by johnnydeep » Sat May 01, 2021 4:23 pm
Glad to see Ingenuity being given more stuff to do, especially now that it's graduating to "operational" goals instead of just "technology demnstration" goals.
And this gives me an excuse to ask again about photos of shadows of the rotors taken from Ingenuity. Here's a video that shows several still shots taken in quick succession, starting at 0:45 with the rotors apparently unblurred but rotating a few degrees during all the shots.
Click to play embedded YouTube video.
Is the reason the images show no blur solely because the shutter speed is great enough to "stop the motion", or is it because of some weird 2D attributes of shadows that almost makes some sense, but is not totally convincing to me? I'm talking specifically about the explanation I got from a reply to my question on the video about why the rotors look stationary. The guy seems like he knows what he's talking about, but if so, I'm having a hard time understanding the explanation. [ Another reply said the frame rate of the video - does Ingenuity even have video capability in it's downward facing camera? - matches that of the rotors, which I replied could make reasonable sense. ]
Me - original question:
Hmm, how is it that the rotors appear almost stationary in that last segment showing Ingenuity in motion from the on-board camera? [ EDIT: ok, it looks like the rotors did rotate a few degrees during the period of motion. Still, why aren't they just a blur? ]
"
Gage":
Videos are a bunch of pictures.
Like, 60 FPS, 60 pictures per second.
The rate it took the pictures matched the rotors RPM
Me:
@Gage Hmm, I guess that makes sense. Per NASA, the rotors spin at 2400 rpm, which is 40 FPS. That seems like a reasonable - if a little fast - video speed. EDIT: and 20, 10, or 5 FPS would also result in apparently stationary rotors, and even 24 FPS might work if we allow for a little rotor image rotation, which there is.
"
One by Land, Two if by Sea Run if by Air":
The key here is understanding the "Shadow". Its 2 dimensional surface. But they are also negative light space, without light bouncing off a moving object. So the light doesn't get scattered or shifted by reflections.
So if you're thinking about a shadow as an object there: We need to look at this a different way.
If a rotating object casts a shadow: the area underneath will simply get lighter and darker (in any fixed spot), but won't move. No motion: no blur.
Or to put it a different way: the "shadow" is not an inherent property of the object (rotor blades). It's not like mass and volume.
Your photographing the ground, which isn't moving.
You can see shadows of hummingbird and insect wings easily, but they blur too much to see them directly in flight.
Shutter speed here is close to an interval of blade rotation. That's not too surprising, since there are 4 blades and each can occupy 2 positions and still be symmetrical in rotation axis.
Me:
@One by Land, Two if by Sea Run if by Air - You’ve said this before in a reply to a similar question I had in another Ingenuity video about the rotors, but I still don’t believe it. Your explanation would seem to imply that it is impossible to take a blurry [photo] of the shadow of a moving object. Is that the right conclusion?
"
One by Land, Two if by Sea Run if by Air":
@Johnny Deep The shadow isn't moving. It's a picture of the ground. If the ground isn't blurry, it is impossible for the shadow to be blurry. They're quite literally the same thing. ...So yes, such a thing would defy all known physics.
The shadow is not an object.
The photos are consistent with this also. The ground simply can't blur under in some spots, and not blur in others, because it's all moving as one object at one speed, regardless of any light that shines on it.
Me:
@One by Land, Two if by Sea Run if by Air - Would the situation be any different if we were photographing a rotor "shadow" cut out of a piece of black paper and spinning at 3200 RPM on the ground? Surely that's a "real" object, so it should be possible for it to be blurred in a photo, no?
So, what's really going on with those rotor shadows?
Glad to see Ingenuity being given more stuff to do, especially now that it's graduating to "operational" goals instead of just "technology demnstration" goals.
And this gives me an excuse to ask again about photos of shadows of the rotors taken from Ingenuity. Here's a video that shows several still shots taken in quick succession, starting at 0:45 with the rotors apparently unblurred but rotating a few degrees during all the shots.
[youtube]https://www.youtube.com/watch?v=qsF3RXsm8mo[/youtube]
Is the reason the images show no blur solely because the shutter speed is great enough to "stop the motion", or is it because of some weird 2D attributes of shadows that almost makes some sense, but is not totally convincing to me? I'm talking specifically about the explanation I got from a reply to my question on the video about why the rotors look stationary. The guy seems like he knows what he's talking about, but if so, I'm having a hard time understanding the explanation. [ Another reply said the frame rate of the video - does Ingenuity even have video capability in it's downward facing camera? - matches that of the rotors, which I replied could make reasonable sense. ]
[b]Me - [i]original question[/i][/b]:
[quote]Hmm, how is it that the rotors appear almost stationary in that last segment showing Ingenuity in motion from the on-board camera? [ EDIT: ok, it looks like the rotors did rotate a few degrees during the period of motion. Still, why aren't they just a blur? ][/quote]
"[b]Gage[/b]":
[quote]Videos are a bunch of pictures.
Like, 60 FPS, 60 pictures per second.
The rate it took the pictures matched the rotors RPM[/quote]
[b]Me[/b]:
[quote]@Gage Hmm, I guess that makes sense. Per NASA, the rotors spin at 2400 rpm, which is 40 FPS. That seems like a reasonable - if a little fast - video speed. EDIT: and 20, 10, or 5 FPS would also result in apparently stationary rotors, and even 24 FPS might work if we allow for a little rotor image rotation, which there is.[/quote]
"[b]One by Land, Two if by Sea Run if by Air[/b]":
[quote]The key here is understanding the "Shadow". Its 2 dimensional surface. But they are also negative light space, without light bouncing off a moving object. So the light doesn't get scattered or shifted by reflections.
So if you're thinking about a shadow as an object there: We need to look at this a different way.
If a rotating object casts a shadow: the area underneath will simply get lighter and darker (in any fixed spot), but won't move. No motion: no blur.
Or to put it a different way: the "shadow" is not an inherent property of the object (rotor blades). It's not like mass and volume.
Your photographing the ground, which isn't moving.
You can see shadows of hummingbird and insect wings easily, but they blur too much to see them directly in flight.
Shutter speed here is close to an interval of blade rotation. That's not too surprising, since there are 4 blades and each can occupy 2 positions and still be symmetrical in rotation axis. [/quote]
[b]Me[/b]:
[quote]@One by Land, Two if by Sea Run if by Air - You’ve said this before in a reply to a similar question I had in another Ingenuity video about the rotors, but I still don’t believe it. Your explanation would seem to imply that it is impossible to take a blurry [photo] of the shadow of a moving object. Is that the right conclusion?[/quote]
"[b]One by Land, Two if by Sea Run if by Air[/b]":
[quote] @Johnny Deep The shadow isn't moving. It's a picture of the ground. If the ground isn't blurry, it is impossible for the shadow to be blurry. They're quite literally the same thing. ...So yes, such a thing would defy all known physics.
The shadow is not an object.
The photos are consistent with this also. The ground simply can't blur under in some spots, and not blur in others, because it's all moving as one object at one speed, regardless of any light that shines on it.[/quote]
[b]Me[/b]:
[quote] @One by Land, Two if by Sea Run if by Air - Would the situation be any different if we were photographing a rotor "shadow" cut out of a piece of black paper and spinning at 3200 RPM on the ground? Surely that's a "real" object, so it should be possible for it to be blurred in a photo, no?[/quote]
So, what's really going on with those rotor shadows?