Flickering in Time-Lapse HDR IBL

I often mention flickering as part of our time-lapse HDRI sky dome products – I don’t want anyone to have any unpleasant surprises prior to purchasing, and I’d like people to know what to expect if they choose a time-lapse that features flickering, so they can factor it into their schedule and workflow. So with that said, lets first have a look at a worst case scenario for a flickering scene using the footage for the full day time-lapse. If you watch the CG animation in the top right you’ll see flickering pretty much as bad as it can get with large, flat, bright surface areas of diffuse material, filling half of the frame. Notably, you can see the flickering stops around 11 seconds as the clouds thin out:

and here’s another test render of mine with Afternoon2Night01¬†that features some flickering, but not as harsh. I actually think it adds something in this case:

What is it exactly?

There are two types and causes of flickering, most assume its the first –

  1. When shooting time-lapse photography (the normal, non HDR kind) with auto settings or anything but wide-open aperture, each frame will be slightly lighter or darker due to slight differences in aperture diameter and the camera choosing different shutter speeds. The time-lapse HDRIs were not shot with auto settings, and there is no flicker due to aperture either.
  2. In time-lapse HDR image based lighting, normally slow changes in cloud cover and lighting conditions are greatly sped up, causing not slow subtle changes in brightness, but fast changes from frame to frame.

This short article will cover number 2 exclusively. Again to re-iterate and state this as simply as possible:

Flickering in IBL animations using time-lapse HDRI is caused by normally slow changes in light levels and cloud cover to be sped up greatly

So in a cloudy sky you would normally (in real-time) notice the sun going in and out of cloud cover, the strength of the sun slowly rising and falling as it is concealed and revealed. If we instead photograph the sky once per minute or so, we can get a situation where in frame A, the sun is completely behind cloud, then frame B shows a bright, fully visible sun. If this continues on, in a worst case scenario, lighting will go from bright to dull every other frame, creating a strobing, flickering effect.

This flickering effect is mostly visible in the time-lapses “Full Day” and “Half Day” as they were both shot on cloudy days with roughly 1 minute intervals. In the other skies, the effect is far more subtle and more short lived, and you can see preview renders of how the lighting/flickering looks in the videos below. Note that the sky itself will never flicker, just the scene due to changes in cloud cover:

Is it “bad”?

Some testers and customers have been put off by the effect, others quite like it and think it looks realistic. I believe you’ll find that depending on your scene composition and material usage you may or may not find it a problem. For example, a scene composed featuring little sky, with large open plains, the lighting will be clearly visible and affect most of the frame. On the other hand if half of your composition is water or a darker/less diffuse material, glass, etc, and the other half of the frame is the sky itself, then the flickering effect is less noticeable. You can always judge for yourself and test it in your own scenes by trying out the full day sample footage, available here.

How do I reduce it?

There are a number of ways you can fix or reduce the effect, if you need/want to. The simplest would be clamping the sun or adjusting gamma to give you less intense lighting. This is an ok solution as its quick and doesn’t affect light colors much, but you’ll probably need to adjust other parts of your lighting solution. On the other end of the spectrum, you can just use the time-lapse for ambient lighting (clamp the sun) and put in your own virtual sun. Another technique that may help is tonemapping, this would work in a similar manner to gamma adjustment where the changes in lighting are lessened slightly. You could even try de-flickering the end render result, though I didn’t have a huge amount of success with this.

Virtual sun settings

If you choose to clamp and replace or compliment the sun light in the HDRI with a virtual sun, and are using any form of daylight solution, you may be able to plugin the following lat/long settings and dates. As I’ve sun aligned the sun to always set in the center of the frame, you’ll probably need to rotate it to match. I’ve had a customer already have success with this.

The time-lapses were shot in Adelaide, South Australia, so the lat/long settings are:
Latitude: 34.9 S
Longitude: 138.6 E

They were shot on the dates:
FullDayCloudy: 29/11/2013
HalfDayCloudy: 10/11/2013

If you tried the samples or purchased one of the full-day or half-day time-lapses (or tried the samples) I’d be keen to know what approach you took in the comments. At some stage I hope to have a more in-depth page perhaps with tutorials and/or making-ofs that show an exact workflow.