Why are interlace/combing artifacts only an issue on progressive displays?

[90sTV]

Since the way interlaced displays work is we see the full "frame" thanks to us images remaining in our vision for a bit, wouldn't we still notice that the fields which are 1/60th of a second apart don't actually line up when things move in that span of time (I realize it would be different for stuff originally shot on film, since the frames are divided up into true half frames instead of fields)? Or is it because we don't perceive that so much when each field is only on the screen for 1/60th of a second?

[jagabo]

Since the way interlaced displays work is we see the full "frame" thanks to us images remaining in our vision for a bit, wouldn't we still notice that the fields which are 1/60th of a second apart don't actually line up when things move in that span of time (I realize it would be different for stuff originally shot on film, since the frames are divided up into true half frames instead of fields)? Or is it because we don't perceive that so much when each field is only on the screen for 1/60th of a second?

It's not true that your visual persistence lets you "see" both fields at the same time on a CRT. Make a video that has one field all white and the other field all black. The flicker will blow you out of the room.

[pandy]

Because progressive display ignore time order for fields and display two fields from different time slots at once.
Interlaced display can be simulated on progressive but usually it is better to deinterlace video than simulate interlace.

[jagabo]

Also, if you look closely at a CRT display you'll see that the size of the electron beam is nearly two lines thick. So the previous field no longer visible and is not black, it's simply overwritten by the current field.

[pandy]

Also, if you look closely at a CRT display you'll see that the size of the electron beam is nearly two lines thick. So the previous field no longer visible and is not black, it's simply overwritten by the current field.

There is many factors involve in CRT resolution - electron beam can be bellow sub-micron (electron lithography is example), there is mask (grill) involved for colour displays, phosphor grain etc but some colour CRT's offered more than 2000 lines resolution and CRT's with microchannel plate and/or fiber-optic screen may offer even higher resolutions.

[90sTV]

Also, if you look closely at a CRT display you'll see that the size of the electron beam is nearly two lines thick. So the previous field no longer visible and is not black, it's simply overwritten by the current field.

So what you're saying is it basically doubles each line? Wouldn't that ruin the image?

[SameSelf]

who cares? CRTs have gone the way of the dodo and interlacing will too hopefully, but we apparently still live in a bandwidth limited world according to broadcasters.

[johnmeyer]

You actually notice nothing on a CRT interlaced display, not only because of persistence of vision, but because the "painting" of the picture by the scanning electron beam gun is continuous. That means that as the beam scans across the face of the tube, every moment in time is different. This is quite different from film or from a typical LCD TV where you are watching a series of still photos, where the entire photo -- top to bottom, and left to right -- comes from the same instant in time.

[pandy]

You actually notice nothing on a CRT interlaced display, not only because of persistence of vision, but because the "painting" of the picture by the scanning electron beam gun is continuous. That means that as the beam scans across the face of the tube, every moment in time is different. This is quite different from film or from a typical LCD TV where you are watching a series of still photos, where the entire photo -- top to bottom, and left to right -- comes from the same instant in time.

Depends from many factors - with beam deflection implemented in digital way all this can be highly accurate - also some CRT's may provide additional feedback so deflection correction can be applied continuously.

[jagabo]

because the "painting" of the picture by the scanning electron beam gun is continuous. That means that as the beam scans across the face of the tube, every moment in time is different.

Of course, that's not true for anyone watching movies on a CRT TV because those where almost always shot on film. So the continuous scanning of the camera has little to do with the lack of comb artifacts.

[johnmeyer]

because the "painting" of the picture by the scanning electron beam gun is continuous. That means that as the beam scans across the face of the tube, every moment in time is different.

Of course, that's not true for anyone watching movies on a CRT TV because those where almost always shot on film. So the continuous scanning of the camera has little to do with the lack of comb artifacts.

Yes, I agree. I was only thinking of video when I wrote that.

[90sTV]

I'm confused about the scan lines mostly displacing each other. Wouldn't that make objects appear to expand and contract, since things would be out of alignment? Or is it that the low pass filter would make it so that the smallest detail in the vertical dimension was two scan lines, so every other scan line would basically be a duplicate of the one before it? I thought I read that the filter only reduced resolution by 30%, though.

[jagabo]

That is why interlaced TV flickered so badly on sharp horizontal edges.