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RJ Emery
10-22-2005, 12:18 PM
When it comes to computer monitors, I am confused about aspect ratios. It seems CRTs have a different ratio than flat panel displays, and I do not know the visual difference. I wish I could put an equiavlent sized CRT and flat planel display side-by-side, each displaying the same image full screen. Perhaps then I would see something.

If someone could help me understand the difference in aspect ratios for the two types of monitors, I would be most appreciative.

Why, for example, IS there a difference?

Does it matter?

Why should it matter or not?

Stephen Owades
10-22-2005, 09:01 PM
When it comes to computer monitors, I am confused about aspect ratios. It seems CRTs have a different ratio than flat panel displays, and I do not know the visual difference. I wish I could put an equiavlent sized CRT and flat planel display side-by-side, each displaying the same image full screen. Perhaps then I would see something.

If someone could help me understand the difference in aspect ratios for the two types of monitors, I would be most appreciative.

Why, for example, IS there a difference?

Does it matter?

Why should it matter or not?
The answer to your question depends on what you mean by "different [aspect] ratio." Almost all CRT and LCD computer displays use "square pixels"--i.e., the number of pixels per inch is the same horizontally and vertically--in their "normal" display modes. And you want square pixels, to avoid seeing everything squished or stretched disproportionately. So the physical aspect ratio of the screen should correspond to the ratio of horizontal to vertical pixels. A "standard" monitor, either CRT or LCD, tends to have an aspect ratio of 4:3 (1600x1200, 1024x768, 800x600, 640x480) or close to it (1280x1024).

Because people are starting to use their computer displays (especially laptops) for widescreen movie viewing, and manufacturers are making LCD panels for video use, we're starting to see more widescreen (16:9) displays on computers. Such displays should be fed with the appropriate ratio of pixels, like 1920x1080 or 1280x720. If such a display receives a signal with 4:3 proportions (e.g., 1024x768), it's either going to show black bars at the ends or stretch everything out; neither option is ideal, but stretching is worse.

If your computer's graphics card and drivers are capable of generating widescreen pixel ratios, you can use a widescreen LCD monitor properly. Whether you want to is up to you. Standard 4:3 displays are available in both CRT and LCD forms, and should produce equivalent images when fed with standard pixel ratios.

If you have something else in mind, please explain your question more clearly and I'll try to help.

Richard Waller
10-22-2005, 11:45 PM
A parallel question. My smart new TV has a screen 23 x 13 which is not 4 x 3 or 16 x 9 so either looses the bottom of the the picture, or shows black top and bottom, or when on Auto has a picture smaller than is acceptable. Who designs these things?

Michael Rowley
10-23-2005, 06:11 AM
Richard:

a screen 23 x 13 which is not 4 x 3 or 16 x 9

It's about as close as you can get: actual, 1.769; required, 1.777.

Richard Waller
10-23-2005, 10:48 AM
OK. So why does the Auto feature screw up here?

Michael Rowley
10-23-2005, 12:20 PM
Richard:

So why does the Auto feature screw up here?

I don't know what 'auto' is supposed to do, but I presume it's detects whether the width:height ratio of the signal fed to the monitor is (much) greater than 1.33. On a CRT, any slight difference between a perfect ratio of 16:9 and the actual ratio is dealt with by adjustment of the width or height.

RJ Emery
10-23-2005, 06:22 PM
If you have something else in mind, please explain your question more clearly and I'll try to help.

I will try to rephrase the question, but not being all that knowledgeable and certainly being no expert, I am having difficulty articulating what I am trying to express.

IIRC, the last time I checked, which was about two years ago, a CRT had a 4:3 aspect ratio, while a "corresponding" flat panel display had a 5:4 aspect ratio. In my mind for a given image, that meant the flat panel monitor would display a "distorted" image vis-a-vis the CRT.

I understand now that some flat panel monitors can be wide screen, therefore displaying a much different aspect ratios.

I DO NOT WISH TO DISCUSS SUCH WIDE SCREEN FLAT PANEL MONITORS IN THIS THREAD. Anyone who has a desire to do just that should feel free to go off and start another thread.

I wish to understand how 4:3 CRTs and 5:4 LCDs can possibly display the same view given the same resolution (e.g., 1024x768). That one issue stopped me from considering LCDs for DTP work, among other projects sensitive to WYSIWYG graphics.

gary
10-23-2005, 07:33 PM
The pixels on the LCD are not perfectly square.

After running Auto-Adjust
The image on my 19" Samsung LCD measures 374mm x 297mm; this is a 1.259 ratio (closer to 1.25 than 1.33)
This means each of the 1280x1024 pixels is 0.292mm wide and 0.290mm tall (1.0069 aspect)
This means 86.9 pixels/inch horizontal versus 87.6 pixels/inch vertical

The viewable area on my 17" ViewSonic CRT is 318mm x 242mm; this is a 1.314 ratio
This means each of the 1024x768 pixels is 0.311mm wide and 0.315 mm tall (0.987 aspect)
This means 81.8 pixels/inch horizontal versus 80.6 pixels/inch vertical

So, do you prefer wide LCD or tall CRT pixels?
1/0.987 = 1.013 so the CRT pixels are less square than the LCD

Of course I never have to tweak my LCD for bowed sides or rhomboid distortion.

RJ Emery
10-23-2005, 07:50 PM
The pixels on the LCD are not perfectly square. ... So, do you prefer wide LCD or tall CRT pixels?


Gary,

I appreciate your response.

The pixels on the CRT are round if not perfect circles. Your calculations, subject to a slight error, demonstrate that. If the LCD pixels are not square, then it suggests a distortion. Perhaps there would be a distortion given square pixels or not.

On your CRT, at any specific resolution, if you had a (JPEG) image of a perfect circle whose diameter was just less than the viewable height of your CRT, and without making any other kind of adjustments, would it not appear and be measurable as a perfect circle?

If you then took that same image at the same resolution and without any further adjustment, displayed it on your LCD (which has a different aspect ratio than the CRT), would that image then also be measurable as a perfect circle? If so, how and why?

gary
10-25-2005, 06:56 AM
Using GIMP I created an 800x800 image and displayed it pixel-for-pixel. It displayed a box 187mm x 187mm (on the LCD) as near I can measure.

I connected the CRT to a system and booted Knoppix 3.9 then used GIMP to create a 720x720 image display pixel-for-pixel. It display a box 138mm x 140mm. Spending a few minutes adjusting horizontal width, position, vertical height, position, trapezoid, pincusion, parallelogram, rhomboid I get a box 147mm x 148mm.

That was a lot of work to get the CRT looking as clean as the LCD. The good news is that you should only have to do this once.

Note that some (Hitachi?) CRTs use rectangular pixels.

And that brings up another issue - there is a one-to-one mapping between virtual and physical pixels on the LCD whereas that is not necessarily true on the CRT. The grey background on the LCD is uniform whereas on the CRT I can see the composition of red, green and blue dots. On the CRT the red pixels are not in the same location as the blue pixels.

RJ Emery
10-25-2005, 07:12 AM
Gary,

Yes, that was a lot of work to perform on my behalf, and I thank you for that herculean effort. I presume you used a square instead of a circle.

I still don't understand matters. It is not you. It is me. I must have some a bad frame of reference that is preventing me from seeing the light.

I still do not understand why the same image displayed on two monitors with different aspect ratios does not produce a distortion of some sort.

Stephen Owades
10-25-2005, 08:47 AM
I still do not understand why the same image displayed on two monitors with different aspect ratios does not produce a distortion of some sort.If the width-to-height ratio of the image area on a monitor doesn't match the horizontal-to-vertical pixel ratio that's being displayed on it, there will be distortion in the image (it will be squeezed or stretched). But if the difference between the ratios is small, the degree of distortion will be small as well, and it shouldn't be too bothersome.

Stephen Owades
10-25-2005, 08:59 AM
The red-green-blue phosphor dots or stripes on a CRT monitor don't line up with the pixels of a displayed image, nor are they intended to. CRTs don't have "pixels" per se, and it's a mistake to think of them that way. The three electron beams reach the phosphor layer of a CRT by passing through a mask, which basically allows each beam to see phosphors of only one color, but the beams are not aimed at particular phosphor spots. If the mask was aligned with the pixels being displayed, you'd probably see moiré problems, so it's just as well that there's no attempt to do that.

Trinitron monitors (Sony) and trinitron-style CRT monitors (many other manufacturers) use vertical stripes of phosphors and a vertical-slit mask, while "conventional" CRT monitors use "dot triplets" and round-hole masks. The main difference this can make in computer applications is the possibility that thin horizontal and vertical lines may appear with somewhat different thickness or brightness on a trinitron-style monitor, but that's usually not bothersome.

On LCD displays, pixels should correspond exactly with display elements, which are usually vertically-aligned slivers of red, green, and blue. It is because of this correspondence, and the vertical color sub-pixels, that technology like Microsoft's ClearType can work so well to improve the perceived quality of type on an LCD. If the LCD is addressed via analog inputs, the matching of display pixels to screen pixels may not be as precise, and a digital interface will produce visibly better images on an LCD display as a result. And if the image you're displaying doesn't correspond with the actual device resolution on an LCD, the image will be rescaled to the device--always use the device's resolution as your display resolution if possible.

fhaber
10-25-2005, 01:44 PM
RJ, as Gary and Stephen have said, there is always some distortion of aspect ratio on a CRT monitor. It gets worse. There's geometric distortion as well. Circles can be subtly egg-shaped, pointed top and bottom, etc. In the very best of circumstances, the best of CRT monitors can be expected to be within 2% of an overall correct point map, in the inner 80% of the raster, only on Thursdays with a tailwind.

When comparing LCD to LCD, you can confine yourself to pixel shape and H/V pixel counts with some assurance of describing the situation accurately.

humpty
11-05-2005, 06:39 PM
I still do not understand why the same image displayed on two monitors with different aspect ratios does not produce a distortion of some sort.

Most 5:4 monitors have 5:4 frames.
A pixel for pixel image will look exactly the same on a 4:3 and 5:4 monitor, albeit the 5:4 monitor will have more vertical space outside the image.

A '4:3' image can be streched full-size onto a 5:4 monitor frame, in which case it will appear taller, and will not be pixel for pixel.

There are rumours that some monitors have 5:4 (e.g 1280 x 1024) content
but have only 4:3 frames. In which case all images will be distorted.