Disk cache formats in ArcGlobe

ArcGlobe supports two disk cache formats: JPEG and DXT.

The data in ArcGlobe caches are compressed to reduce disk space. The default compression applied is JPEG, with 16 Bit color format. DXT is an alternative format that is supported by graphics card hardware. This overview should help you determine which format will be best for your data.

What are the differences between the JPEG and DXT formats?

On machines with recent graphics cards, DXT caches do not need to be uncompressed prior to rendering. JPEG caches, however, do need to be uncompressed before rendering and therefore have a performance overhead.

JPEG cache data with 16 Bit color format requires 2 Bytes per texel in memory, while DXT cache data requires only 1 Byte per texel in memory. This means that only half as much graphics memory is used by DXT data when compared to JPEG 16 Bit color data.

DXT caches are generally 8 to 12 times larger than JPEG caches on disk.

When should I use JPEG and when should I use DXT?

If your machine has an older graphics card it may not natively support the DXT format. In these cases, ArcGlobe will use emulation software to support DXT caches, instead of the hardware, and there will be no performance benefits in using the DXT disk cache option.

Newer machines will include hardware support of the DXT format and are therefore candidates for using the DXT cache option. This choice is more important if application performance is a concern.

One key factor in determining the performance differences between JPEG and DXT caches is the extent of your data. DXT caches can render up to 40% faster (in frames per second) than JPEG caches on image data with local area extents. However, there is little difference between the two formats when the data has a global extent. The cache generation time is similar for the DXT and JPEG disk cache formats, regardless of the data extent.

Another key factor when deciding between the formats is the amount of physical memory that your machine has. ArcGlobe will consume between 10% and 30% less overall memory when using DXT caches instead of JPEG caches, which translates into better performance if you are rendering large datasets and have limited memory (RAM and graphics card texture memory) on your machine.

The third and final factor to consider is disk space. Although DXT caches generally provide better interactive performance than JPEG caches, they also consume significantly more storage disk space. Depending upon the data, a DXT cache can be between 8 and 12 times larger on disk than the JPEG equivalent. The trade-off between better performance and larger disk space requirements has to be considered.

Special considerations when using a DXT cache?

DXT is dependent on the OpenGL graphics driver and graphics card hardware. Occasionally, some OpenGL implementations may lack or incorrectly support DXT compression. This could introduce artifacts in DXT caches. Updating to a newer graphics card driver or using a different graphics card tends to fix these issues.

Does changing the setting invalidate my existing cache?

Yes. Selecting a different cache format entails re-creating the entire cache in the selected format.