The HOOPS/3dGS provides extensive texture mapping capabilities, and takes advantage of underlying 3D hardware acceleration whenever possible. The texture functionality is flexible and easy to use, and is documented in Section 5.2 of the HOOPS/3dGS Programming Guide. This article reviews some key texture topics.

How to Enable Texture Mapping

Use of textures requires the following steps:

1. Insert a named image into the scene-graph using HC_Insert_Image.
2. Use HC_Define_Texture to create a texture using the image.
3. Assign texture-coordinate parameters to the shell or mesh. These control the mapping between points in the object and points in the texture.
4. Specify a parameterization source. Refer to HC_Define_Texture to see a list of valid sources. (The default is 'uvw')
5. Use the texture as the color of the object; for example, HC_Set_Color("faces = (diffuse = my_new_texture)");
6. Make sure that the "texture interpolation" Rendering_Option is enabled (it is enabled by default).

Applying Multiple Textures Per Color Channel

Different texture maps can be used for different components of an object’s color. For example, you could use one texture for the diffuse color, and another texture for the specular color. See the HC_Set_Color command for more information.

Local and Global Textures

The usage of HC_Define_Texture described aboves makes the texture a 'global' texture. (Think of HC_Define_Texture as a regular #define) You can't have 'global' textures with the same name in different parts of the scene-graph. However, you can use HC_Define_Local_Texture to make the texture 'local'. This setting makes the texture scoped to a segment and thus allows you to have textures with the same name in different parts of the scene graph.

Image Downsampling and Textures

When a user is inserting an image to be used as a texture, it is not uncommon for it to be of a non-uniform size, or larger than necessary to adequately depict the material of interest. Therefore, when an image is used create a texture, HOOPS will automatically ‘down-sample’ the image to a power of 2 and usually to a size of 512x512. An image with these properties helps ensure optimum performance, particular when the display driver is set to OpenGL. In most cases, down-sampling the image is acceptable if the texture is being used to depict some material property of an object.

Draping Imagery over Terrain Data

More and more HOOPS customers are interested in draping satellite imagery over terrain data. This is achieved by converting the imaging information to a HOOPS/3dGS image, defining a texture, and applying that texture to a HOOPS/3dGS mesh primitive that represents the terrain data. If the terrain data is non-uniform, then a HOOPS/3dGS shell must be used.

The starting images can sometimes be quite large, since a significant amount of resolution is desirable in the final rendering. Therefore, HOOPS/3dGS’ automatic down-sampling (discussed in the previous section) can result in significant loss of critical image resolution.

The "no downsample" Define_Texture option is designed to address the requirement of preserving the full image data when texture mapping it to a shell/mesh. When this option is set, HOOPS/3dGS will use the largest power of 2 size that both fits the image data and the limits of OpenGL. Hardware and software OpenGL both have limits, and HOOPS/3dGS will stay within the limit.

The fact that HOOPS/3dGS stays within image-size limits means that image resolution could still be lost of the limit has been reached. To help ensure that image resolution is never lost, the application could chop up the image into tiles and remap them to subsections of the shell/mesh. This functionality may be added to a future release of HOOPS/3dGS. Additionally, a HOOPS/3dGS option may be added in future releases which instructs HOOPS/3dGS to always use the full-sized image, and fallback to software rendering of the 3D hardware can’t handle the large image (however, this would result in a significant performance drop since no 3D HW acceleration would be used).

Currently, texture parameters must be explicitly set when you wish to drape an image to a HOOPS/3dGS mesh. Future releases of HOOPS/3dGS may include an implicit ‘drape’ parameterization source option. This would instruct HOOPS/3dGS to drape a texture across an entire mesh by automatically setting the texture parameters.