Using custom sprite shaders for point cloud renderer #7681

banesullivan · 2025-07-08T18:29:43Z

banesullivan
Jul 8, 2025
Maintainer

Describe something you've made

Here's a demo of using customer shader logic to display different kinds of sprites when rendering points (default in VTK/PyVista is just squares)

Links to Documentation, Examples, or Class Definitions.

No response

Pseudocode or Screenshots

import pyvista as pv
import numpy as np
import vtk

symbol_shaders = {
    "triangle": """
//VTK::Color::Impl

vec2 p = gl_PointCoord * 2.0 - 1.0;

// Define upright triangle in NDC
float a = 0.5;
float b = 0.8660254;
vec2 v0 = vec2(0.0, a);
vec2 v1 = vec2(-b, -a);
vec2 v2 = vec2(b, -a);

// Point-in-triangle test
float area = abs((v1.x - v0.x)*(v2.y - v0.y) - (v2.x - v0.x)*(v1.y - v0.y));
float a1 = abs((v1.x - p.x)*(v2.y - p.y) - (v2.x - p.x)*(v1.y - p.y)) / area;
float a2 = abs((v2.x - p.x)*(v0.y - p.y) - (v0.x - p.x)*(v2.y - p.y)) / area;
float a3 = abs((v0.x - p.x)*(v1.y - p.y) - (v1.x - p.x)*(v0.y - p.y)) / area;

if ((a1 + a2 + a3) > 1.01) {
  discard;
}
""",
    "circle": """
//VTK::Color::Impl

vec2 p = gl_PointCoord * 2.0 - 1.0;  // center at (0,0), range [-1, 1]
float d = dot(p, p);                 // squared distance from center

if (d > 1.0) {
  discard;                           // outside the circle
}
""",
    "hexagon": """
//VTK::Color::Impl
vec2 p = gl_PointCoord * 2.0 - 1.0;
p = abs(p);

if (p.x > 1.0 || p.y > 1.0) {
  discard;
}

float dx = 0.866 * p.y + 0.5 * p.x;
float dy = 0.866 * p.x + 0.5 * p.y;

if (p.x + 0.577 * p.y > 1.0) {
  discard;
}""",
    "diamon": """
//VTK::Color::Impl
vec2 p = gl_PointCoord * 2.0 - 1.0;
if (abs(p.x) + abs(p.y) > 1.0) {
  discard;
}
""",
    "asterisk": """
//VTK::Color::Impl

vec2 p = gl_PointCoord * 2.0 - 1.0;
float r = length(p);
if (r > 1.0) discard;

float theta = atan(p.y, p.x);

// Number of star points
float N = 5.0;

// Inner-to-outer radius ratio (0.0 to 1.0)
float inner = 0.5;

// Modulate radius in polar space
float star_radius = mix(1.0, inner, abs(cos(N * theta)));

if (r > star_radius)
  discard;
""",
    "star": """
//VTK::Color::Impl

vec2 p = gl_PointCoord * 2.0 - 1.0;
float r = length(p);
if (r > 1.0) discard;

float theta = atan(p.y, p.x); // [-π, π]
theta = mod(theta + 6.2831853 + 1.5707963, 6.2831853); // rotate so tip points up

float N = 5.0;
float inner = 0.4;

// Sharpen the cosine peaks for crisp star tips
float radial = pow(0.5 * (1.0 + cos(N * theta)), 3.0);
float radius = mix(inner, 1.0, radial);

if (r > radius)
  discard;
""",
}


points = np.random.rand(1000, 3)
cloud = pv.PolyData(points).elevation()

pl = pv.Plotter(shape=(2, 3))


def plot_points(symbol):

    actor = pl.add_mesh(
        cloud,
        style="points",
        render_points_as_spheres=False,
        point_size=25,
        show_scalar_bar=False,
    )

    prop = actor.GetShaderProperty()

    shader_code = symbol_shaders[symbol]
    prop.AddShaderReplacement(
        vtk.vtkShader.Fragment, "//VTK::Color::Impl", True, shader_code, False
    )
    return actor


for i, symbol in enumerate(symbol_shaders.keys()):
    if i < 3:
        pl.subplot(0, i)
    else:
        pl.subplot(1, i - 3)
    plot_points(symbol)


pl.link_views()
pl.show()

user27182 · 2025-07-08T19:01:13Z

user27182
Jul 8, 2025
Maintainer

Cool! Should we use circles by default instead of squares? And the star-generating functions are nice, maybe add a new pv.Star function to geometric_objects.py ?

5 replies

banesullivan Jul 8, 2025
Maintainer Author

Personally I like circles a lot more than squares. I'd be up for changing the default behavior here but I'd want to do some validation around performance for this first

banesullivan Jul 8, 2025
Maintainer Author

Actually. I wonder what the implementation of render_points_as_spheres is doing under the hood. Because we can effectively do the same thing with this shader code:

//VTK::Color::Impl

vec2 p = gl_PointCoord * 2.0 - 1.0;
float d = dot(p, p);               // distance² from sprite centre

if (d > 1.0)
{
  discard;                         // outside the circle mask
}
else
{
  // ----- faux 3-D lighting ---------------------------------------
  vec3  N = normalize(vec3(p, sqrt(max(0.0, 1.0 - d)))); // fake normal
  vec3  L = normalize(vec3(-0.4, 0.6, 1.0));             // light direction

  float ambientK  = 0.25;                                // 25 % ambient
  float diffuseK  = max(dot(N, L), 0.0);                 // Lambert term
  float shade     = ambientK + (1.0 - ambientK) * diffuseK;
  // ---------------------------------------------------------------

  // 'texColor' comes from VTK just a few lines earlier
  ambientColor = texColor.rgb * shade;   // shade the mapped colour
  diffuseColor = vec3(0.0);              // disable extra lighting
  opacity      = texColor.a * opacityUniform;
}

import pyvista as pv
from pyvista import examples
import vtk
import numpy as np

point_cloud = pv.PolyData(np.random.rand(10000, 3)).elevation()

pl = pv.Plotter(shape=(1,2))
actor = pl.add_mesh(point_cloud.copy(), scalars="Elevation", point_size=10, style='points')
# actor = pl.add_mesh(pv.Sphere())

shader_code = """
//VTK::Color::Impl

vec2 p = gl_PointCoord * 2.0 - 1.0;
float d = dot(p, p);               // distance² from sprite centre

if (d > 1.0)
{
  discard;                         // outside the circle mask
}
else
{
  // ----- faux 3-D lighting ---------------------------------------
  vec3  N = normalize(vec3(p, sqrt(max(0.0, 1.0 - d)))); // fake normal
  vec3  L = normalize(vec3(-0.4, 0.6, 1.0));             // light direction

  float ambientK  = 0.25;                                // 25 % ambient
  float diffuseK  = max(dot(N, L), 0.0);                 // Lambert term
  float shade     = ambientK + (1.0 - ambientK) * diffuseK;
  // ---------------------------------------------------------------

  // 'texColor' comes from VTK just a few lines earlier
  ambientColor = texColor.rgb * shade;   // shade the mapped colour
  diffuseColor = vec3(0.0);              // disable extra lighting
  opacity      = texColor.a * opacityUniform;
}

"""

prop = actor.GetShaderProperty()
prop.AddShaderReplacement(
    vtk.vtkShader.Fragment,
    "//VTK::Color::Impl",
    True,
    shader_code,
    False
)

pl.subplot(0,1)
pl.add_mesh(point_cloud.copy(), scalars="Elevation", point_size=10, style='points', render_points_as_spheres=True)

pl.link_views()
pl.show()

However, I know the render_points_as_spheres option fails on some GPUs. In all likelihood render_points_as_spheres is doing something similar with the shader but if this approach works more universally, then perhaps this should be the default?

user27182 Jul 8, 2025
Maintainer

Actually. I wonder what the implementation of render_points_as_spheres is doing under the hood.

I believe it's here:

https://github.com/Kitware/VTK/blob/d49ad79ab7d7cf9ec1f3cf96ba608755441af9af/Rendering/OpenGL2/vtkOpenGLPolyDataMapper.cxx#L1976

Rather than //VTK::Color::Impl, it replaces //VTK::Normal::Dec and //VTK::Depth::Impl

user27182 Jul 8, 2025
Maintainer

The shader color is set here:

https://github.com/Kitware/VTK/blob/d49ad79ab7d7cf9ec1f3cf96ba608755441af9af/Rendering/OpenGL2/vtkOpenGLPolyDataMapper.cxx#L762

and it looks like special handling for actor properties (diffuse, specular, etc.) are all appended, and so these should still work, even after setting a custom shader

user27182 Jul 21, 2025
Maintainer

Personally I like circles a lot more than squares. I'd be up for changing the default behavior here but I'd want to do some validation around performance for this first

FYI: Support for rendering circles instead of squares was officially added for WebGPU rendering as part of VTK 9.5
https://docs.vtk.org/en/latest/release_details/9.5.html#rendering

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Using custom sprite shaders for point cloud renderer #7681

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banesullivan Jul 8, 2025 Maintainer

Describe something you've made

Links to Documentation, Examples, or Class Definitions.

Pseudocode or Screenshots

Replies: 1 comment · 5 replies

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user27182 Jul 8, 2025 Maintainer

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