Why mask the screen as opposed to the projector itself? I have a 4:3 projector that I'm going to mask for 16:9. I figured I'd use some thick black paper or something, build a small attachment that fits right on the front of the projector. Thoughts?
Alex is right, but masking at the projector may still be something that you want to do. There's quite a bit of area to mask off in a 4:3 image to let through the 16:9 part. So just err on the side of caution and let through a slightly larger area than the full 16:9. That way, the fuzzy edges will be just off your screen, and you will block out most of the unused part of the original 4:3 image.
On my DLP projector, masking at the projector is only marginally successful at masking off just the halo (while still viewing a 4:3 image), which is very narrow. It makes the edges of the image fuzzy. But with the 16:9 image there's definitely room to work to make sure the fuzzy edges are just barely off screen. Definitely a smart and easy way to do it when masking at the screen is for some reason difficult or undesirable in your setup. -- Herb
[This message has been edited by hsitz (edited 07-02-2001).]
I was wondering the same thing. My G11 has quite a distinctive 4:3 leakage around the 16:9 image. I am planning on using a velvet mask around the screen but I would like this to be 3-4" all around, not the 18" on bottom that I would require without some kind of 2 stage setup that Herb describes.
Why mask the screen as opposed to the projector itself?
I think both offer benifits for different reasons. I think the best choice is to do both. The masking inside the projecor helps preserve the source and the masking at the screen helps the final projected image.
I am masking my Proxima DX3 with a third hand for soldering and a black cardboard. The reason for the third hand is that it gives some distance between the lens and the cardboard (20, 25 cm)so the edges of the mask are a little bit sharper. My strategy is to have the 2.35 image at the bottom of the screen (via YXY or Dscaler A/R position) and only mask the upper part of the screen.
Altough i have to leave a 10 cm non-masked area (due to the unsharpness of the masking) the results are definitely worth the hassle.
I often wondered about the physics behind the fact that you cannot mask with a cut out due to the fuzzy image on screen. I am curious does any body know why the mask (even with a very sharp) causes this. I couldn't imagine diffraction around the edge from the light waves but then again my knowledge of light isn't all that large. Maybe a decent URL (I've searched but found nothing pertaining to this particular question) somebody might know of!
The physics of this are pretty straightforward. Diffraction has nothing to do with it. It's simply because the image is not focused at the point where you are trying to mask it.
Just turn on your projector and take a piece of white paper and hold it a couple inches in front if it. You'll see a fuzzy rectangle of light. It's fuzzy because it is out of focus. At this point, the photons that are headed to the top row of pixels on your screen are not all in a clean line. Some are coming from a point a bit lower on the lens and some are coming from a point a bit higher. The angle in which they are traveling is slightly different, however, so they will all converge right at your screen's surface. Similarly, the photons that are headed to the second row of pixels are also spread out when they are near the lens. At this point, the fuzzy band of light that is the first row of pixels overlaps the fuzzy band that is the second row. Now if you take a sharp edge and block the light at this point, you will be blocking only part of the light from the top row of pixels. You will also be blocking part of the light from the other rows of pixels in the vicinity. The effect is that the sharp line near the lens becomes a fuzzy line at the screen.
With my G11, its not just a case of fuzziness, unless you call it a case of *extreme* fuzziness. Let me explain:
My current setup does have masking around the edge of the screen (and in particular, 11" of masking at the top of the screen).
My screen is 16:9. I shift the letterboxed image to the bottom of the 4:3 panel. What this means is that, above the 11" of masking is still a significant amount of light from the letterbox-bar. My idea was to crudely block this light at the projector, and then the 11" of masking at the screen would still give me the sharp edged border.
What I found was that the focal point was so different (my projector is ~18 feet away from the screen) that the image itself started getting darker well before the light spill was eliminated. In fact, the *bottom* of the image started getting affected before the light spill (from the top portion of the digital panel!) was even fully eliminated.
I suppose in a short-throw situation you might get better results.
I have the lp350 which has a nice 4:3 halo as well. I fixed it and it is completely gone, you cant see any light aroud the image at all. I have done it by putting a plate about 1 foot in front of the projector, the plate takes templates with rectangles cut out to match the aspect ratios, I.e 1.85 and 2.35 for me. With this you can see a _faint_ border near the picture. By putting velour masking around the screen that extra light is completely gone and I'm really happy about it.
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