Are You Using OpenEXR for Your Rendered frames?

Despite the hours that are often spent rendering out frames of 3D animation, many 3D content creators and visualisers are still unsure of the best options when it comes to an outputted file format.

Native Video Formats?

When rendering out large volumes of 3D data I would always advise people to steer clear of animating out to native video files such as .avi or .mpeg. Although valid formats, trouble occurs when you’ve already waited 10 hours for a sequence to render and on 98% completion your system crashes and you have to start the render all over again!

Still Picture Images

The safer approach to adopt is to render out each frame as a “still picture” image. Take the scenario from above: say the system crashes at 98%. If rendering out individual frames then there is no need to start the render from the beginning again. Instead, all that needs to be done is to re-render from frames say, 990 to 1000, leaving you without the hassle of an incomplete or corrupt video file.

This then begs the question of “What image file format should I be using?” There are numerous options here, the first being the good old .jpg. There are many advantages and uses for JPEG images but as far as I’m concerned, 3D is not one of them! Not when you’re producing an animated sequence anyway! The problem with JPEGs is the loss of colour clarity during compression. Now, I hear what you’re saying: “My 3D application allows me to turn off .jpg compression!” Well that might be the case, but does it really eliminate all image compression when writing the file? I don’t think so – I certainly haven’t ever found a piece of software that successfully does this anyway!

Your next route is either TARGA or TIFF, again other popular formats. Although TARGA only offer 8 bits per channel – nowhere near enough for the type of colour quality that is being demanded today – and despite the high-resolution nature of TIFF files, there is a different byte order between Mac and PC. Creating colour conflicts in post-processing on different systems.

Recommended Image File 

A solid choice amongst artists has been the use of 24 bit PNGs. Providing good solid colour depth without any loss as well as being able to store alpha channels directly in the file, PNGs make great files for compositing. As popular as PNGs are there is another file format that really should be used as your standard for all final renders; the OpenEXR.

Created by Industrial Light and Magic (the guys behind Narnia and Pirates of the Caribbean) and around since 2003, OpenEXR is a 32 bit per channel file format with unlimited channels. It’s a high dynamic range imaging file format and is available to use under a free software licence. Key to OpenEXR’s success is its 32 bit per channel; essentially the greater the bits the more colours you can have in your image and the better your image will look.

Increased colour ranges make for an easier job in post-production or when colour grading, with simple yet accurate correction of variables such as exposure, hue / saturation or brightness / contrast. Again, because of increased colour information, OpenEXR permit more channels than your standard Red, Green, Blue and Alphas. In fact you have the choice of any number of channels, opening up the possibilities of rendering out things like diffuse, specula or shadows, for example, to their own buffers within one HDR image. These options give greater flexibility and increased control.

The Verdict

In short, for flexibility and colour clarity OpenEXR really should be your file format of choice when it come to final renders, with the option to use PNG as a back up when compatibility or size is an issue.

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