EXR is a high dynamic range format – the best format to use when working with 32-bit HDR imagery. It supports several compression modes that can be either lossless or lossy. Here are the main compression algorithms, including two different methods of Zip compression practiced by EXR and supported by ImageConverter Plus:
None—disables all compression.
Run Length Encoding (RLE)—a basic form of compression which is lossless.
Zip (per scanline)—zip-style compression applied to individual scanlines.
Zips (16 scanline blocks)—zip-style compression applied to blocks of 16 scanlines at time. This tends to be the most effective style of compression to use with rendered images that do not have film grain applied.
PIZ ( wavelet compression )—uses a combined wavelet/Huffman compression. This form of compression is best for grainy images.
For most images without a lot of grain, the two Zip compression methods seem to work best, while the PIZ compression algorithm is better suited to grainy images. PIZ does not work well for tiled files, unless the tiles are fairly large – maybe 256×256 or 512×512 pixels. For smaller tiles we recommend that you use ZIP or PXR24. If you deal with a situation when your computer won’t open an EXR file the best advice we can give is to convert your EXR image to JPEG or other image format in order to make it more universally recognizable. Compressing an image with a lossless method preserves the image exactly: The pixel data are not altered. Compressing an image with a lossy method preserves the image only approximately: image quality can degrade with multiple saves.