Forums Technical Discussion OpenCL Voxel Space to World Space position?

OpenCL Voxel Space to World Space position?

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cs_pix
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Hi everyone,

I'm trying to get the world space position of a voxel in a heightfield volume using OpenCL. There is a option in the bindings to get “Volume Transform to World”, which returns a float16 OpenCL matrix but I don't understand how to extract a voxel position from this.

My guess is that I have to use gdix and gdiz and multiply them with the matrix but the matrix itself I don't understand.

height_xformtoworld.hi.hi = 0, 0, 0, 4?
height_xformtoworld.hi.lo = 0, ~15?, 0, 0
height_xformtoworld.lo.hi = ~15?, 0, 0, 0
height_xformtoworld.lo.lo = 0, 0, ~15?, 0

There is this weird approx 15 value in there. (I figure it out by dividing the matrix value by 15 to get 1 showing on the heightfield mask). These values are also out of place. I would expect matrix.lo.lo to return the translation part of the matrix but…

Any help to figure this out would be appreciated.
Thx!

Christian
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jlait
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From attribvolume.hip of the OpenCL masterclass:

    float4 voxelpos = pos.x * density_xformtovoxel.lo.lo +
                      pos.y * density_xformtovoxel.lo.hi +
                      pos.z * density_xformtovoxel.hi.lo +
                      density_xformtovoxel.hi.hi;

In this case you are going the other way, but it should be a matter of using the gidx.xyz rather than pos.xyz.

So hi.hi is the translate component, not lo.lo. Note heightfields are stored always with a xy layout in memory, but usually displayed with a zx layout on screen. This would be why the 15s are not along a diagonal.
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postas
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Hi Jeff,
And what is gidx.xyz in this case?

In default kernel we have three ids…
int gidx = get_global_id(0);
int gidy = get_global_id(1);
int gidz = get_global_id(2);

But they are integer ids - we are unlikely to multiply them by xform matrix to get world coordinate….
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postas
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Sorry, i've got it…
We have to multiply the voxel idx by voxel_x_size, idy by voxel_y_size, idz by voxel_z_size…
this will be the xyz to transform by xformtoworld matrix..
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Konstantin Magnus
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Could you perhaps provide a code example of getting the world position of voxels in openCL?

So far I have got this, but I would not know how to multiply float3 pos by float16 density_xformtoworld. 

float pos_x = gidx * density_voxelsize_x;
float pos_y = gidy * density_voxelsize_y;
float pos_z = gidz * density_voxelsize_z;
float3 pos = {pos_x, pos_y, pos_z};
https://procegen.konstantinmagnus.de/ [procegen.konstantinmagnus.de]
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gorrod
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It's just like jlait wrote. What you need to do is going the other way around from the world pos to the index.

This is what works for me:

kernel void get_world_P( 
                 int world_Px_stride_x, 
                 int world_Px_stride_y, 
                 int world_Px_stride_z, 
                 int world_Px_stride_offset, 
                 float16 world_Px_xformtoworld, 
                 global float * world_Px ,
                 global float * world_Py ,
                 global float * world_Pz 
)
{
    int gidx = get_global_id(0);
    int gidy = get_global_id(1);
    int gidz = get_global_id(2);
    int idx = world_Px_stride_offset + world_Px_stride_x * gidx
                               + world_Px_stride_y * gidy
                               + world_Px_stride_z * gidz;
    float4 world_pos = gidx * world_Px_xformtoworld.lo.lo +
                    gidy * world_Px_xformtoworld.lo.hi + 
                    gidz * world_Px_xformtoworld.hi.lo +
                    world_Px_xformtoworld.hi.hi;
        world_Px[idx] = world_pos.x;
        world_Py[idx] = world_pos.y;
        world_Pz[idx] = world_pos.z; 
                    
}

Attachments:
openCL_xformtoworld.hipnc (119.0 KB)

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Konstantin Magnus
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Thank you so much gorrod! I think this works for me, too ; )
https://procegen.konstantinmagnus.de/ [procegen.konstantinmagnus.de]
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