Simeon Schmauß
@65dBnoise @PaulHammond51 I agree that color perception can vary quite a lot between different people, however there is a pretty well established process for digital cameras here on earth to produce natural looking colors.
NASA doesn't apply any of those established processes to the Mastcam images.
And what they are doing instead is no substitute to this process, from my understanding it's plainly wrong to call the "natural color images" true to what the human eye would see.
Aug 16, 2023 at 3:26pmWeb
Show threadSimeon SchmaußAug 16, 2023
@65dBnoise @PaulHammond51 First of all, they are completely ignoring white balancing. White balancing is important when the scene light color doesn't match the environment the image is looked at. Our eyes always try to balance to the local light conditions, so if we display a non whitebalanced image on our screen, our eyes won't adjust to the colors of the image because they are adjusted to our surroundings.
Show threadSimeon SchmaußAug 16, 2023
@65dBnoise @PaulHammond51 So while their approach to show the images this red, may not be completely wrong, as in the light on mars is overall much redder than on earth, our eyes can't adjust to the colors like they would standing on mars.
Show threadSimeon SchmaußAug 16, 2023
@65dBnoise @PaulHammond51 Therefore it is important to white-balance an image to simulate the color adjustments of our eyes to the normal white point of our displays.
Whitebalancing is not a perfect method though, our eyes don't adjust in a linear manner to different lighting conditions. But it's at least a better approximation than showing the unbalanced image.
Show threadSimeon SchmaußAug 16, 2023
@65dBnoise @PaulHammond51 That said, there is another big issue with the natural color images. Their calibration completely ignores color spaces.
A color space defines how much real life color saturation an RGB triplet value corresponds to. This depends on the capabilities of the display your viewing the image on and it also depends on the sensitivity curves of the camera sensor.
Show threadSimeon SchmaußAug 16, 2023
@65dBnoise @PaulHammond51 If you don't account for that, and just interpret the raw color values as sRGB colors, an image will usually look less saturated than the real scene.
This also applies to the "natural color" images from Mastcam. So even if those images would show the reddish lighting conditions on Mars correctly, they don't show the saturation (and therefore the color separation) correctly.
Show threadSimeon SchmaußAug 16, 2023
@65dBnoise @PaulHammond51 Producing images that show what our adjusted eyes would see on Mars is certainly not easy. I'm trying my best to do that with my calibrated images, but it's definitely not perfect.
The real answer to this question will likely only come once we eventually visit there ourselves.
What I'm sure of however is that the Mastcam calibration (as described here: https://www.hou.usra.edu/meetings/lpsc2023/pdf/2504.pdf) is not accurate in this way.
Show threadSimeon SchmaußAug 16, 2023

@65dBnoise @PaulHammond51 In contrast, here is the color calibration for Insight, which is doing things a lot better and in a similar way to my own process.
https://doi.org/10.1029/2020EA001336

I described my own process here: https://twitter.com/stim3on/status/1649000065350893568
Once I find the time I will do a better writeup on this.

Show threadSimeon SchmaußAug 16, 2023
@65dBnoise @PaulHammond51 Oh, and I think there is another issue with the natural color images, I'm not completely sure about this since the calibration (Edit: the documentation of it) is not very detailed.
The issue concerns gamma. Our eyes don't react to light in a linear way, but in a logarithmic fashion. That means we are more sensitve to differences in dark parts of a scene than bright ones.
Show threadSimeon SchmaußAug 16, 2023
@65dBnoise @PaulHammond51 To account for this, sRGB images usually apply a "gamma curve", i.e. an exponential brightness encoding to map more dark values to the 8-bit brightness range than bright ones.
Calculations usually happen in linear brightness range, so if you want to create an image that should be displayed correctly, you need to apply a gamma curve to it.
It's not a very intuitive topic, but this video explained it well: https://www.youtube.com/watch?v=LKnqECcg6Gw
Computer Color is Broken

YouTube
Show threadSimeon SchmaußAug 16, 2023
@65dBnoise @PaulHammond51 It is my understanding, that the "natural color" images don't apply this gamma curve after the calibration is applied. Therefore the brightness values are not at all true to our perception of brightness. Rather these images look like what we would see if we had a linear brightness perception. 😬