By Chris Deighton, CTO
LED driver chips operate using linear PWM output. Unfortunately, our eyes (and cameras) sense brightness very differently, in a non-linear way with much more sensitivity to small changes at low brightness. As a result, reproducing accurate and detailed low-brightness imagery is one of the most difficult challenges for an LED panel.
Many LED processing systems are simply unable to output low-brightness levels without various artefacts showing up, such as banding, colour shifts, speckle or ‘patchwork-quilt’ blockiness. Rather than fixing these problems, some of these LED processing systems implement a deliberate ‘fudge-up’ to artificially boost the brightness of dark parts of the image. Measurements have shown such systems can in some cases make dark parts of the image over ten times brighter than they should be. This has the benefit of simply ‘skipping over’ the problematic low-brightness levels, and to the casual observer can initially look ‘better’ because dark parts of the image appear brighter. However, in a professional environment, this creates two serious problems:
First, the brightness levels being reproduced are extremely inaccurate, so the image will look very different to a reference monitor. This has the unfortunate effect of revealing detail that was meant to stay hidden, which can often include compression artefacts (such as blocks), camera or render noise (such as dither or speckle), and other artefacts which interfere with the image quality. This effect is exacerbated when using High Dynamic Range (HDR) content, which deliberately offers content creators the ability to accurately reproduce very low-brightness levels.
Second, even in higher brightness parts of the image, the colour accuracy is adversely impacted, as any attempt to add a small amount of red, green or blue light to bright pixels will result in substantially more light being added than is intended. This introduces unpredictable colour shifts which are typically impossible to correct for. This effect is exacerbated when using a Wide Colour Gamut (WCG) such as Rec.2020 or ACEScg – as is also often employed for HDR content and for on-set environments.
“A key benefit of using LED panels for virtual production is the realistic lighting effects available. However, even the best LED panels have a comparatively low peak brightness compared to conventional lighting fixtures.”
Given both of these issues seriously reduce overall image quality and accuracy, Brompton Technology avoids these kind of ‘dark fudge-ups’, instead choosing to spend considerable time researching, testing and developing the ability to achieve superior low-end brightness accuracy. This originally led to our ‘Dark Magic’ feature, enabling previously unachievable low-brightness levels to be accurately reproduced in LED panels using the Brompton R2 receiver card. The further addition of ‘Studio Mode’ enables panels to be run at lower brightness levels while preserving PWM bit depth, which helps with the reproduction of low-brightness imagery, but does reduce the overall brightness the panels can achieve, making this technique less useful for HDR content.
Performance at 1% brightness: Camera footage of 2000 Nits panels running at just 20 Nits.
Brompton’s ongoing research has now led to the creation of a brand new feature – ‘Extended Bit Depth’ – to further improve the low-end performance of LED panels. In conjunction with ShutterSync (which ensures the LEDs are appropriately driven to avoid artefacts at any desired camera shutter angle), Extended Bit Depth enables panels to increase their PWM bit depth, depending on the video framerate and camera shutter angle in use. For example, a panel running at 24 fps with a camera shutter angle of 180° might offer an additional 2.3 bits of PWM precision, giving five times the number of achievable brightness levels at the low end, with 125 times the number of achievable colours. The result of this is that the panels can achieve an additional 2.3 stops of dynamic range at the low end – which is crucial when filming an LED screen displaying content such as dark, shadowy environments. Moreover, there is no reduction in the peak brightness of the panel, ensuring HDR content continues to be displayed to its full potential.
“Making the panels (and any lighting from them) appear much brighter on camera traditionally had the drawback of revealing artefacts in dark areas of the image due to the panels’ limited bit depth. Extended Bit Depth overcomes this issue, enabling higher apparent brightness lighting while maintaining background image quality.”
A key benefit of using LED panels for virtual production is the realistic lighting effects available, which firmly anchor any foreground elements within the virtual environment. However, even the best LED panels have a comparatively low peak brightness compared to conventional lighting fixtures, so it’s a common scenario to want to achieve higher brightness output. While Extended Bit Depth doesn’t increase the overall brightness of the panels, it can effectively achieve a similar result by enabling the camera exposure to be substantially increased (typically by using a wider aperture or higher sensor gain). This has the benefit of making the panels (and any lighting from them) appear much brighter on camera, but traditionally had the drawback of revealing artefacts in dark areas of the image due to the panels’ limited bit depth. The additional precision and detail available in dark areas of the image when using Extended Bit Depth overcomes this issue, enabling higher apparent brightness lighting while maintaining background image quality.
New camera sensors are offering ever-increasing numbers of stops of dynamic range. Ensuring that LED panels are able to keep up with this improvement is critical to maintaining the quality of the final output for virtual production and other applications using ‘in-camera’ capture of content displayed on LED screens.
Extended Bit Depth is available as a free firmware update for all existing and new LED panels containing the Brompton R2 or R2+ receiver card, further extending their market-leading colour accuracy and image quality.