17 September 2013 - 06:51 AM

I think that the method you described is correct. I asked about the row/column matrices since they make a big difference.

Did you try transposing each matrix (as the matrix below) ?

float4x4 translate = float4x4(
1, 0, 0, 0,
0, 1, 0, 0,
0, 0, 1, 0,
3, 3, 3, 1);

CHeers!

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