Question

I am trying to change a C++ project, which is currently drawing some lines when I click on the view port. This functionality is perfectly fine, but what I am trying to change is when I click on “UP” or “Down” keys the color for next lines to change. Currently if I click on those keys the color changes for all the lines including the old ones (already drawn).

Please give me an idea of what to do. Here is some of the code:

void drawPrimitive() {
Vertex *temp;

//  Set the primitive color
glColor3fv(primitiveColor);

//  Set the point size in case we are drawing a point
if (type == POINT)
    glPointSize(pointSize);

//  Display results depending on the mode
glBegin(mode);
    for(temp = head; temp != NULL; temp = temp->np)
    {
        if (smoothShading)
            glColor3f(temp->r, temp->g, temp->b);

        glVertex2f(temp->x, temp->y);
    }
glEnd(); }


void mouse(int button, int state, int x, int y) {
if(button == GLUT_LEFT_BUTTON && state == GLUT_DOWN)
{
    float pointX, pointY;

    pointX = (float)x/window_width * world_width;
    pointY = (float)(window_height - y)/window_height * world_height;

    //  Add a vertex to the list of vertices...
    addVertex(&head, &tail, pointX, pointY, 0.0f, primitiveColor[0], primitiveColor[1], primitiveColor[2]);

    //  automatically calls the display function
    glutPostRedisplay();
}
else if(button == GLUT_MIDDLE_BUTTON && state == GLUT_DOWN)
{
    deleteVertex(&head, &tail);
    glutPostRedisplay();
} }


void special(int key, int x, int y) {
switch (key)
{
    //  change primitive color
    case GLUT_KEY_UP :
        changePrimitiveColor(1);
        break;
    case GLUT_KEY_DOWN :
        changePrimitiveColor(-1);
        break;
}

glutPostRedisplay(); }


void changePrimitiveColor(int step) {
primitiveColorId += step;

if (primitiveColorId < 0)
    primitiveColorId = COLOR_COUNT - 1;

if (primitiveColorId >= COLOR_COUNT)
    primitiveColorId = 0;

setColor(primitiveColor, primitiveColorId); }

Answer 1

Your code is sort of unclear; is primitiveColor a global variable?

Assuming you’re calling drawPrimitive() for all your lines each redraw, the same primitiveColor would be used for all the lines. As soon as you change the color when you press up or down, the redisplay function is called, and all the lines will be redrawn using the same color.

What you might want to do is have a list containing both the primitives and their respective colors. When you iterate through this list, you can set a color for each line.