LCD 'moiré'

[kr236rk]

hi,

i have been experimenting with filming off LCD screens. although LCD has no refresh rate - like CRT - there is a definite 'moiré effect' in dv, whatever shutter speed is chosen: 1/50, 1/120, 1/250.

what is cause, the technical reason for this lcd/dv moiré effect, please - and can it be avoided?

thanks,

[jagabo]

The moire patterns are caused by the mask (electrical conductors) on the front surface of the LCD interacting with the digitizing of the image. Get a magnifying glass and look at the surface of your LCD monitor, you'll see the mask pattern. This is little fuzzy, and you can't see the individual RGB cells, but you can see the mask pattern (the background was pure white):



To reduce the moire patterns get farther away from the LCD or don't focus so sharply.

[kr236rk]

hi Jagabo,

To reduce the moire patterns get farther away from the LCD or don't focus so sharply.

yes, that was the sort of thing i was seeing - will experiment



many thanks,

[kr236rk]

hi.

The moire patterns are caused by the mask (electrical conductors) on the front surface of the LCD interacting with the digitizing of the image.

just a thought - what would the effect be filming from a digital projection, on a screen - filming imagery from a digital projector, please?

thanks again,

[BJ_M]

forget it with a DLP with a color wheel ... bad results usually as you are not sync'd with the wheel

a three chip DLP works well ..


a lcd projector is ok - IF you can turn off the blasted auto iris and manually set up a slightly over saturated image , but you have to play with it ...

to reduce moire you can defocus your image or add dithering in the source media ...

lcd flat screens - are not the best at any time for solid backgrounds like sky - though with a really good source HD, ive been somewhat impressed with the new sharp lcds

[kr236rk]

hi,

forget it with a DLP with a color wheel ... bad results usually as you are not sync'd with the wheel

a three chip DLP works well ..

i must admit - you got me there!

a lcd projector is ok - IF you can turn off the blasted auto iris and manually set up a slightly over saturated image , but you have to play with it ...

i think this would drive me completely nuts - DIY ciné transfers have got me half way there! btw - now you got me interested - are you telling me, no moiré with dv projectors?

thanks -

[BJ_M]

moiré can be caused by either or both the source media and the viewing equipment ...

so i cant say you will get rid of it ... plus lcd in general is more likely to have it to begin with ... you will have less of it though with a high resolution chip (true HD vs. low end lcd projectors) ..

i would think HD plasma would work well if you could avoid any glare (not sure - never tried) , ED plasma suxs i am sure though ..

really a good crt is the least likely to have any or little ....


also - bear in mind that there are two sources of moire in the viewing screen - a visible type that you can plainly see (where we use dithering to reduce) and an optical effect of filming vs. the on/off effect of the grid (as well mentioned above) ...

ive had the same issue with a perforated screen and Digital projectors ... the grid of the chip would cause interference with the perforations and cause the same moire issues ..

[jagabo]

Yes, many front projectors have the "screendoor" problem. This is where you can see the same wiring mask as with direct view LCDs.

[kr236rk]

many thanks for your help, guys -

moiré can be caused by either or both the source media and the viewing equipment ...

so i cant say you will get rid of it ... plus lcd in general is more likely to have it to begin with ... you will have less of it though with a high resolution chip (true HD vs. low end lcd projectors) ..

i would think HD plasma would work well if you could avoid any glare (not sure - never tried) , ED plasma suxs i am sure though ..

really a good crt is the least likely to have any or little ....

ive had the same issue with a perforated screen and Digital projectors ... the grid of the chip would cause interference with the perforations and cause the same moire issues ..

hm, when i first dicovered lcd i thought - wow - no screen refresh! - the blurb forgot to mention lcd moiré. never mind, i wouldn't be without my lcd monitors now!

jagabo wrote:

Yes, many front projectors have the "screendoor" problem. This is where you can see the same wiring mask as with direct view LCDs.

i've actually encountered both types of lcd moiré, the grid pattern and rainbow-type parallel bands which flip across the screen at various focal lengths.

yes, it's the lens focusing narrowly on the screen instead of making sense of the wider visual context. interestingly, this doesn't happen with digital stills, although the individual pixels begin to stand out.

crt is a different kettle of fish. i can vhs video cam off crt's, and get no visual interference at all; but you cannot use ciné on a crt because the refresh clashes with 99% of all ciné cameras' shutter speeds ...

[SingSing]

Did you consider capture off the PC directly with a software player ? or play full screen, then press alt-prt screen and get a screen capture ?

[jagabo]

i've actually encountered both types of lcd moiré, the grid pattern and rainbow-type parallel bands which flip across the screen at various focal lengths.

The grid pattern isn't a moire. It's just a physical property of the display. Moire and aliasing are the interaction of grid/line patterns with digital sampling when the pattern is near the sampling resolution and can happen with both output and input devices.

http://en.wikipedia.org/wiki/Moire_pattern
http://www.wfu.edu/~matthews/misc/DigPhotog/alias/index.html

crt is a different kettle of fish. i can vhs video cam off crt's, and get no visual interference at all; but you cannot use ciné on a crt because the refresh clashes with 99% of all ciné cameras' shutter speeds ...

You can get moire with CRT displays too. You can even see it happen when watching broadcast TV. For example, someone wearing a shirt with horizontal stripes at just the right size will cause moire. If you look closely at the screen of any CRT you will see the vertical or hexagonal (depending on design) grid. This grid will cause moire patterns if you zoom in closely enough with a digital camera. Here's a close-up picture of my monitor while typing this message:

[kr236rk]

hi SingSing,

Did you consider capture off the PC directly with a software player ? or play full screen, then press alt-prt screen and get a screen capture ?

would this be for still or moving image capture, please?

thanks

[kr236rk]

hi Jagabo,

The grid pattern isn't a moire. It's just a physical property of the display. Moire and aliasing are the interaction of grid/line patterns with digital sampling when the pattern is near the sampling resolution and can happen with both output and input devices.

ok - there is no cure for this in a cam, apart from 'soft focus' ?

You can get moire with CRT displays too. You can even see it happen when watching broadcast TV. For example, someone wearing a shirt with horizontal stripes at just the right size will cause moire. If you look closely at the screen of any CRT you will see the vertical or hexagonal (depending on design) grid. This grid will cause moire patterns if you zoom in closely enough with a digital camera.

yes, i've seen that 'shirt effect' - must be a real headache for tv producers!

luckily, the vhs video cam - being analogue - records neither horizontal refresh rate clashes (it's shutter speed matches the crt frequency) nor moiré

[SingSing]

hi SingSing,

Did you consider capture off the PC directly with a software player ? or play full screen, then press alt-prt screen and get a screen capture ?

would this be for still or moving image capture, please? thanks

By nature all image capture are still, or else you get a composite of multiple pictures.
1. Player likes PowerDVD has screen capture feature, just click on the camera icon.
2. Pause any player with the image you want, press ALT and PRT SCREEN together, will capture the image to the clipboard. You can open an image editor, and use "paste as new image", then store into jpeg file.

[kr236rk]

hi SingSing,

Did you consider capture off the PC directly with a software player ? or play full screen, then press alt-prt screen and get a screen capture ?

would this be for still or moving image capture, please? thanks

By nature all image capture are still, or else you get a composite of multiple pictures.
1. Player likes PowerDVD has screen capture feature, just click on the camera icon.
2. Pause any player with the image you want, press ALT and PRT SCREEN together, will capture the image to the clipboard. You can open an image editor, and use "paste as new image", then store into jpeg file.

hi,

this software is unknown to me. i use Pinnacle 9 av/dv (+ PCI card) to capture ciné film or analogue video to HDD, which is excellent.

ciné is a perennial problem however, because the film speeds - 18 frames per second, 24fps - clash with digital cams (1/50th shutter speed) and result in a strobe or flicker effect. the only fail-safe solution to this is mega technical hardware like -

http://www.moviestuff.tv/wp_xp.html

the sheer cost of which i could not justify for the limited film-making i indulge in: although, at times, i am sorely tempted ...

other solutions are the excellent anti-flicker softwares available on the internet - but you are going to compromise image quality, if only marginally, following this path.

thanks for your help!

[SingSing]

Taking picture off a LCD screen is as awkward camcording movie in theater.

Digital photo/video are meant to be transfer with ease and witout loss of quality.

[kr236rk]

Taking picture off a LCD screen is as awkward camcording movie in theater.

using a heavy tripod can help. in a theatre, you'd be hampered by the lightness of modern dv cams, which are going to twitch around in your hand - a heavier cam is better for stability - filming off the shoulder, for example ...

Digital photo/video are meant to be transfer with ease and witout loss of quality.

yep, dv, vhs are no worries; 8mm is a different story though.



bests,