Please note, this doc contains strobing items with a strobing effect that may affect photosensitive readers.
As Netflix Original Productions grow throughout the world, we aim to provide a stunning user experience, which includes being mindful of user safety. This document aims to outline how Photosensitive users may be affected by flashing or strobing material in your content, and the steps that can be taken to avoid harmful flashes and QC failures during the delivery process.
Since we cannot guarantee safety in all scenarios, and are committed to creative freedom in storytelling, we are offering this document as a best practice guideline to help minimize photosensitivity issues with our audiences.
What is Photosensitivity?
Photosensitivity is any negative physical effect that is caused or aggravated by strobing, flashing or quickly changing patterns, luminance or chrominance. It can lead to headaches, eyestrain, nausea and, in some users, seizures. Further information can be found here via the Epilepsy Foundation
Why does Netflix care about Photosensitivity in content?
We want to provide a stunning viewing experience for all users, on all devices, in any viewing setting. Limiting the occurrence of flashes or patterns that could cause ill-effects in viewers is a part of that. Our delivery specifications currently only state that Animated content must be tested. However, we are also taking a closer look at VFX heavy programs like science fiction titles as well as live concerts that might feature flashing or strobe effects. We feel that avoiding photosensitivity issues will provide a greater experience to all our users, regardless of content type.
Is my content required to pass a photosensitivity test?
Our current delivery specifications only require testing for Animated shows. While other content is not required to pass a photosensitivity test, if a test is applied and the content fails, a visual warning during playback may be applied for our audiences, allowing them to make an informed decision on how and what they watch. Even in content other than Animation, flashing material that is found to be particularly risky may be tested and a warning applied. Our goal is to avoid issues like these, and in later sections of this document, we offer suggestions on how to do just that.
What elements in a piece of content may aggravate photosensitivity in audiences?
- Rapidly changing, tight (moving shapes or lines with little space between light and dark portions) patterns that take up a majority of the viewing field
- Rapidly changing chrominance patterns that take up a majority of the viewing field
- Fast changes from blue to red are the most risky
- Rapidly changing brightness at a rate of more than 3 times per second and a brightness change of 20 nits or more, that take up a majority of the viewing field
- More in depth information can be found in the ITU-R BT-1702 Recommendation
How are photosensitivity risks detected?
Video content can be run through software tools to analyze if it contains potentially harmful flashes, patterns or chrominance changes. These tools are integrated into many commercially available Auto QC tools, as well as available online. More information can be found in the Netflix article, “Photosensitive Epilepsy Guidelines: What Can I Do If My Content Fails?”
What are some examples of potentially harmful flashes?
The definition of a harmful flash is any change in luminance over 20 nits at a rate of more than 3 times per second, that takes up a majority of the viewing area on screen. However, some elements in a program may trigger or be potentially harmful flashes, and may not be expected.
Some examples of this may include but are not limited to:
- Large flashy explosions
- Strobe lights in scenes
- Many camera flashes going off at once
- Any strobing source of brightness above the brightness and flash rate parameters
- This could even include things like shots of a sunset while driving through a row of trees.
- Changes in brightness on a dark background
- Bright stars with a dark space background
- Moving patterns that take up a large amount of frame
- Tight close up on a crowd moving where background actors are wearing vividly colored clothing
- Fast cuts between bright and dark scenes
- Bright sunny shots cutting quickly to someone in a dark room
- Bright red flashes that take up a majority of the screen
- Changes from Red to Blue are the most risky, as there is a large spectrum change
What are some examples of potentially triggering patterns on screen?
Tight patterns that rotate, flash or oscillate quickly can cause a photosensitive response in viewers.
Please note, these clips contain items with a strobing effect that may affect photosensitive readers.
- Clips below show more apparent examples of potentially triggering flashes that can result from VFX heavy story elements or characters:
- Flashing practical lights or rapid cuts between highly contrasting shots (as noted above) can cause PSE test failures for similar reasons to the examples provided above. This is amplified in highly lit or darker settings, such as clubs.
- Action scenes often fail testing due to potentially triggering flashes caused by gunshots, explosions, fast moving action, or elements of the setting, such as reflections on windows creating rapidly changing patterns/reflections in the background of a shot. Note how arches in the setting below cause rapidly alternating flashes during a car chase:
What steps can be taken to avoid harmful flashes, patterns or chrominance changes in content?
A great viewing experience is top of mind for all our storytellers at Netflix. In order to avoid potentially harmful flashes, patterns or chrominance changes, be mindful of the elements of the story during the script, production and post production phases.
- Does the title sequence include heavy flashing or moving patterns?
- Are there scenes that take place in a dance club, discoteque or red carpet event?
- Are there call outs to strobe lights or flashing lights?
- Are there close ups of police sirens, logos that are flashing red and blue, or broken lights that are flickering?
- Are there motion graphics planned that will take up a majority of the screen?
- Do the words strobing, flashing, pattern, or other related terms drive a main story point?
If the answer to any of the above questions is yes, then we ask you to be mindful of the usage of flashing elements on screen in production, visual effects, post production and mastering.
What if flashes are central to the story, and a necessary element in my production?
This is OK, as we want to protect the creative intent of our storytellers. However, steps can be taken to minimize the flashing light risk, while still achieving the desired story elements.
These can include:
- Changing the rate at which a light flashes to be less than 3 times per second
- Changing the overall brightness change of the flash through either camera exposure, power window in grading, or visual effects matting.
- Decreasing the overall area on screen that the flashing takes up
- Changing the colors that oscillate on screen to be less harmful, and a smaller frequency change on the visual spectrum
What should productions be mindful of during their process to protect photosensitive viewers?
Avoiding harmful flashes during the production process can be tricky, as the way elements are lit and exposed on set are going to end up appearing differently once they are graded and finished. To best avoid photosensitivity issues during the production process, we ask that you avoid any quickly strobing lights that are prominently featured on set. These could come from strobe lights, broken lights, lightning effects, neon signs, or camera flashes.
To reduce the risk of these, space out the time between flashes, reduce the overall brightness of each flash or reduce the amount of screen space the flash occurs in.
Avoiding harmful flashes during Post Production and DI may prove to be a bit easier. On content that is in the grading process, use a scope for any flashing scenes to measure the changing in brightness on each flash. If the luminance change is more than 20 nits from dark to bright, consider placing a power window over the flash to reduce the brightness, or add frames in between each flash to space out how many times per second the flash occurs.
The same goes for any chrominance changes or patterns. Reducing the speed or rate at which the patterns, colors or regions of the screen change can greatly reduce the risk of potentially harmful flashes or patterns, while still achieving your desired creative effect.
But how can you know that your content has potentially harmful flashes during the DI process? There are commercially available software tool solutions that can analyze a variety of video formats, including ProRes, DNx and J2K. Tools like Aurora, Q Scan, Vidchecker or Baton contain Flash Pattern Analysis algorithms to detect and flag any potentially harmful flashes, patterns or color changes. Running a file through this during the Post Production or cut phase could prove very beneficial in avoiding a harmful flash failure later on in the delivery process.
Delivery/QC of Final Deliverables
During the delivery process, if harmful flashes haven’t been avoided, the content may be subjected to Photosensitivity testing at a QC Vendor or internal at Netflix. If failed, the External Post team will be notified and have the opportunity to adjust the content via color, adding frames to reduce the frequency of flashes or VFX. The scope of the fix will vary based on the severity and location of the flashes.
The External Post team will be given the chance to review and decide if a fix will be possible or if it will negatively impact the Creative Intent of the show. If the team decides that a fix will not be pursued, Netflix may elect to include an on screen warning on the content during playback.
If adjustments are made to the IMF, we expect that the same adjustments be made to any and all parent assets of the video files. The best approach would be to apply them on the original color timeline and save them in the color projects. If a VFX fix is applied, the original projects and shots should be updated to reflect this. As Netflix may remaster content into new resolution or dynamic range formats, the fixes to flashing content should travel to those new assets as well. By applying any and all changes to the source materials or applying them via the original working projects, any remaster or recreated assets will still have the flashing fixes applied. If you have any questions about this process, reach out to your Netflix Post Contact for more information.
- 3/11/21 - Updated wording in several sections per input from Netflix Legal team.