More Uses For Alpha Maps

Introduction
In order to show some of the aspects of Alpha Maps and how they can be used to control an object's surface we are going to make a chrome whiffle ball. A whiffle ball is a small light plastic ball with round holes cut in it.

Step One
In the modeller add a ball. Using the numeric requester make the sides and segments both 32. Ensure that the center is 0m,0m,0m for XYZ and radii is 10cm, 10cm, 10cm for XYZ. Make the sphere and in the Change Surface panel make the surface name ball and check the double-sided and smooth check boxes. Finally, save the object as WhiffleBall.lwo.

Step Two
In layout add the WhiffleBall.lwo object, bring up the surface panel and ensure the current surface is ball. Initially lets create a basic chrome surface so set the Color to 120,130,150 for red, green and blue. Then set the Luminosity to 40%, the Diffuse to 0%, the specularity to 150%, the Glossiness to 16 and the Reflectivity to 75%. Set the Color Highlights to 100% and set the Reflect Options to Raytrace and Backdrop. To give the chrome something to reflect go to the Effects Panel and check the Gradient Backdrop check box. Now, add IFW2_Gradient and select Dimples. Set the size to 30mm, 30mm, 30mm for XYZ and the Bump to 0.0. To get the right spot layout right it is usually best to do a color test so uncheck the Do Blend check box and set Blend1 = 80% and all other blends to 100%. By turning off the blending we have forced the texture not to interpolate the colors (and ultimately the alpha map) so that we have a sharp transition. Finally, set Color 1 to Yellow and Color 2 to Blue. The figure on the left shows what the ball should now look like.

Step Three
In order to make the whiffle ball holes we need to make the spots which are currently yellow transparent. To do this we are going to use an alpha map. This is basically the same as the color map except it has single values rather then colors. These values will be interpolated against the texture function in the same way as the color map. This can then be used to control all surface parameters using the controls under the Affect Tab. The first thing to do is enable the Transparency so, under the Affect Tab, enable the check box next to the transparency. Now to make the holes transparent we want to make the alpha map full on (transparent) where the color is yellow and turned off (opaque) where the color is blue. So make Alpha 1 = 100% and all the other Alphas 0%. Notice how the alphas line up to the colors. Also turn off the application of color by unchecking all eight color affect check boxes. The figure on the left shows what the ball should now look like.

Step Four
As you can see the texture is not quite there yet as the holes are still reflective and have a specular highlight. What we want to do is use the alpha map to switch these off in the holes. The problem is we only have one alpha map. However, this can be overcome by making the alpha map control these parameters but inverting them where appropriate. This can be done by enabling both the Reflectivity and Specularity on the Affect section but making their values -75% and -100% respectively. By making them negative it has the same affect as inverting the alpha map. The figure on the left shows what the completed ball should be like. The final scene is shown at the top of the tutorial page.


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