How do I take screenshots at my displays native pixel density? ...NOT 72 PPI!

lonesdav000 · Jul 9, 2011

My Mac (I assume all Mac's, and most computers in general) always takes screenshots at 72ppi... why is this? Also, it seems 72ppi is the default pixel density for just about everything. Again, WHY??? Doesn't this seem counter-intuitive when it comes to image size? The difference, although small, can be very noticeable when viewed at 100% actual size.

Screenshot 72ppi ...here is a regular screenshot, unedited.
Screenshot 99.28ppi ...here is a screenshot modified to 99.28ppi (my displays native pixel density), the way I want screenshots to be taken by default.
Comparison ...when compared in Preview with the preference option "Define 100% scale as:" set to "Size on screen equals size on printout" (I prefer a true WYSIWYG experience).

Let me rephrase my question:
It's been explained to me and I've read several articles on the subject. I still disagree with the reasoning for why 72ppi is the default and I'm asking for a hack or third-party app to take screenshots at a pixel density of my choosing.

...or do I have to resort to manually changing it after the fact using something like Pixelmator or Photoshop? At this point I'm having to settle with a folder action that takes a new screenshot and modifies the pixel density in Pixelmator. Adds about 5 seconds of "lets stare at the screen while automator moves the mouse around".

tersono · Jul 9, 2011

You seem to be misunderstanding the whole concept of pixel density vs. resolution.

Yes, 72dpi is an arbitrary value and isn't the same pixel density as your screen (which will vary dependent on the monitor you use). However, a 1920x1200 screenshot will be EXACTLY THE SAME NUMBER OF PIXELS whether the file is assigned the nominal value of 72dpi, 96dpi or 300dpi (the latter usually used for print).

The reason why screenshots work this way is that IT DOESN'T MATTER what the pixel density is - 1920x1200 is always going to have the same number of pixels - and therefore the same resolution - regardless.

wordoflife · Jul 9, 2011

May I ask how you got your menu bar like that?
and I noticed it actually spells out the day of the week ... how?

simsaladimbamba · Jul 9, 2011

wordoflife said:
May I ask how you got your menu bar like that?
and I noticed it actually spells out the day of the week ... how?

iStat Menus can do that.

lonesdav000 · Jul 9, 2011

tersono said:
You seem to be misunderstanding the whole concept of pixel density vs. resolution.

Yes, 72dpi is an arbitrary value and isn't the same pixel density as your screen (which will vary dependent on the monitor you use). However, a 1920x1200 screenshot will be EXACTLY THE SAME NUMBER OF PIXELS whether the file is assigned the nominal value of 72dpi, 96dpi or 300dpi (the latter usually used for print).

The reason why screenshots work this way is that IT DOESN'T MATTER what the pixel density is - 1920x1200 is always going to have the same number of pixels - and therefore the same resolution - regardless.

Yes... I KNOW IT WILL HAVE EXACTLY THE SAME NUMBER OF PIXELS!!!!!!
That is irrelevant!!!

...and no, I am not misunderstanding the concept of pixel density.
Resolution is the number of pixels, pixel density is the number of pixels per inch.

The point is I prefer to view images in their real world dimensions (inches). A very literal example would be viewing an image of a ruler at actual size, or perhaps a product I wish to buy, or a 4x6 inch photo. Another example would be taking a screenshot of my screen, being 20 inches, and viewing it on a different computer with a larger screen and different pixel density, and still have it be presented as a 20 inch diagonal image.

wordoflife said:
May I ask how you got your menu bar like that?
and I noticed it actually spells out the day of the week ... how?

I've had it for a while... I'm pretty sure I followed the directions here.
I use "MenuCalendarClock for iCal" for a custom clock.

lucidmedia · Jul 10, 2011

Does your mac even know what the PPI of your monitor is? You can set a resolution in preferences, but unless the PPI information is "handshake-ed" between the monitor and computer on startup, your mac would have no way to know the native PPI of a device.

I suppose you could write a script that sets all screenshots to a certain PPI...

lonesdav000 · Jul 10, 2011

lucidmedia said:
Does your mac even know what the PPI of your monitor is? You can set a resolution in preferences, but unless the PPI information is "handshake-ed" between the monitor and computer on startup, your mac would have no way to know the native PPI of a device.

I suppose you could write a script that sets all screenshots to a certain PPI...

Actually, I have no way of knowing the exact pixel density of my screen. The screen isn't really exactly 20 inches, so just doing the math doesn't work. 99.28 is the closest number I could come up with by eye and even that still seems to be off just enough to make the image blurry.
If you, or any one else, knows the exact pixel density of the 20 inch, early 2008, iMac, I'd appreciate it.

Consultant · Jul 11, 2011

lonesdav000 said:
Actually, I have no way of knowing the exact pixel density of my screen. The screen isn't really exactly 20 inches, so just doing the math doesn't work. 99.28 is the closest number I could come up with by eye and even that still seems to be off just enough to make the image blurry.
If you, or any one else, knows the exact pixel density of the 20 inch, early 2008, iMac, I'd appreciate it.

You can calculate it with basic math.

fluffyroo · Jul 11, 2011

Hey there, was just reading over this thread and thought it was an interesting topic. I'm no expert on this topic at all, but I know a bit about resolutions and dpi and ppi from some photography and journalism courses. For your iMac, if you go to this link:

http://www.everymac.com/systems/app...20-inch-aluminum-early-2008-penryn-specs.html

It shows that the LCD is 20" and the resolution is 1680x1050. You probably already know this much. I found a link to a PPI calculator here:

http://thirdculture.com/joel/shumi/computer/hardware/ppicalc.html

After inputting this information the result was 99.06ppi, so I'm assuming you've measured closer and got that different number. However, I think we're getting DPI and PPI confused. DPI refers to printed dots and the space between them, where PPI refers to square pixels in a digital image. The reason this is important is, say you have a 300ppi image you shot with your DSLR. When you convert PPI to DPI you basically halve it because you now have to account for dots and spaces.

However, more to your point, displays differ in pixel density or PPI because of the manufacturing process from each company. For instance, my 13" macbook pro's resolution is 1280x800 at 13" or 116.11 PPI. My Dell 24" monitor connected to my 27" iMac has a resolution of 1920x1200, or 94.34 PPI. My 27" iMac has a resolution of 2560x1440, or 108.79. As you can see, all of these monitors have different PPI, so if I take a screenshot of my 13" MBP and copy the image to my iMac, viewing it on both monitors it will be at a differing size.

As far as an answer about the 72 DPI Tiff Images, I can only assume the entire OS defaults to 72 DPI when you are printing something, an overall standard so that there's no confusion. For example, if you printed out a webpage order confirmation on a 13" macbook pro, it'd be larger than printing it on a 27" iMac because the DPI would be different.

Take the iPhone for instance. The iPhone 3GS has a 163 PPI at 480x320. The iPhone 4 has 326 PPI at 960x640. Yet they're both 3.5" LCDs so the menus and dock size and icon size are all exactly the same ratios, just one's higher resolution. I hope this helps, and let me know if I got anything wrong.

iphonepiephone · Jul 11, 2011

Forums make people lazy. Google makes people lazy. If you come to a forum and expect answers that align with your (sometimes erroneous) perceptions, and they end up not doing so, you may get angry, and cause resentment and irritation.

In short, I recommend you work it out for yourself, OR be civil.

Designer Dale · Jul 11, 2011

lonesdav000 said:
Actually, I have no way of knowing the exact pixel density of my screen. The screen isn't really exactly 20 inches, so just doing the math doesn't work. 99.28 is the closest number I could come up with by eye and even that still seems to be off just enough to make the image blurry.
If you, or any one else, knows the exact pixel density of the 20 inch, early 2008, iMac, I'd appreciate it.

This web page will tell you the dpi of the screen for you.

Screen dpi Calculator

Dale

lonesdav000 · Jul 11, 2011

Consultant said:
You can calculate it with basic math.

Designer Dale said:
This web page will tell you the dpi of the screen for you.

Screen dpi Calculator

That only allows for a rough approximation, I've tried it and the difference, although small, bugs me too much. The 20" early 2008 iMac display isn't really exactly 20 inches, Apple apparently rounded up for maketing. In my situation, I need exact measurements to get an exact pixel density and 99.28 is the closest I could come up with short of spending hours going through every single decimal point or taking the thing apart and measuring it with lasers.

iphonepiephone said:
Forums make people lazy. Google makes people lazy. If you come to a forum and expect answers that align with your (sometimes erroneous) perceptions, and they end up not doing so, you may get angry, and cause resentment and irritation.

In short, I recommend you work it out for yourself, OR be civil.

How am I not being civil?

I would prefer screenshots be taken at a pixel density of my choosing, not 72ppi, and I do not know where this particular variable can be changed in Apple's software. I have come to this forum hoping that some one else might know of a hack to accomplish this or perhaps of a third-party screen capture application that would allow it.

If you consider this to be "lazy" and your only advice for me is to essentially figure it out myself, I thank you for this advice, but, it is unfortunately of no help to me as I simply do not know how. If I did, I would not be asking for assistance in a forum. Do you know where and how Apple's software assigns pixel density data to a screenshot?

fluffyroo said:
As you can see, all of these monitors have different PPI, so if I take a screenshot of my 13" MBP and copy the image to my iMac, viewing it on both monitors it will be at a differing size.

If the screenshot included accurate pixel density data instead of a generalized number like 72ppi, the image could be viewed at the same apparent (original) size on any display ...which is the point I am trying to make.
I assume you understand that, though. If so, you and I are the only one's that seem to get it.

jwhazel · Jul 11, 2011

This is not meant to be a confrontational post. It's just that I still don't think you understand the difference between ppi and resolution as it pertains to screenshots. 72 is an arbitrary number, it doesn't matter what it is. Could be 1, could be 1 million. Screen capture programs can only record the value per each pixel in the frame buffer and record it to a file. If you don't believe me, go back and look at your original question:

Also, it seems 72ppi is the default pixel density for just about everything. Again, WHY???

Exactly. Why? Why is it than in 20+ years of screen capture software, non of them bothered to include the option of setting or automatically acquiring native ppi? Answer: because it doesn't matter. The DPI that you're setting manually is for print, it's irrelevant to screen viewing. In the example your posting, you set the option "Size on screen equals size on printout". This is because it's resampling the image to give you an approximation of what the printout will look like relative to the size of your screen.

...or do I have to resort to manually changing it after the fact using something like Pixelmator or Photoshop?

This is exactly what you have to do because this is exactly what these programs are designed to do: resample the image, which is what you're doing by attempting to "view an image of a ruler at actual size" regardless of ppi. Screenshot programs just copy the framebuffer based on resolution, they have no clue what ppi is.

I tried to explain this the best I could. Sorry if I failed.

Edit:

If the screenshot included accurate pixel density data instead of a generalized number like 72ppi, the image could be viewed at the same apparent (original) size on any display ...which is the point I am trying to make.
I assume you understand that, though. If so, you and I are the only one's that seem to get it.

I get what you're trying to say. What I don't think you understand is that there are 3 things here: resolution, ppi, dpi. All of them are independent of the other. Screenshots don't understand what ppi is. The programs you are using to change the dpi are resampling the image and changing the resolution. If you have photoshop, go into image size and change the resolution from 72 to 300 but uncheck "resample image". Notice that the pixel dimensions did not change, the image will look the same on a 72 ppi monitor as it will a 99 ppi, as in it will occupy the same amount of pixels on either monitor. The only thing that changes is the print size. Wikipedia sums this up pretty nicely:

http://en.wikipedia.org/wiki/Dots_per_inch
A digitally stored image has no inherent physical dimensions, measured in inches or centimetres. Some digital file formats record a DPI value, or more commonly a PPI (pixels per inch) value, which is to be used when printing the image. This number lets the printer know the intended size of the image, or in the case of scanned images, the size of the original scanned object.

lonesdav000 · Jul 11, 2011

jwhazel said:
If you have photoshop, go into image size and change the resolution from 72 to 300 but uncheck "resample image". Notice that the pixel dimensions did not change . . .

Thats exactly what I've been doing, using Pixelmator. Tripple-check the images I uploaded.

jwhazel said:
. . . the image will look the same on a 72 ppi monitor as it will a 99 ppi, as in it will occupy the same amount of pixels on either monitor.

...except for when you apply the option "Size on screen equals size on printout", in which case the software resamples the image giving an approximation of the actual size when "printed on paper". As I've said, this ability can be very useful when the original screen size is important in sharing screenshots. A screenshot taken on a 20 inch screen can then always be viewed as a 20 inch image, regardless of the pixel density of the screen its being viewed on.

Wikipedia Post
Some digital file formats record a DPI value, or more commonly a PPI (pixels per inch) value, which is to be used when printing the image. This number lets the printer know the intended size of the image, or in the case of scanned images, the size of the original scanned object.

Imagine, if you will, taking a screenshot is like making a "scanned image" of your screen. One would expect the original size of said object, the screen, to be accurately represented in the image files pixel density/ppi/dpi/resolution, or whatever. (...I'm not sure why the specific wording "pixel density" seemed confusing before. Even the Wikipedia entries for Pixel Density and DPI makes it seem as though the terms are interchangeable. Sorry if I'm still not getting it right, but my use of the terms "pixel density" and "ppi" still seem appropriate. ...I might be wrong, either way it's beside the point.)

jwhazel said:
Screenshot programs just copy the framebuffer based on resolution, they have no clue what ppi is.

Somewhere along the line, 72ppi is applied as the default pixel density used in screenshots. That is a very specific number. Where does it come from? It has to be changeable somewhere. Unless, of course, 72ppi is used as the default in all image files when no pixel density data is available?? Is that the case????

Edit:
On a side note, I think I now understand the issue with my wording. Its become sort of a habit, but I'll try to use the term "resolution" instead. Sorry again for what ever confusion it caused. (still not sure why it was such a huge issue)

Jim Campbell · Jul 12, 2011

lonesdav000 said:
Somewhere along the line, 72ppi is applied as the default pixel density used in screenshots. That is a very specific number. Where does it come from?

Back in the mists of pre-history, Apple made the only machines on the market that gave you true WYSISYG for desktop publishing. The original Apple Laserwriter printer output at 144dpi -- having a screen resolution of 72dpi made the maths for WYSIWYG easier.

Cheers

Jim

jwhazel · Jul 12, 2011

lonesdav000 said:
Somewhere along the line, 72ppi is applied as the default pixel density used in screenshots.

No, it isn't. This is what we're trying to tell you. Screenshots have no idea what ppi/dpi is. what your seeing as 72 is the default setting in whatever graphic program you're using being applied for printing. Changing it does nothing (unless you're printing).

http://forums.adobe.com/message/2910161
Read some of the replies in this thread. I think it will drive my point home better.

Edit- also this:
http://answers.google.com/answers/threadview/id/138915.html
The answer to the first of your questions relies on the fact that Pixels Per Inch in graphics files is just a
simple value written into the file, and changing the PPI merely changes this number, and nothing else.
...
But
merely changing the PPI will not affect the display on the image on
your computer monitor.

dmz · Jul 12, 2011

Where does it come from?

I hate to correct our illustrious colleague, Jim, but the Apple ImageWriter had a 144 dpi native resolution, the LaserWriter was 300 dpi. However, the original 72 dpi specification was based on the original Macintosh's native dpi - which was correlated to the traditional printer's measure of ~72 points to the inch.

This allowed a direct correspondence between the size of an object on the screen and the printed size - part of the WYSIWYG philosophy introduced by Apple and others. All the early Apple monitors were based on this same assumption - that the screen pixels were approximately 1/72 inch square. You can see this if you examine the specs of the early Apple monitors, e.g.:

9-inch - 512x342 ~72 dpi
12-13-inch - 640x480 ~69-76 dpi
15-inch portrait - 640x870 ~77 dpi
19-inch two-page - 1152x870 ~77 dpi

This convention has been lost as pixel densities have gone up, but lingers on as the "default" resolution for screen graphics on Macs - some Windows programs will default to 96 dpi, closer to what more computers were displaying by the mid-90s, but as someone has already pointed out - this spec is rather meaningless - what really matters is how many pixels there are in the source image.

There are many, many discussions of image resolution here at MR, and some good references available online. There is a difference between pixel-density, PPI and DPI, and resolution is a rather arbitrary term as used in digital imaging. Even wikipedia is not authoritative on these definitions, so check as many sources as you can. Good luck, and my apologies to Jim for being so pedantic as to "correct" him.

Cheers!

dmz

Jim Campbell · Jul 12, 2011

dmz said:
I hate to correct our illustrious colleague, Jim, but the Apple ImageWriter had a 144 dpi native resolution

My apologies post in haste; remove egg from face at leisure.

Cheers!

Jim

lonesdav000 · Jul 12, 2011

jwhazel said:
No, it isn't. This is what we're trying to tell you. Screenshots have no idea what ppi/dpi is. what your seeing as 72 is the default setting in whatever graphic program you're using being applied for printing. Changing it does nothing (unless you're printing).

http://forums.adobe.com/message/2910161
Read some of the replies in this thread. I think it will drive my point home better.

Oh... Ok, thank you. Sorry for being so stubborn.
Now: Is there a way make what ever OS X uses to capture the screen, apply a specified resolution value?

lonesdav000 · Jul 12, 2011

jwhazel said:
. . . merely changing the PPI will not affect the display on the image on
your computer monitor.

...unless you apply the option "size on screen equals size on printout", in which case the software resamples the image giving an approximation of the actual size when "printed on paper".

iconmaster · Jul 12, 2011

lonesdav000 said:
The point is I prefer to view images in their real world dimensions (inches).

What you want to do is view a screenshot taken of your own display at the actual dimensions of your screen? And you're worried about resolution?

No, I'm sorry. I can't help you at all.

lonesdav000 · Jul 12, 2011

iconmaster said:
What you want to do is view a screenshot taken of your own display at the actual dimensions of your screen? And you're worried about resolution?

No, I'm sorry. I can't help you at all.

...then just don't post anything. ?????
You don't seem to understand the point I'm trying to make, anyway.

jwhazel · Jul 12, 2011

lonesdav000 said:
Oh... Ok, thank you. Sorry for being so stubborn.

It's not a problem. Just as long as you truly understand: that 72 number that you keep seeing has nothing to do with your screenshots... unless you're printing them. It's not initially embedded into the image in anyway. It's a somewhat arbitrary number the program you're viewing them in is using to try and tell you how big it will be when you hit the print button and look at the sheet that comes out.

lonesdav000 said:
Now: Is there a way make what ever OS X uses to capture the screen, apply a specified resolution value?

You mean like converting from a native res of 1360x1768 up to 1920x1200 or down to 800x600? (I just used arbitrary values there). Theres no way that the native tools in OSX will allow you stretch or resample an image. Again, these tools just capture the screen (or a selection) and dump it into a file. You need image manipulation software like preview, pixelmator, photoshop, etc... to actually perform any modifications to the file. One option would be to write a script that uses the terminal command 'screencapture' to output the screen to a file, then use imagemagick's 'mogrify' command to resize it.
http://www.manpagez.com/man/1/screencapture/
http://www.imagemagick.org/script/mogrify.php?ImageMagick=onb9jmq8pp4raqb7iaqlkrovv5

I have to point out here though that by applying "a specified resolution" you are resizing/resampling the original image. This will slightly distort it depending on a variety of factors.

Quote:
Originally Posted by jwhazel
. . . merely changing the PPI will not affect the display on the image on
your computer monitor.
...unless you apply the option "size on screen equals size on printout", in which case the software resamples the image giving an approximation of the actual size when "printed on paper".

I admit, now maybe I am confused. Are you trying to resize screenshots for desktop viewing or for printing? If it's for printing, then you are on the right track with trying alter the DPI to get 1:1 sized images.

If you're just trying to view these pictures on your screen then why would you select "size on screen equals size on printout"? I was wrong before. Looking at your example it doesn't even resample the image, it just zooms in on it. Like if you opened pixelmator/photoshop and just zoomed to 120%. This is even supported by the fact that the horizontal and vertical scroll bars of each window are different lengths in that picture (because each image contains the same amount of pixels, the right window is just zoomed in and therefore has more "canvas" area). All preview is doing is zooming in on it to give you a rough idea of what it would look like if you print it out on paper. I can't see any reason for wanting to view images with that option on a regular basis (and not planning on printing them).

flipster · Jul 12, 2011

I'm good with computers and hardware, but not when it comes to graphics.

Is this why when I take a full screenshot of my desktop, it comes out ugly and blurry? Is there any way around this to make it look clean? I don't see why not, it looks like it's been compressed!

How do I take screenshots at my displays native pixel density? ...NOT 72 PPI!

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