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This chapter is from the book

This chapter is from the book

Setting the Point Size

When you draw a single point, the size of the point is one pixel by default. You can change this size with the function glPointSize:

void glPointSize(GLfloat size);

The glPointSize function takes a single parameter that specifies the approximate diameter in pixels of the point drawn. Not all point sizes are supported, however, and you should make sure the point size you specify is available. Use the following code to get the range of point sizes and the smallest interval between them:

GLfloat sizes[2];  // Store supported point size range
GLfloat step;     // Store supported point size increments


// Get supported point size range and step size
glGetFloatv(GL_POINT_SIZE_RANGE,sizes);
glGetFloatv(GL_POINT_SIZE_GRANULARITY,&step);

Here, the sizes array will contain two elements that contain the smallest and largest valid value for glPointsize. In addition, the variable step will hold the smallest step size allowable between the point sizes. The OpenGL specification requires only that one point size, 1.0, be supported. The Microsoft software implementation of OpenGL, for example, allows for point sizes from 0.5 to 10.0, with 0.125 the smallest step size. Specifying a size out of range is not interpreted as an error. Instead, the largest or smallest supported size is used, whichever is closest to the value specified.

Points, unlike other geometry, are not affected by the perspective division. That is, they do not become smaller when they are further from the viewpoint, and they do not become larger as they move closer. Points are also always square pixels, even if you use glPointSize to increase the size of the points. You just get bigger squares! To get round points, you must draw them antialiased (coming up in the next chapter).

OpenGL State Variables

As we discussed in Chapter 2, OpenGL maintains the state of many of its internal variables and settings. This collection of settings is called the OpenGL State Machine. You can query the State Machine to determine the state of any of its variables and settings. Any feature or capability you enable or disable with glEnable/glDisable, as well as numeric settings set with glSet, can be queried with the many variations of glGet.

Let's look at a sample that uses these new functions. The code in Listing 3.3 produces the same spiral shape as our first example, but this time, the point sizes are gradually increased from the smallest valid size to the largest valid size. This example is from the program POINTSZ in the CD subdirectory for this chapter. The output from POINTSZ shown in Figure 3.4 was run on Microsoft's software implementation of OpenGL. Figure 3.5 shows the same program run on a hardware accelerator that supports much larger point sizes.

Figure 3.4Figure 3.4 Output from the POINTSZ program.

Figure 3.5Figure 3.5 Output from POINTSZ on hardware supporting much larger point sizes.

Listing 3.3 Code from POINTSZ That Produces a Spiral with Gradually Increasing Point Sizes

// Define a constant for the value of PI
#define GL_PI 3.1415f


// Called to draw scene
void RenderScene(void)
  {
  GLfloat x,y,z,angle;  // Storage for coordinates and angles
  GLfloat sizes[2];  // Store supported point size range
  GLfloat step;    // Store supported point size increments
  GLfloat curSize;  // Store current point size
  ...
  ...


  // Get supported point size range and step size
  glGetFloatv(GL_POINT_SIZE_RANGE,sizes);
  glGetFloatv(GL_POINT_SIZE_GRANULARITY,&step);

  // Set the initial point size
  curSize = sizes[0];

  // Set beginning z coordinate
  z = -50.0f;

  // Loop around in a circle three times
  for(angle = 0.0f; angle <= (2.0f*GL_PI)*3.0f; angle += 0.1f)
    {
    // Calculate x and y values on the circle
    x = 50.0f*sin(angle);
    y = 50.0f*cos(angle);

    // Specify the point size before the primitive is specified
    glPointSize(curSize);

    // Draw the point
    glBegin(GL_POINTS);
      glVertex3f(x, y, z);
    glEnd();

    // Bump up the z value and the point size
    z += 0.5f;
    curSize += step;
    }
  ...
  ...
  }

This example demonstrates a couple of important things. For starters, notice that glPointSize must be called outside the glBegin/glEnd statements. Not all OpenGL functions are valid between these function calls. Although glPointSize affects all points drawn after it, you don't begin drawing points until you call glBegin(GL_POINTS). For a complete list of valid functions that you can call within a glBegin/glEnd sequence, see the reference section at the end of the chapter.

If you specify a point size larger than what is returned in the size variable, you also may notice (depending on your hardware) that OpenGL uses the largest available point size but does not keep growing. This is a general observation about OpenGL function parameters that have a valid range. Values outside the range are clamped to the range. Values too low are made the lowest valid value, and values too high are made the highest valid value.

The most obvious thing you probably noticed about the POINTSZ excerpt is that the larger point sizes are represented simply by larger cubes. This is the default behavior, but it typically is undesirable for many applications. Also, you might wonder why you can increase the point size by a value less than one. If a value of 1.0 represents one pixel, how do you draw less than a pixel or, say, 2.5 pixels?

The answer is that the point size specified in glPointSize isn't the exact point size in pixels, but the approximate diameter of a circle containing all the pixels that are used to draw the point. You can get OpenGL to draw the points as better points (that is, small filled circles) by enabling point smoothing. Together with line smoothing, point smoothing falls under the topic of antialiasing. Antialiasing is a technique used to smooth out jagged edges and round out corners; it is covered in more detail in Chapter 6, "More on Colors and Materials."

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