OpenGL Programming Guide

Chapter 5Color

Specifying a Color and a Shading Model

Specifying a Shading Model

A line or a filled polygon primitive can be drawn with a single color (flat shading) or with many different colors (smooth shading, also called Gouraud shading). You specify the desired shading technique with glShadeModel(). void glShadeModel (GLenum mode);

Sets the shading model. The mode parameter can be either GL_SMOOTH (the default) or GL_FLAT.

With flat shading, the color of one vertex of a primitive is duplicated across all the primitive's vertices. With smooth shading, the color at each vertex is treated individually. For a line primitive, the colors along the line segment are interpolated between the vertex colors. For a polygon primitive, the colors for the interior of the polygon are interpolated between the vertex colors. Example 5-1 draws a smooth-shaded triangle, as shown in Figure J-11 .

Example 5-1 : Drawing a Smooth-Shaded Triangle: smooth.c

#include <GL/gl.h>
#include <GL/glu.h>
#include "aux.h"

void myinit (void)
{
    glShadeModel (GL_SMOOTH);   /* GL_SMOOTH is the default */
}

void triangle(void)
{
    glBegin (GL_TRIANGLES);
        glColor3f (1.0, 0.0, 0.0);
        glVertex2f (5.0, 5.0);
        glColor3f (0.0, 1.0, 0.0);
        glVertex2f (25.0, 5.0);
        glColor3f (0.0, 0.0, 1.0);
        glVertex2f (5.0, 25.0);
    glEnd ();
}

void display(void)
{
    glClear (GL_COLOR_BUFFER_BIT);
    triangle ();
    glFlush ();
}

void myReshape(GLsizei w, GLsizei h)
{
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    if (w <= h) 
        gluOrtho2D (0.0, 30.0, 0.0, 
             30.0 * (GLfloat) h/(GLfloat) w);
    else 
        gluOrtho2D (0.0, 30.0 * (GLfloat) w/(GLfloat) h, 0.0,
             30.0);
    glMatrixMode(GL_MODELVIEW);
}


int main(int argc, char** argv)
{
    auxInitDisplayMode (AUX_SINGLE | AUX_RGBA);
    auxInitPosition (0, 0, 500, 500);
    auxInitWindow (argv[0]);
    myinit();
    auxReshapeFunc (myReshape);
    auxMainLoop(display);
}

With smooth shading, neighboring pixels have slightly different color values. In RGBA mode, adjacent pixels with slightly different values look similar, so the color changes across a polygon appear gradual. In color-index mode, adjacent pixels may reference different locations in the color-index table, which may not have similar colors at all. Adjacent color-index entries may contain wildly different colors, so a smooth-shaded polygon in color-index mode can look psychedelic.

To avoid this problem, you have to create a color ramp of smoothly changing colors among a contiguous set of indices in the color map. Remember that loading colors into a color map is performed through your window system rather than OpenGL. For the moment, however, assume you have an auxSetOneColor() routine that loads a single index in the color map with specified red, green, and blue values. The first argument for this routine is the index, and the others are the red, green, and blue values. To load thirty-two contiguous color indices (from color index 16 to 47) with slightly differing shades of yellow, you might call

for(i=0; i<32; i++){
   auxSetOneColor(16+i, 1.0*(i/32.0), 1.0*(i/32.0), 0.0);
}

Now, if you render smooth-shaded polygons that use only the colors from index 16 to 47, those polygons have gradually differing shades of yellow.

With flat shading, the color of a single vertex defines the color of an entire primitive. For a line segment, the color of the line is the current color when the second (ending) vertex is specified. For a polygon, the color used is the one that's in effect when a particular vertex is specified, as shown in Table 5-2 . The table counts vertices and polygons starting from 1. OpenGL follows these rules consistently, but the best way to avoid uncertainty about how a flat-shaded primitive will be drawn is to specify only one color for the primitive.

Table 5-2 : How OpenGL Selects a Color for the ith Flat-Shaded Polygon

Type of Polygon	Vertex Used to Select the Color for the ith Polygon
single polygon	1
triangle strip	i+2
triangle fan	i+2
independent triangle	3i
quad strip	2i+2
independent quad	4i

OpenGL Programming Guide