Yellow Triangle that should be 3 colours OpenGL

Question

This is probably a silly mistake but I cant see it?! I have classes defining geometry and classes that render that geometry. Right now it is basic triangles and colours for each vertex.

Here is the code defining said geometry objects data:

CGeometry* g = new CGeometry();
g->vertexes = new double[3*3];

g->vertexes[0] = 0;
g->vertexes[1] = 0;
g->vertexes[2] = 0;

g->vertexes[3] = 100;
g->vertexes[4] = 100;
g->vertexes[5] = 0;

g->vertexes[6] = 100;
g->vertexes[7] = 0;
g->vertexes[8] = 0;

g->colors = new double[12];

g->colors[0] = 1;
g->colors[1] = 1;
g->colors[2] = 0;
g->colors[3] = 1;

g->colors[4] = 1;
g->colors[5] = 0;
g->colors[6] = 1;
g->colors[7] = 0;

g->colors[8] = 0;
g->colors[9] = 1;
g->colors[10] = 1;
g->colors[11] = 0;

And here is the code that renders said data:

CGeometry* g = object->geometry;

int j = object->endIndex - object->startIndex;
double* vertexes = g->vertexes;
double* colors = g->colors;

glBegin(GL_TRIANGLES);
{
 for(int i = 0; i < j; i++){
  int coord = object->startIndex+i;
  int colorind = coord*4;

  double r,g,b,a;
  r = colors[colorind];
  g = colors[colorind+1];
  b = colors[colorind+2];
  a = colors[colorind+3];

  glColor4d( r,g,b,a);
  glVertex3d( vertexes[coord*3],
     vertexes[coord*3+1],
     vertexes[coord*3+2]);
 }
}
glEnd();

Yet regardless of what I put my triangle is always yellow, or the value of the first color in the colors array. I have gone into the debugger and checked the values on each individual loop iteration and the values of the r g b and a variables do infact change accordingly and are not always yellow, yet the result is a yellow triangle.

Yet if I put the following nabbed from the neheGL tutorials:

glClearColor(0.1f,0.1f,0.1f,1);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // Clear Screen And Depth Buffer
glLoadIdentity();         // Reset The Current Modelview Matrix
//glTranslatef(1.5f,0.0f,0.0f);      // Move Left 1.5 Units And Into The Screen 6.0
glBegin(GL_TRIANGLES);        // Drawing Using Triangles
 glColor3f(1.0f,0.0f,0.0f);      // Set The Color To Red
 glVertex3f( 0.0f, 1.0f, 0.0f);     // Top
 glColor3f(0.0f,1.0f,0.0f);      // Set The Color To Green
 glVertex3f(-1.0f,-1.0f, 0.0f);     // Bottom Left
 glColor3f(0.0f,0.0f,1.0f);      // Set The Color To Blue
 glVertex3f( 1.0f,-1.0f, 0.0f);     // Bottom Right
glEnd();           // Finished Drawing The Triangle
glTranslatef(160.0f,0.0f,0.0f);      // Move Right 3 Units
glColor3f(0.5f,0.5f,1.0f);       // Set The Color To Blue One Time Only
glBegin(GL_QUADS);         // Draw A Quad
 glVertex3f(-1.0f, 1.0f, 0.0f);     // Top Left
 glVertex3f( 1.0f, 1.0f, 0.0f);     // Top Right
 glVertex3f( 1.0f,-1.0f, 0.0f);     // Bottom Right
 glVertex3f(-1.0f,-1.0f, 0.0f);     // Bottom Left
glEnd();

I get a nice blended triangle with 3 colours from each vertex

Answer 1

The alpha values of your second and third color are set to zero, so they are fully transparent. The first color has alpha=1 and is the only one that can be seen in the resulting triangle...

Answer 2

Actually, I think I got it: you're only seeing a small portion of the bottom left corner of your triangle. You need to move away from it to view it entirely: your coordinates are too large.