Attic:Old/BlenderDev/OpenEXRFeats

Compression available in OpenEXR

OpenEXR features four state-of-the-art compression algorithms for large, high resolution images. The four are PIZ, ZIP, RLE and Pxr24. PIZ, ZIP and RLE are lossless codecs, but Pxr24 is lossy. ZIP and Pxr24 compress the most, however, of all the algorithms. Take this example:

A Blender scene was rendered at 1024 x 768 resolution and saved into four different OpenEXR image files, each using a different codec. Here are the results:

- No Codec:....10000.0 kilobytes (10 megabytes)
- RLE Codec:...251.0 kilobytes
- PIZ Codec:...108.0 kilobytes
- Pxr24 Codec:.55.7 kilobytes
- ZIP Codec:...55.1 kilobytes

As is fairly obvious here, a codec can significantly reduce the amount of disk space an OpenEXR image takes. And when saving and loading compressed images, there is virtually no overhead, which gives another reason to use compression.

High Dynamic Range

The most obvious benefit of an OpenEXR image is that it is an HDR image. HDR stands for high-dynamic-range. Here is a brief overview of what HDR is: A normal picture (LDR - low-dynamic-range) uses 8 bits to describe each component(R, G, B, or A) in a pixel. For your average low-end picture this is fine. A problem arises, however, with luminance (brightness). Because a normal 8 bit color cannot extend beyond a value 1.0 (white), all brighter luminosities are rounded down to 1.0. This creates problems for images with a high range of luminosity (for example, a photograph). If the image is processed, but all luminosity values over 1.0 are cut off, it could significantly hinder the quality of the image. This is where HDR comes in. Instead of only 8 bits for each component, OpenEXR uses 16 bits. This expands the range of the component allowing luminosities well over 1.0 to be kept. This technique is especially good for post-processing motion pictures, bringing Blender even closer to Hollywood than it ever has been before.

More information about HDR and OpenEXR can be found on Wikipedia.

Versatility

OpenEXR was developed by Industrial Light & Magic, the largest visual effects studio in the USA. This makes it so OpenEXR is a very common format among high-end projects. According to www.openexr.com, the first movies ever to use OpenEXR were the following:

- Harry Potter and the Sorcerer's Stone
- Men in Black II
- Signs
- Gangs of New York

It now remains the most prominent image format among high-end motion picture production, and will continue to grow and expand over the years. You can be assured that with OpenEXR as your main format you will be able to use all the major post-production software with ease.

-- Austin|Benesh - 09 Aug 2005