NeHe Productions: OpenGL Lesson #18
Date de publication : 31/03/2006 , Date de mise à jour : 31/03/2006
Lesson: 18 Quadrics
Quadrics are a way of drawing complex objects that would usually take a few FOR loops and some background in trigonometry.
We'll be using the code from lesson seven. We will add 7 variables and modify the texture to add some variety :)
Okay now move down to InitGL(), We're going to add 3 lines of code here to initialize our quadratic. Add these 3 lines after you enable light1 but before you return true. The first line of code initializes the Quadratic and creates a pointer to where it will be held in memory. If it can't be created it returns 0. The second line of code creates smooth normals on the quadratic so lighting will look great. Other possible values are GLU_NONE, and GLU_FLAT. Last we enable texture mapping on our quadratic. Texture mapping is kind of awkward and never goes the way you planned as you can tell from the crate texture.
Now I decided to keep the cube in this tutorial so you can see how the textures are mapped onto the quadratic object. I decided to move the cube into its own function so when we write the draw function it will appear more clean. Everybody should recognize this code. =P
Next is the DrawGLScene function, here I just wrote a simple if statement to draw the different objects. Also I used a static variable (a local variable that keeps its value everytime it is called) for a cool effect when drawing the partial disk. I'm going to rewrite the whole DrawGLScene function for clarity.
You'll notice that when I talk about the parameters being used I ignore the actual first parameter (quadratic). This parameter is used for all the objects we draw aside from the cube, so I ignore it when I talk about the parameters.
The second object we create is going to be a Cylinder. The first parameter (1.0f) is the radius of the cylinder at base (bottom). The second parameter (1.0f) is the radius of the cylinder at the top. The third parameter ( 3.0f) is the height of the cylinder (how long it is). The fouth parameter (32) is how many subdivisions there are "around" the Z axis, and finally, the fifth parameter (32) is the amount of subdivisions "along" the Z axis. The more subdivisions there are the more detailed the object is. By increase the amount of subdivisions you add more polygons to the object. So you end up sacrificing speed for quality. Most of the time it's easy to find a happy medium.
The third object we create will be a CD shaped disc. The first parameter (0.5f) is the inner radius of the disk. This value can be zero, meaning there will be no hole in the middle. The larger the inner radius is, the bigger the hole in the middle of the disc will be. The second parameter (1.5f) is the outer radius. This value should be larger than the inner radius. If you make this value a little bit larger than the inner radius you will end up with a thing ring. If you make this value alot larger than the inner radius you will end up with a thick ring. The third parameter (32) is the number of slices that make up the disc. Think of slices like the slices in a pizza. The more slices you have, the smoother the outer edge of the disc will be. Finally the fourth parameter (32) is the number of rings that make up the disc. The rings are are similar to the tracks on a record. Circles inside circles. These ring subdivide the disc from the inner radius to the outer radius, adding more detail. Again, the more subdivisions there are, the slow it will run.
Our fourth object is an object that I know many of you have been dying to figure out. The Sphere! This one is quite simple. The first parameter is the radius of the sphere. In case you're not familiar with radius/diameter, etc, the radius is the distance from the center of the object to the outside of the object. In this case our radius is 1.3f. Next we have our subdivision "around" the Z axis (32), and our subdivision "along" the Z axis (32). The more subdivisions you have the smoother the sphere will look. Spheres usually require quite a few subdivisions to make them look smooth.
Our fifth object is created using the same command that we used to create a Cylinder. If you remember, when we were creating the Cylinder the first two parameters controlled the radius of the cylinder at the bottom and the top. To make a cone it makes sense that all we'd have to do is make the radius at one end Zero. This will create a point at one end. So in the code below, we make the radius at the top of the cylinder equal zero. This creates our point, which also creates our cone.
Our sixth object is created with gluPartialDisc. The object we create using this command will look exactly like the disc we created above, but with the command gluPartialDisk there are two new parameters. The fifth parameter (part1) is the start angle we want to start drawing the disc at. The sixth parameter is the sweep angle. The sweep angle is the distance we travel from the current angle. We'll increase the sweep angle, which causes the disc to be slowly drawn to the screen in a clockwise direction. Once our sweep hits 360 degrees we start to increase the start angle. the makes it appear as if the disc is being erased, then we start all over again!
In the KillGLWindow() section of code, we need to delete the quadratic to free up system resources. We do this with the command gluDeleteQuadratic.
Now for the final part, they key input. Just add this where we check the rest of key input.
Thats all! Now you can draw quadrics in OpenGL. Some really impressive things can be done with morphing and quadrics. The animated disc is an example of simple morphing.
Everyone if you have time go check out my website, TipTup.Com 2000 .
GB Schmick ( TipTup )
Jeff Molofee ( NeHe )
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