SAMPLE: Demonstrates OpenGL Texture-Mapping Capabilities

ID: Q160817

The information in this article applies to:

• Microsoft Win32 Application Programming Interface (API) for Windows NT & Win95, version 4.00

SUMMARY

GLTEXTUR.EXE provides a demonstration of how to use a Device-independent Bitmap (DIB) as a texture-map for OpenGL by pasting a DIB (chosen by the user) onto three different OpenGL objects.

GLTex allows you to modify texture settings so you can see quickly and easily the visual effect created.

Please see the REFERENCES section of this article for more information on resources.

MORE INFORMATION

The following file is available for download from the Microsoft Software Library:

 ~ GLTEXTUR.EXE (size: 91394 bytes) 

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The new GL_BGRA_EXT image format makes using DIBs as textures easy because BGR is the same ordering used for DIBs. For convenience's sake, GLTex converts all DIBs it reads into 32-bpp DIBs (by calling StretchDIBits). This makes the DIB handling and texture handling much cleaner throughout the sample. OpenGL textures require that the image used be of size 2^n by 2^n. GLTex's ScaleDIB function performs the conversion to 32-bpp and scales the image to a square one of size 2^n by 2^n.

When the OpenGL scene is rendered, three simple three-dimensional shapes are drawn with the converted and scaled texture mapped onto *each* face. This mapping is handled (initially) by the glTexCoord function, which is called as the shape vertices are specified. The drawing of the shapes is handled by GLTex's BuildShapes function.

Settings

Once you have loaded a DIB and see how it maps to the shape faces, GLTex allows you to change the texture mapping settings in order to see how these changes would effect the image. The following list gives modifications that GLTex allows you to make, a brief description of their effect, and explains which functions are used to change them:

• "Scene Distance from Viewer" allows you to push the scene farther away or bring it closer. This is achieved with glTranslate.
• "Shape Rotation" allows you to modify the angle of rotation around the x, y, and z axes. This is achieved with glRotate for each axis.
• "Light Position" allows you to move the single light source used. Note that by changing the w coordinate, you can modify whether you are using a directional (w = 0.0) or positional (w = 1.0) light source. This is achieved with glLight(...GL_POSITION...).
• "Texture Mode" allows you to use two of the three texture modes available with OpenGL. Decal mode essentially means that the texture is applied directly to the object without any calculation of material properties or lighting. Modulate mode blends the underlying object (that is effected by material properties and lighting) with the texture. This gives the effect of a lighted texture. The third possibility, Blending, doesn't make sense for a three-component (RGB) image. This is achieved with glTexEnv.
• "Perspective Hint" should be changed to Nicest if the texture appears to be projected incorrectly. You should try it anyway because it will allow you to see the difference. This is achieved with glHint.
• "Minification Filter" controls how a screen pixel should be mapped to a collection of texels (texture elements). This is achieved with glTexParameter(...GL_TEXTURE_MIN_FILTER...).
• "Magnification Filter" controls how a screen pixel should be mapped to a portion of a texel (texture element). This is achieved with glTexParameter(...GL_TEXTURE_MIN_FILTER...).
• "Texture Wrap S" and "Texture Wrap T" allow you to specify whether the image should repeat in the S and T directions. (These can be thought of as the X and Y directions in the texture's world.) This is achieved with glTexParameter(...GL_TEXTURE_<S or T>_WRAP...).
• "Texture Coordinates" specifies how the texture is mapped to the object. Because 0.0 is the default for the first parameter, one side of the texture is always mapped to the edge of the cube's face. This should be used in conjunction with Texture Wrap S and T for repeating the texture across the cube's faces. This is achieved with glTexCoord.
• "Sphere Complexity" allows you to modify the number of faces used to create the sphere. Initially the complexity is 0, which is a very rough 20-sided approximation of a sphere. You can increase this to a point, but keep in mind the complexity increases rapidly and can slow down the rendering (especially when spinning the objects). See GLSphere.c for the details.
• "Transparency" allows you to choose an RGB color that you would like to treat as transparent for the chosen texture. An alpha function is set up to let you see through the texture onto the underlying object where this color exists. Because it is difficult to chose one exact color to use, you can modify how exact the color-matching should be. No variance means only the exact RGB color is made transparent, while total variance means all colors are treated as transparent.
• "S Generation" and "T Generation" allow you to instruct OpenGL to ignore the texture coordinates specified for the individual shape faces by GLTexCoord, and generate textures based on a mode (Object Linear, Eye Linear, or Sphere Map) and a plane (specified by X, Y, Z, W). You can achieve some interesting effects quickly by playing with texture coordinate generation. Note that you will probably want to keep "Texture Wrap S" and "Texture Wrap T" set to repeat. Also, spinning the objects is often the easiest way to see what has been affected.
• "Spin!" speaks for itself. Keep in mind that the "Shape Rotation" setting is a constant that "Spin!" doesn't modify. Also, depending on the speed of your processor and video card, the spinning objects can become quite slow if you have lots of effects turned on at once.

REFERENCES

NOTE: Most of the following references can be found on the Microsoft Developer Network (MSDN) Developer Library CD-ROM. Some of these references are published by publishers independent of Microsoft; we make no warranty, implied or otherwise, regarding the reliability of these resources.

Some code for this GLTex sample was borrowed from the ShowDIB sample for DIB-handling (Dib.c) and from Dale Rogerson's articles/samples for OpenGL palette creation (Glpalette.c). The DIB code has been enhanced in order to handle the loading of 16-bpp and 32-bpp DIBs.

For more information on the settings and OpenGL in general, please see the following resource:

• Neider, Jackie, Tom Davis, and Mason Woo. OpenGL Programming Guide: The Official Guide to Learning OpenGL, Release 1. Reading, MA: Addison- Wesley, 1993. ISBN 0-201-63274-8. (This book is also known as the "Red Book.")

• Rogerson, Dale. "OpenGL V: Translating Windows DIBs." January 1995. (Development Library, Technical Articles)
• Rogerson, Dale. "OpenGL VII: Scratching the Surface of Texture Mapping." April 1995. (Development Library, Technical Articles)

• Gery, Ron. "DIBs and Their Use.". March 1992. (Development Library, Technical Articles)
• Gery, Ron. "Using DIBs with Palettes." March 1992. (Development Library, Technical Articles)

Keywords          : kbNTOS400 kbWinOS95 kbSDKWin32 kbsample
Version           : 4.00
Platform          : NT WINDOWS

Last Reviewed: June 15, 1999