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Gamma correction in MPF

MPF includes functionality to allow you to adjust the gamma of the color information that is sent to physical DMDs (RGB and mono) and to RGB LEDs. (You don't need to set the gamma of an LCD display since that's handled by your OS.)

You can read full details in the Gamma correction article on Wikipedia, but the quick explanation is that the human eye doesn't not perceive a change in brightness at the same ratio that an LED sets its brightness.

When you're setting colors in MPF, you expect that 100% brightness looks fully bright, and that 50% looks like 50%, etc. Here is a screenshot of a slide which has 16 bars which fade from off to fully white, in a more-or-less even fashion:

image

However if you show this slide on your physical DMD with no gamma correction, it looks something like this:

image

Even though the individual pixels are showing their "correct" brightness, the human eye can't really tell a different between 50% and 100%, and pretty much everything on the right half of the DMD looks fully white.

So you can adjust this by setting the gamma value. By default, MPF uses a gamma value of 2.5 for RGB LEDs, and 2.2 for RGB DMDs. (It also uses a value of 1.0 for mono DMDs since some of the hardware controllers do their own internal gamma correction, though others don't, so you might have to change them.

We recommend you read the documentation for the dmds:, rgb_dmds:, and light_settings: (for LEDs) to set the proper gamma.

Tuning your DMD gamma

MPF includes a built-in gamma test slide (the one used in the images above) which you can use to dial-in your gamma setting.

The easiest way to show this slide on your physical DMD is to make a temporary addition to your machine config to add a slide player, like this:

slide_player:
  mode_attract_started:
    dmd_gamma_test:
      priority: 10000000
##! test
#! start_game
#! assert_slide_on_top dmd_gamma_test

This will just show the gamma test slide at a crazy high priority so it shows on top of everything else. (Remember if your DMD is not your default display, you'll also have to add target: dmd or whatever you use to target slides to your DMD.)

Now you can play with different gamma settings for your DMD in either your dmds: or rgb_dmds: section. (Note you'll have to restart MPF after each change you make.)

Note that you might also have to adjust brightness: along with gamma:. For example, some people had to set the brightness of their RGB DMDs to a super low value, like 0.1 or 0.2 before MPF had gamma control, but with proper gamma settings, you can probably take your brightness up to somewhere around 0.5.

We like to use the gamma test slide and set the brightness first based on the right-most brightest block, and then once that's set, we start messing with the gamma. It will probably be some trial-and-error, but once it's dialed in it's a "set it and forget it" type of thing.

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