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Does darkvision allow a creature to distinguish visual contrast? The rules as written, in both Pathfinder and 3.5e, state simply that "Darkvision is black-and-white only (colors cannot be discerned)." This makes it seem like contrast can indeed be discerned, as in a black-and-white picture.

However I seem to remember an old rule (possible 3e?) which implied that darkvision worked similarly to blindsight in this regard, which states specifically that it "never allows a creature to distinguish color or visual contrast. A creature cannot read with blindsight." I believe this rule was accompanied by a comparison between normal vision and darkvision on a picture of a mindflayer.

Does anyone remember this picture and the rules associated with it, or can find any official rules which seem to contradict what the current rules seem to imply?

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According to D&D 3.5 FAQ, a character can distinguish contrast with darkvision.

Does darkvision allow a character to read and write in the dark? Can he see his reflection in a mirror?

Darkvision is described as the “ability to see with no light source at all” (DMG 292). This suggests that a character with darkvision can read, write, and otherwise perform a normal range of actions despite being in complete darkness.

The simplest answer to the mirror question is yes. To avoid delving too deeply into real-world physics—something it’s best to resist, particularly in fantasy gaming—it’s easiest to treat darkvision as allowing a character to perceive anything that a normal person could see with light.

There was an idea going around during the run of 3e that dwarves couldn't read scrolls with darkvision because they can't discern colour. This idea may come from earlier editions, where infravision was literally thermal vision. For example, in AD&D 1st edition:

They do not "see" things which are the same temperature as their surroundings. Thus, a room in a dungeon might look completely blank, as walls, floor, ceiling and possibly even some wooden furniture within are all of the same temperature.

AD&D 2nd edition kept this as an optional rule:

...characters can see the degrees of heat radiated by an object as a glowing blob translated into colors like a thermagram... Does the ink of a page radiate heat differently enough from the paper to be noticed? Probably not.

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Excellent, canonical answer! Also, interesting point about mirrors - I hadn't even thought of that one. –  dlras2 Dec 20 '11 at 15:05
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There's no mention anywhere of darkvision being binary black-and-white. Contrast is implied.

Blindsight is distinctively non-visual in nature and explicitly excludes visual contrast and reading.

Both are (extraordinary) senses, they are not directly relatable.

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Visual contrast describes the differences between hues of the same shade. For example, people with red-green colorblindness cannot see the visual contrast between red and green, and so could not read red words on a green background if they were the same shade. I was not referring to binary black-and-white (which would still allow you to read black-on-white text) but rather grey scale images based on shade, not hue. –  dlras2 Dec 19 '11 at 6:13
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Warning. This is real world physics and visual perception. I have no idea about how D&D canon describes darkvision. If you want a fantasy answer, just skip over this one.

If darkvision works in another band of the electromagnetic spectrum, that should probably be in the far infrared range, where room temperature objects are emissive, that is they emit light. This would allow for perceiving contrast in both temperature differentials, and different reflectivities of objects. However, the luminance values should probably be much different than what we see with the visible spectrum, so contrast levels will also be very different. It is possible to lose some details while some things that are lost to us will be easily visible. And transparency will also work differently. Some transparent things may be opaque down there while some other normally opaque things will be transparent.

It is also possible to discern color at that range, but those colors will also be very different than our colors, and no information about our colors will be available. So it is logical to say that while darkvision enabled characters see some sort of color information, that color has no correlation to our visible spectrum color.

In other words, it will be a different and magnificent world.

If darkvision works like our own real-world darkvision, which is just light-sensitive and color-insensitive rod cells in the retina, then it needs light, however little that may be. We have a low number of rod cells so our darkvision isn't very good compared to say, cats or owls. They have a high number of rod cells in their eyes, so they can get by easily in starlight. Pitch darkness, which is really hard to find(magic?) would disable such darkvision.

Rod cells have a sensitivity curve that peaks at a single color. For us, that happens to be the color blue. At night, blue appears brighter and red things appear darker than all others as our rods are completely insensitive to red. So while there is no real color data in real-world darkvision, it is possible to deduce color based on luminance, but it is also possible to be mistaken about color in the dark.

Both kinds of darkvision would be overwhelmed in the presence of sunlight, so creatures possessing darkvision would either be nocturnal like owls, or have dual vision systems like us humans.

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+1, as this is a great answer for anyone looking to introduce extra realism. It's also interesting, and I love science! –  dlras2 Dec 20 '11 at 15:09
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You are probably thinking of 2e's infravision, which allowed you to see heat. Too many players used it as 'see in dark' so there was all sorts of stuff about how a door would be the same temperature as a wall, no contrast could be seen etc. That is why they brought darkvision in.

That said, I remember seeing that picture of the mindflayer, but have no idea what book it is from-- That was definitely a post-3e picture. I don't remember it being pure binary B&W though, so I suspect that you might be mixing 2e rules with a 3e image.

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Infrared vision sounds similar to what I'm remembering. And I'm glad someone else remembers the mindflayer picture - I can't find it anywhere. –  dlras2 Dec 19 '11 at 6:11
    
I think it was 3.0 DMG under the index of abilities, but I could easily be wrong on that. Let me know? –  Canageek Dec 19 '11 at 6:22
    
I looked through the 3 core 3.5 books, but think I have a copy of the 3.0 DMG somewhere I can look through when I get home. –  dlras2 Dec 19 '11 at 15:47
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It's in the 3.0 DMG on pages 74 and 75. –  Jonathan Drain Dec 20 '11 at 12:37
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