shader-point-cloud-geometry-shader.cpp

#include "al/core.hpp"
using namespace al;
using namespace std;

#define N (1000)
#define CLOUD_WIDTH (5.0)

string fullPathOrDie(string fileName, string whereToLook = ".") {
  SearchPaths searchPaths;
  searchPaths.addSearchPath(whereToLook);
  string filePath = searchPaths.find(fileName).filepath();
  if (filePath == "") {
    fprintf(stderr, "FAIL\n");
    exit(1);
  }
  return filePath;
}

struct AlloApp : App {
  ShaderProgram shader;
  Mesh pointMesh;
  Texture texture;

  void onCreate() override {
    pointMesh.primitive(Mesh::POINTS);
    for (int i = 0; i < N; i++) {
      pointMesh.vertex(
          Vec3f(rnd::uniformS(), rnd::uniformS(), rnd::uniformS()) *
          CLOUD_WIDTH);
      pointMesh.color(HSV(rnd::uniform(), 1.0, 1.0));
    }

    // Generate a texture with an alpha channel for transparency
    texture.create2D(256, 256, Texture::RGBA8);
    int Nx = texture.width();
    int Ny = texture.height();

    // prepare vector for pixel data
    std::vector<Colori> pixel;  // byte color: rgba, [0:255]
    pixel.resize(Nx * Ny);

    for (int j = 0; j < Ny; ++j) {
      float y = float(j) / (Ny - 1) * 2 - 1;
      for (int i = 0; i < Nx; ++i) {
        float x = float(i) / (Nx - 1) * 2 - 1;
        float m = exp(-13 * (x * x + y * y));
        Color c = RGB(m);
        c.a = m;
        pixel[j * Nx + i] = c;
      }
    }
    texture.submit(pixel);

    // these shaders are from the _OpenGL 4.0 Shading Language Cookbook_
    shader.compile(
        R"(
#version 400
layout (location = 0) in vec3 VertexPosition;
uniform mat4 al_ModelViewMatrix;
uniform mat4 al_ProjectionMatrix;
void main() {
  gl_Position = al_ModelViewMatrix * vec4(VertexPosition, 1.0);
}
)",
        R"(
#version 400
in vec2 TexCoord;
uniform sampler2D SpriteTex;
layout( location = 0 ) out vec4 FragColor;
void main() {
  FragColor = texture(SpriteTex, TexCoord);
}
)",
        R"(
#version 400
layout( points ) in;
layout( triangle_strip, max_vertices = 4 ) out;
uniform float Size2;   // Half the width of the quad
uniform mat4 al_ProjectionMatrix;
out vec2 TexCoord;
void main() {
  mat4 m = al_ProjectionMatrix;  // Reassign for brevity
  gl_Position = m * (vec4(-Size2,-Size2,0.0,0.0) + gl_in[0].gl_Position);
  TexCoord = vec2(0.0,0.0);
  EmitVertex();
  gl_Position = m * (vec4(Size2,-Size2,0.0,0.0) + gl_in[0].gl_Position);
  TexCoord = vec2(1.0,0.0);
  EmitVertex();
  gl_Position = m * (vec4(-Size2,Size2,0.0,0.0) + gl_in[0].gl_Position);
  TexCoord = vec2(0.0,1.0);
  EmitVertex();
  gl_Position = m * (vec4(Size2,Size2,0.0,0.0) + gl_in[0].gl_Position);
  TexCoord = vec2(1.0,1.0);
  EmitVertex();
  EndPrimitive();
}
)");
  }

  void onAnimate(double dt) override {}

  void onDraw(Graphics& g) override {
    g.clear(0.23);
    // g.texture();
    // g.meshColor();
    g.depthTesting(false);
    g.blending(true);
    g.blendModeTrans();
    texture.bind();
    g.shader(shader);
    g.shader().uniform("Size2", 0.1);
    g.draw(pointMesh);
    texture.unbind();
  }
};

int main() { AlloApp().start(); }