Color gradients are elements of an image that can be difficult to render satisfactorily on the screen, whether it’s with a 4K TV or a 8K TV or aprojector. The TV or projector has to be able to:
1 / receive and interpret small color differences from the video source.
2 / reproduce these shades of colors progressively.
To effectively reproduce a gradient is to display a range of color that changes gradually, in a nuanced way. Color gradient performance is very important to obtain an optimal display of HDR images.
If you are particularly fond of HDR wide dynamic range content (HDR10, HDR10+, Dolby Vision) and want to make the most of the large color space they can offer, then you will be careful to choose a 4K HDR TV or a 4K video projector with solid color gradation display performance.
Posterization, solarization, Color Banding: definition
Posterization, also called solarization or color banding, is characterized by a loss of image quality on color gradients. Instead of being smooth and progressive, they appear in the form of successive patches or bands with clearly defined shades. This unwanted effect is usually due to the television or projector’s insufficient color depth compared to the source, or sometimes to poor management of the color space by the distributor (TV or projector) which fails to properly sample the colors.
Why is there a posterization effect on my screen?
If the gradients in the image are reproduced with separate bands of colors instead of showing smooth transitions between similar hues, there are three possible explanations:
- The source signal is not encoded with enough bits to allow a nuanced image.
- The bit depth of the TV screen is not high enough to properly display the many tones of the source image.
- The television’s video processing generates bands of color.
The concept of color depth or quantization
Color depth, also called quantization, is the number of bits of information used to tell a pixel which color to display. The greater the quantization, the higher the color fidelity and the greater the level of nuance of each color, resulting in smoother and more progressive gradations, and more realistic colors.
- 8 bits = 256 shades of color per sub-pixel (256 shades of red, 256 shades of green, 256 shades of blue), that is to say a little more than 16 million colors (256 x 256 x 256).
- 10 bits = 1024 shades of color per sub-pixel (1024 shades of red, 1024 shades of green, 1024 shades of blue), or a little more than 1 billion colors (1024 x 1024 x 1024).
With the development of HDR video content, images with 10-bit encoded colors are more and more common. It is therefore important to have a UHD 4K TV capable of supporting 10-bit colors (this is the case for most HDR10 and Dolby Vision compatible TVs).
What are the solutions in case of posterization?
The first thing to do is to check the specifications of your television to see if it is able to display images with significant color depth. Deep Color compatible TVs and models that support HDR and Dolby Vision content are capable of this.
When watching an HDR movie with a 4K UHD Blu-ray player, you should also make sure that the output settings of the player and those of the HDMI input it is connected to are correct. It is necessary to activate the “Deep Color”, “full color” or “extended color” (the name can be different according to the manufacturers) feature. If you do not activate this option, there is a risk of posterization.
If you have adjusted these settings accordingly and still see bands instead of color gradients, try disabling any video processing features that are enabled. Options such as “dynamic contrast” can generate color banding in the image.
Some non-HDR televisions have a setting called Wide Color Gamut (WCG) which allows them to display a wider than average color gamut and should usually be enabled with HDR content. However, the colors of SDR content may be too saturated with this function. This setting is independent of bit depth, but activating it can sometimes accentuate poor color gradation reproduction. It should be tested to see if it is appropriate to activate it.
What are the recommended settings?
Optimal settings for sources
- Resolution/frequency: Auto
- Deep color: 10 bits/enabled
- HDR/Dolby Vision: enabled
- Color space: generally Y’CbCr 4: 4: 4 or Y’CbCr 4: 2: 2, but it is necessary to check because there is no absolute rule: depending on the TV and the source, some settings offer better results than others.
Recommended settings for the signal format at the amplifier’s HDMI output
Not all amps allow you to adjust these parameters. The names of settings/functions may vary from brand to brand.
- Enhanced 4K signal/Upscaling: enabled if you don’t want the TV to perform upscaling / disabled if you want the TV to perform upscaling.
- Full HDMI Mode
- 10-bit colors
Recommended settings for TVs
- 4K Pure Direct/Direct
- Wide color gamut
- Deep Color enabled
To ensure that you have the best settings, it may be useful to calibrate the television using a calibration disc such as the Spears & Munsil UHD HDR Benchmark.
The reproduction of color gradients reveals a television’s ability to display detailed colors. This is very important with HDR images, so if you want to make the most of content with a wide dynamic range (HDR10, Dolby Vision, etc.), you must make sure you have a TV capable of displaying color gradients without posterization. This is generally the case with HDR compatible televisions, as long as they support at least the HDR10 format. However, if your TV meets these requirements but you still see bands of color in the gradients, try disabling the various video processing features one by one to see which ones generate banding.