[HN Gopher] How to (and how not to) fix color banding
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How to (and how not to) fix color banding
Author : asicsp
Score : 101 points
Date : 2023-12-27 06:54 UTC (1 days ago)
(HTM) web link (blog.frost.kiwi)
(TXT) w3m dump (blog.frost.kiwi)
| Pathogen-David wrote:
| > Many Laptop screens are in fact 6-bit panels performing
| dithering to fake an 8-bit output. This includes even high-priced
| workstations replacements, like the HP Zbook Fury 15 G7 and its
| 6-bit LCD panel, that I sit in front of right now.
|
| This made me double-check the date of the article (it's from
| 2023.)
|
| The author's laptop appears to have launched in 2020. I'm
| astounded any manufacturer would think this is even remotely
| acceptable this day and age, much less on such a high-end device.
| (Going off the current generation, these laptops start around
| $3k.)
|
| Is this actually that common or did the author just get unlucky?
| jeffbee wrote:
| The "panel lottery" is still a thing, especially from the major
| brands like HP, Lenovo, and Dell. They don't say what you are
| going to get, they change panel suppliers without changing the
| model number, and in general they don't give a damn about how
| the thing looks. I guess it's why people buy Apple.
| cma wrote:
| Apple at least used to often use multiple suppliers, I
| remember people saying one was better than the other, maybe
| somewhere back in the first few generations of laptops with
| retina displays.
| dspillett wrote:
| It is surprisingly common even on desktop monitors, though not
| high-end ones.
|
| The panel is something that will often get changed over time in
| given laptop model without any visible change in the model
| number etc. and there have been cases of early versions
| (including review models, of course) having better panels and
| later revisions having 6-bit ones. I'd be really irritated if
| it happened in a high-end model but I'd not be surprised to
| discover there are examples where it has.
| matheusmoreira wrote:
| > there have been cases of early versions (including review
| models, of course) having better panels and later revisions
| having 6-bit ones
|
| How is this not fraud?
| dspillett wrote:
| They are very careful not to state parts of the panel spec
| that they don't absolutely have to, then they can legally
| (though obviously not morally) change the unstated parts
| without any issue. With early review units there are other
| extra excuses available to them.
|
| It is rare that a screen will get such a significant
| downgrade, though it has happened. More common is
| downgrades in RAM and drive performance. Drive makers
| themselves play fast and lose with this sort of trick,
| sometimes changing both controllers and memory to the
| detriment of performance multiple times without outward
| change in product codes.
| some1else wrote:
| Apple UI blur & vibrancy[1] look smooth without having to
| introduce noise. They have the advantage of owning the entire
| pipeline with phones & laptops, but the effect is decent even on
| budget external displays.
|
| 1: https://developer.apple.com/design/human-interface-
| guideline...
| londons_explore wrote:
| This is great... but it's at the wrong level.
|
| The LCD itself should ideally be a nice HDR LCD, but if it isn't,
| it should apply time-dependant error diffusion dithering. Ie. if
| you took a photograph, you would see a 6-bit or whatever error
| diffused image, but that dither pattern changes at 60fps. That
| should happen entirely within the firmware of the screen, and is
| actually easy to do (some types of dithering require no RAM,
| others require 1 line of pixels as a buffer).
| zokier wrote:
| Plenty of displays use various temporal and spatial dithering
| methods
| londons_explore wrote:
| I guess we only notice the ones that don't do it properly...
| tedunangst wrote:
| We draw on the screens we have, not the screens we want.
| rsaxvc wrote:
| Another GPU friendly approach is mixing with blue noise. I used
| it in a display driver with XRGB8888 framebuffer and Y2 pixels,
| and it worked great.
| quag wrote:
| When generating images also mix in a little blue noise to avoid
| banding effects. A simple lookup table goes a long way. I found
| it worked much better than adding uniformly random noise (white
| noise) to each pixel.
| lynndotpy wrote:
| Are there any debanding methods that add a point to the color
| channels?
|
| E.g. A delta of +(1, 1, 1) in RGB space would have six
| intermediary (but not perceptually evenly spaced) values, e.g.
| (1,0,0), (0,0,1), (0,1,0), (1,0,1), (0,1,1), and (1,1,0).
|
| This might be something dithering already does (and I just don't
| understand it).
| klodolph wrote:
| It's more or less linear--what you can do is dither in each of
| the R,G,B channels separately. To remove banding in an image,
| you dither in R, dither in G, dither in B.
|
| If you take a gray value that is 50% between two gray values,
| and you use uncorrelated noise to dither, then the result will
| contain all (+-0.5,+-0.5,+-0.5) delta from the original gray.
| If you use ordered (correlated) dither, you'll get a subset of
| those 8 colors.
| mpabst wrote:
| Something similar is used in visual psychophysics:
| https://www.spiedigitallibrary.org/conference-proceedings-of...
| petermcneeley wrote:
| This can also be done temporally not simply spatially.
| https://www.shadertoy.com/view/tslfz4
| kevingadd wrote:
| My recommendation is to do the equivalent of FRC in your shaders.
| I use one of the dither functions from
| https://developer.oculus.com/blog/tech-note-shader-snippets-...
| to break up banding in my geometry rasterization shaders that I
| use for rendering all my UI elements. It looks good in static
| images and fantastic in motion since the dithering pattern shifts
| every frame.
|
| One thing to watch out for is that if you're alpha blending, you
| need to be fairly cautious about how you dither and multiply.
| Premultiplied alpha does not get along with dithering, so the
| output from your shaders needs to be dithered but NOT
| premultiplied, with premultiplication performed by the GPU's ROPs
| (they typically seem to operate above 8 bits of precision.) If
| you don't do this you will get really nasty banding on alpha
| fades even though you dither. Also, counter-intuitively, you may
| not want to dither the alpha channel (test it yourself, ymmv)
|
| When dealing with sRGB and linear space it can also be important
| whether you dither before or after the color-space conversion.
| Sunspark wrote:
| Changing the dither pattern every frame subjectively increases
| noise so I have change dither pattern every frame off for
| videos.
|
| I also turn off colored noise as well, because even though
| having colored noise on lowers luma noise, it does increase
| chroma noise.
|
| I don't want to see noisy colors moving around on videos, esp.
| anime.
|
| My monitor does do FRC, so all this software stuff is added
| noise which is why I like to turn it off.
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