# How to compute 64 bit integer in vtkShaderProgram?

**URL:** <https://discourse.vtk.org/t/how-to-compute-64-bit-integer-in-vtkshaderprogram/14046>\
**Category:** Development\
**Created:** [June 13, 2024, 7:29pm UTC](https://discourse.vtk.org/t/how-to-compute-64-bit-integer-in-vtkshaderprogram/14046 "2024-06-13T19:29:16Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Michael\_Jackson](https://discourse.vtk.org/user_avatar/discourse.vtk.org/michael_jackson/32/61_2.png) [@Michael\_Jackson](https://discourse.vtk.org/u/Michael_Jackson)\
**Post date:** [June 13, 2024, 7:29pm UTC](https://discourse.vtk.org/t/how-to-compute-64-bit-integer-in-vtkshaderprogram/14046/1 "2024-06-13T19:29:16Z")

</div>

I’m really don’t know what I am doing when it comes to OpenGL and OpenGL Shader and all of that but I have a custom piece of code that was written for us:

```auto
    std::string FSSource = shaders[vtkShader::Fragment]->GetSource();
    vtkShaderProgram::Substitute(FSSource, "//VTK::Picking::Impl",
                                 "//DREAM3D-NX::Picking::Impl\n"
                                 " ivec2 size = textureSize(actortexture,0);\n"
                                 " vec2 fsize = vec2(size);\n"
                                 " vec2 fcoord = tcoordVCVSOutput * fsize;\n"
                                 " ivec2 coord = ivec2(fcoord);\n"
                                 " int idx = coord.y * size.x + coord.x;\n"
                                 " fragOutput0 = vec4(double(idx % 256) / 255.0, double((idx / 256) % 256) / 255.0, double((idx / 65536) % 256) / 255.0, 1.0);\n");
    shaders[vtkShader::Fragment]->SetSource(FSSource);

```

and the issue is that somewhere in that code we are overflowing at 24 bits for the ‘idx’ calculation. I have tried to use `dvec2` but that just throws errors about reserved words.

What version of OpenGL Shader Language does Vtk 9.2.6 use? Would it be any better with a newer version of VTK?

How could I adjust the code to allow for the proper calculation of the “idx” variable without over flowing the 24 bit of precision.

Thanks.

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<div class="post-metadata">

**Author:** ![Paulo\_Carvalho](https://discourse.vtk.org/user_avatar/discourse.vtk.org/paulo_carvalho/32/370_2.png) [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)\
**Post date:** [June 14, 2024, 1:25pm UTC](https://discourse.vtk.org/t/how-to-compute-64-bit-integer-in-vtkshaderprogram/14046/2 "2024-06-14T13:25:36Z")

</div>

Hello,

The `dvec`_n_ type is supposed to be used in that case. I think you should work on the errors you’re getting. How about sharing them here?

best,

PC

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<div class="post-metadata">

**Author:** ![dgobbi](https://discourse.vtk.org/user_avatar/discourse.vtk.org/dgobbi/32/18_2.png) [@dgobbi](https://discourse.vtk.org/u/dgobbi)\
**Post date:** [June 17, 2024, 5:37pm UTC](https://discourse.vtk.org/t/how-to-compute-64-bit-integer-in-vtkshaderprogram/14046/3 "2024-06-17T17:37:08Z")

</div>

I think you’ll do fine if you just use unsigned integer math instead of signed integer math.

For unsigned integers, overflow for multiplication and addition is well defined: the overflowed bits are lost, but the lower 32 bits that actually fit in the output are fine. (Overflow for signed integers, by comparison, is a mess.)

The “`fragOutput0`” line only actually needs the lower 24 bits of the integer to compute the result, so the 32 bits provided by `uint` is plenty. No need for 64 bit ints.

My suggested code:

```c++
uint idx = uint(coord.y) * uint(size.x) + uint(coord.x);
fragOutput0 = vec4((idx % 256) / 255.0,
                   ((idx / 256) % 256) / 255.0,
                   ((idx / 65536) % 256) / 255.0,
                   1.0);

```

* * *

Edit: This is just a shot in the dark, but depending on what this code is actually for, you might need to change the “`fragOutput0`” so that it consumes 30 bits instead of only 24 bits:

```c++
uint idx = uint(coord.y) * uint(size.x) + uint(coord.x);
fragOutput0 = vec4((idx % 1024) / 1023.0,
                   ((idx / 1024) % 1024) / 1023.0,
                   ((idx / 1048576) % 1024) / 1023.0,
                   1.0);

```