# GOAL: accurate standard for screenshots

531 Views 11 Replies 5 Participants Last post by  bob2010
This is a splinter thread from the Superplex topic:
http://www.avsforum.com/avs-vb/showt...&pagenumber=36

Quote:
 Originally posted by CMRA marcorsyscom, Your reposting of the applicable screen shot sent a shockwave through the other thread. Maybe you can help clear things up. Perfect blacks, I gather are registered at 0,0,0 and perfect whites as 255,255, and 255 for each RGB channel. Am I correct so far? (shows what I know about Photoshop) By this, does this encompass the 256 step gray scale we read and hear so much about? If so: Can I assume 0 is blackest black and 255 is whitest white? Thats only a 256 wedge scale, correct. CR, or contrast ratio, is defined as the range between the whitest white and the blackest blacks. So: IF a projector has a CR of 500:1, how does it equate to to a 256 grayscale? What if a projector has a CR of 1000:1? What if a projector has a CR of 2000:1? The real question is if the scale goes from 0 to 255 wouldn't a CR of 300:1 be all that is needed to get the job done? Please educate me/us.
Whoa, I'm a student, not an educator! I barely have a layman's understanding of the "256" numbers, and I'm the wrong person to ask about the science behind the specs and numbers. But I'm willing to start with some simple rambling and sit back to learn the real truth from those who really know...

I think "256" is practically arbitrary. Especially since we're more interested in the high/low ends (and the ratio between the two), instead of the number of slices (256) between white and black.

My understanding is that a digital camera's Auto White Balance (AWB) looks for gray and then divides the steps from the middle (gray) to the top (white) and bottom (black). So in a really bright scene, half of the color steps can be used in a small range to depict the subtle shades of white, without "clipping" (which would otherwise happen if the camera just said "everything above this brightness I'll call 'white'", so a lot of subtle shades and detail would end up getting called the same value of "white").

And if you didn't have AWB, then a dark scene would only have half the bits available to describe "dark", which means several shades of dark would get grouped together into the same color band, which would "crush" the blacks.

But back to "256"... A higher number of color bands used to describe the levels of gray (or in our case, levels of red green and blue) would be better, but 256 is usually accepted as "close enough for human eyes".

Consider the extremes: If you divide the range from black to white into only 10 steps, it's easy to differentiate between 20% black and 30% black. But let's say you divide the range between black and white into a million shades of gray. Could you tell the difference between gray#500000 and gray#500001? Probably not. That's why a JPG with 16.7 million colors looks better than a GIF with 256 colors.

But if you use 256 steps, you approach the reasonable ability to barely tell the difference between gray#125 and gray#127. So PSP uses a range of 256 steps.

Digital cameras do a great job of finding the brightest white it senses, and the darkest black it senses, then finding a balanced gray and dividing the steps to depict the shades and color.

But your question keyed on contrast ratio...

For contrast ratio, let's consider the low end and high end separately. CRT tube projectors do a great job at the low end (dark) because they can project black by just turning their electron beam OFF, thus not illuminating the phosphor, thus no light is generated.

But a bulb projector (LCD/DLP) has a harder time showing black because it's hard to stop light from a high wattage bulb with just a pixel. However, LCDs have an easier time with the high end (brighter whites). But even bulbs have their limits on brightness, so when you divide the brightest white (for LCD, let's say 1000 lumens), by the darkest black (let's say 2 lumens), you get 1000:2 which is the same as a 500:1 contrast ratio (all high:low values are only notional).

But CRTs aren't as bright, so let's say their brightest white is 250 lumens. However, CRTs can make great blacks (by turning the tube phosphor energy OFF), so let's call their black 0.01 lumens, so the CRT's contrast ratio could be 250:0.01 or 25000:1.

But black on a screen can only be as black as the ambient light. So if ambient light is 1 lumen, it moots the 0.01 lumen black of the CRT, so the effective CRT CR becomes 250:1. Conversely, LCD CR is less affected by ambient light.

And now back to how I propose any of this relates to the white and black values I measured in PaintShopPro from our screenshots...

This is not an attempt to debunk screenshots, but I think AWB and auto-exposure settings in digital cameras can compensate for whites that aren't really white and blacks that aren't really black.

The digicam looks for the brightest pixel it sees and calls it "white". Then it looks for the darkest pixel it sees and calls it "black". Then it does AWB and decides how to divvy up the steps between black and white to best depict the subtle shades and details without clipping the bright parts or crushing the dark parts.

My PaintShopPro (PSP. aka a poor man's Photoshop) was only measuring what the digicam had already adjusted for and recorded as a JPG... it found the brightest parts of the screenshot and saw that the camera stored it as "white", so PSP reported "255" (white), even though the screen was really a light shade of gray.

Then PSP saw the darkest part of the screenshot that the digicam called "black" (even though there was ambient, reflected, and bleed light from a very high wattage bulb in the room), so PSP dutifully reported the darkest pixels of the screenshot as 0,0,0 (black).

Phew. Sorry to ramble. I honestly look forward to better explanations...

-Clarence

There are a few other posts, but they're starting to get lost in the 37 pages of the original superplex discussion, so I'm going to start a new thread here.

My intent is to try to learn how most of the variables can be adequately accounted for in order to produce a screenshot as "accurately" as possible.

This is not a "debunking" thread!
See less See more
Status
Not open for further replies.
1 - 12 of 12 Posts
Quote:
 Originally posted by CMRA From those last posts I can tell somebody knows something. In an effort to solve my own curiosoty I recaptured that same scene, this time with my other digicam, an Oly 2100uz. Settings= 'auto' white balance, ISO 100, Aperture prioity (same as the ES-100). I leave it to you experts to determine the difference between the two. I'm posting both shots for comparison. (download and do your thing). Here is the UZ 2100 photo. Have at it, experts!
CMRA's Oly 2100uz screenshot retake
http://members.aol.com/marcorsyscom/cmra-2100.jpg

Quote:
 Originally posted by CMRA Ok, here is the controversial and offending photo from the ES-100 previously posted. Download, compare, analyze, manipulate, what have you. Tell me you findings.
CMRA's original ES-100 screenshot
http://members.aol.com/marcorsyscom/cmra-orig.jpg
See less See more
Instead of auto white balance (AWB), can someone describe the most accurate manual way to accurately set a digicam's brightness levels? I saw mention earlier of a \$10 photographer's gray card?

Again, this isn't an accusational thread, CMRA & I understand that. I trust that his screens look great! And we've already established there's no post-prcessing (e.g. Photoshop) involved. I'm hoping to learn more of CMRA's excellent screenshooting technique and talent. I'm also hoping to learn more about screen options since I want to build a new screen when we move in a few months. However, I have a CRT, so I'm not too sure these high-gain DIY screens will work as well for me.

Here's the same frame I extracted from the DVD:
http://members.aol.com/marcorsyscom/mouse1.jpg

and the full-size file . CMRA, if you send me the full size image from your digicam, I'll host that too.

Man, I still drool over that blue sky reflection everytime I see your results. Did you use any overlay color enhancements (ZP pre-processing or sharpening) on either of these?

Does anybody care to give me a tutorial on the difference between CONTRAST and BRIGHTNESS settings on a projector? (Brightness seems easy, but what are you really adjusting when you increase contrast?)

Suggestion from the screenshot galleries: too many variables? thread:
Quote:
 Everyone seems to have a Da-lite pull-down floating around... if half the screen of the screenshot was matte white, we'd all have a better frame of reference.
How about something simpler... I don't have a Dalite screen, but how about if people used a standard 89c piece of 24"x36" white poster board.

Isn't 1.0 gain based on white paper? (EDIT: actually magnesium carbonate , but paper's close enough) It's cheap, flat, globally available, and would also provide a sense of scale (e.g. 72" screen vs 10'+ screens )
See less See more
I can see a big difference. It should be plain to all who look closely.

Hopefully, the difference won't start any trouble. I'm sure somone will point out the obvious difference, so I'll beat them to it.

Stewart is leaning in closer to the Phone.

Oh......, and there is no stuck pixel.

......that was fun.
See less See more
There are 71876 unique colors in the ES-100 pic and only 62041 colors in the UZ 2100 pic.

Is that from the camera's abilities or method of photography?
Quote:
 Stewart is leaning in closer to the Phone. Oh......, and there is no stuck pixel.
That's an interesting observation. Or is it common knowledge (and probably a sore subject) that CMRA has a burnt pixel (I'm assuming it's that white speck by Stuart's backpack)?

Are stuck pixels common with LCD projectors? (I have a CRT)

Also, does anybody know of a software DVD player that has "by frame reference" instead of h:mm:ss? (like TMPGEnc's Setting.. Advanced.. Source Range). It'd be convenient when several people try to replicate the results of the same scene on multiple screens.
Quote:
 Originally posted by marcorsyscom That's an interesting observation. Or is it common knowledge (and probably a sore subject) that CMRA has a burnt pixel (I'm assuming it's that white speck by Stuart's backpack)? Are stuck pixels common with LCD projectors? (I have a CRT)
They can be, but in this case, the stuck pixel is in the camera, not the PJ. Otherwise, it would be on both screen shots.
Quote:
 Originally posted by marcorsyscom I'm also hoping to learn more about screen options since I want to build a new screen when we move in a few months. However, I have a CRT, so I'm not too sure these high-gain DIY screens will work as well for me. Man, I still drool over that blue sky reflection everytime I see your results.
To quote Tryg loosely, "I have never seen images of this quality on any screen through any PJ." unquote "loosely".

Your CRT might not need the Cr boost, but it can certainly benefit from many of the SD/MM Light Fusion screen's other attributes that work to make the image look as exacting and as bright as it does. IMO. Of Course.

The Future is so bright ya gotta wear to get along!
See less See more
Quote:
 Originally posted by SDB There are 71876 unique colors in the ES-100 pic and only 62041 colors in the UZ 2100 pic. Is that from the camera's abilities or method of photography?
Well, you have my attention. Is this smoke or real? I wouldn't put anything past the software gurus of today. Please explain.
Quote:
 Originally posted by CMRA Well, you have my attention. Is this smoke or real? I wouldn't put anything past the software gurus of today. Please explain.
I did a unique color count and that's what PSP 7 and Freehand both spit out. Some of it could be attributed to the 2 different frames of the movie but with that high a #, I'm guessing it's the cameras. We would need 2 pics from the exact same frame with the cameras set up in the same place to get any meaningful figures.

Someone else might see this differently, but I'm not too sure what can be had from the 2 existing color counts other than to be amusing.
Quote:
 Is this smoke or real?
A 24 bit image can have up to 16.7 million colors.

I checked a non-screenshot file directly from my digicam and it contained 199,280 colors.

I expect that the JPG compression and size reduction used to upload pictures to this forum will combine similar colors.

I checked my full sized uncompressed PNG which I extracted directly from the DVD and it contained 50902.

So I don't see anything unusual about the color count from either of CMRA's screenshots.

-Clarence
See less See more
Guy's

I'd like to suggest this site for alot of info on dig. photography

Check the "Exposure strategies" section, I think you'll see there's

quite a few variables to deal with.

Also, how different is a SS going to look on my 21" NEC monitor,

compared to someone sitting in front of a flat panel display, from

experience, very different.

Bruce
See less See more
1 - 12 of 12 Posts
Status
Not open for further replies.