Simple 2D plot program displays blank

OpenGL OpenGL: Basic Coding
1

I came across pseudo code in steve harrington old textbook that displays 3D f(u,v) function on 2D using x=u+0.5v and y=f(u,v)+0.5v.
I thought it would be easy to implement in modern opengl and connect lines of constant u and constant v to get the shape of the function but I got a blank screen, then I decided to just print points and still got blanks.
1225 vertexData points are generated with x and y between 0.0 and 0.75 for the chosen function and z is set to -1.0. I stared at the program for hours but could not figure out why nothing is displayed. the relevant parts are pasted below:

void drawScene()
{
  glClear(GL_COLOR_BUFFER_BIT);
  glDrawArrays(GL_POINTS, 0, vertexData.size());
  glutSwapBuffers();
}
void resize(int w, int h)
{
  glViewport(0, 0, w, h);
}

void setup()
{  // Create and compile our GLSL program from the shaders
  programID = LoadShaders( "plot3D_Harrington.vshader", "plot3D_Harrington.fshader" );//"fshader.fshader");
  // Use our shader
  glUseProgram(programID);
  //glPolygonMode(GL_FRONT_AND_BACK, GL_POINT);
  glClearColor(0.0, 0.0, 0.0, 1.0);
  generate_vertexData();
  GLuint MVPID = glGetUniformLocation(programID, "MVP");
  GLuint colorID = glGetUniformLocation(programID, "Color");
  mat4 proj = ortho(-1.0f, 1.0f, -1.0f, 1.0f, 0.1f, 2.0f);//frustum(-1.0f, 1.0f, -1.0f, 1.0f, 0.1f, 2.0f);  
  glUniformMatrix4fv(MVPID, 1, GL_FALSE, &proj[0][0]);
  float red[3] = {1.0, 0.0, 0.0};
  glUniform1fv(colorID, 3, red);
  glGenVertexArrays(1, &vao);
  glBindVertexArray(vao);

  glGenBuffers(1, &buf);
  glBindBuffer(GL_ARRAY_BUFFER, buf);
  glBufferData( GL_ARRAY_BUFFER, sizeof(vec3)*(vertexData.size()), &(vertexData[0][0]), GL_STATIC_DRAW);
  glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, (void *)0 );
  glEnableVertexAttribArray( 0 );
}

the shaders are simple. vshader setting the gl_Position and the fragment shader setting the color to red…

Need an expert eye to spot what’s wrong with the code…

2

I need some more info to know how your data is actually structured.

  1. What exactly does generate_vertexData do? How does it generate the data, some code would be really helpful
  2. How is vertexData defined? I’m assuming it’s an array of arrays by this expression &(vertexData[0][0]), but of what type?
3

Here is the required info:

static vector<vec3> vertexData;

float F(float u, float v) {return 0.5*sin(8.0*M_PI*u)*sin(4.0*M_PI*v);}
void generate_vertexData()
{
	float Maxtable[3*NumCurves];
	for (int i=0; i<3*NumCurves;i++) Maxtable[i]=0.0;
	float dv = 0.5/(NumCurves-1);
	float du = 0.5*dv;
	float v = 0.0;
	for (int t=0; t<NumCurves;t++) {
		float u =0.0;
		for (int k=0;k<2*NumCurves-1;k++) {
			float w = F(u,v);
			float x = u+0.5*v;
			float y = w+0.5*v;
			Maxtable[t+k] = (y>=Maxtable[t+k]?y:Maxtable[t+k]);
			vertexData.push_back(vec3(x,Maxtable[t+k],-1.0f));
			//indxV.push_back(t*(2*NumCurves-1)+k);
			u+=du;
		}
		v+=dv;
	}
/*	for (int k=0; k<2*NumCurves-1; k++) {
		for (int t=0; t<NumCurves; t++) {
			indxU.push_back(k+t*(2*NumCurves-1));
		}
	}
*/
}
6

a vector of vec3 is the type of vertexData

8

I can’t really test generate_vertexData because it has a few formatting issues which I can’t resolve by just guessing.
However, everything else seems to work, I assume the problem is somewhere inside generate_vertexData.
I can give you a tip, try to use simpler points. Instead of generating them, try manually creating a list of points, at positions you recognize.
I.e.

vertexData =
{
    glm::vec3(0.0f, 0.0f, 0.0f),
    glm::vec3(0.25f, 0.0f, 0.0f),
    glm::vec3(0.50f, 0.0f, 0.0f),
    glm::vec3(0.75f, 0.0f, 0.0f),
};

This will help you verify that your initialization is at least correct

9

Actually, your program runs fine. Can you show me your shader code?
Vertex and fragment shader, please

10

Here is the vertex shader:

#version 450

layout( location = 0 ) in vec3 vPosition;
uniform mat4 MVP;
void
main()
{
    gl_Position = MVP*vec4(vPosition.xyz, 1.0);
}

and here is the fragment shader:

#version 450 core

uniform vec3 Color;
out vec4 fColor;

void main()
{
    fColor = vec4(Color.rgb, 1.0);
}
11

Try changing glUniform1fv(colorID, 3, red) to glUniform3fv(colorID, 1, red), in setup function

12

this is how i did it (in 2D, y=f(x))
https://sites.google.com/site/john87connor/graph-plotting/example-y-fx

13

I did, also I had the call generate_vertexData() after allocating the buffer. Now everything is working as expected. Thanks for your help, it was a great asset…

14

Great work, but the problem here is w=f(u,v), that is in 3D and its plot is projected into 2D. u(x axis) moves the plot right(x axis) and v(y axis) moves the plot right and up(both x and y) and w(z axis) moves the plot up (y axis); hence the formula x = u + 0.5v and y=w + 0.5v.