OpenGL® colors may have unexpected results due to color depth, shading mode, lighting, materials, and the alpha channel. Proper variable settings and understanding of attributes can avoid frustration.
On the surface, and in many basic applications, OpenGL® colors appear to be very easy to use. There are situations and subtleties in the renderer that can sometimes be overlooked, however, causing unexpected results in a scene. These can be caused by color depth or hardware settings. Other times, simple things like the shading mode can be overlooked and the results of a render will be unexpected. Finally, the more complex mechanisms that operate on OpenGL® colors, such as lighting and materials, can drastically affect the appearance of the final output.
One thing to be sure of in a program that uses OpenGL® is that all the correct variables are set. This means making sure the shading mode is set to flat or smooth as needed before rendering. The color order should also be set to the expected value. While nearly all programs use a red, green, and blue (RGB) order, there are image formats and other factors where it might be easier to reverse the order and use a blue, green, and red (BGR) pattern.
A sometimes overlooked and sometimes difficult element of OpenGL® color is color depth. Although a program you are developing may be created to use the color depth of your development systems monitor, the depth of another user’s monitor may not be the same. This means that if a program is developed with a 24-bit color depth and runs on a system with a 16-bit color depth or an indexed color model, the scene will not render correctly and it may appear that the program not working. Establishing and adjusting the depth of color available can avoid this type of frustration.
Two areas of OpenGL® colors that have a large range of shades that can produce undesirable results are lighting systems and materials. When using lighting, it is important to take into account how light will interact with an object and the color potential of a light, if any. With improper placement and settings, a light source can wash colors or alter their appearance. Experimentation may be the only way to find the right balance between preserving the mood of a scene and accurately rendering colors.
The material system can completely change OpenGL® colors on the surface of an object. Setting the Ambience, Diffuse, or Specular values too high can result in objects being rendered completely white, while other settings can render objects black or invisible. The light source settings for a scene can also complicate the material system. When using materials, many problems can be avoided by knowing in detail what each attribute does.
Finally, it can be important to understand the alpha channel used in OpenGL® colors. This is a fourth value in addition to the RGB values of a color that determines the transparency of an object. Many programmers get frustrated trying to set the alpha value to create a translucent object, only to find out much later that the color of the diffuse attribute is only used to determine the alpha value of an entire object. The drawing order of the objects, regardless of subsequent transformations, also determines the behavior of the alpha value. This means that objects that are transparent and thus transformed into a position in front of a later drawn object will not show the object behind it due to the render order.
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