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Re: SOLVED - blurry screen captures - Snagit for Mac (4.1.2) -- DOES ANYBODY REALLY UNDERSTAND PNGs?
Subject:Re: SOLVED - blurry screen captures - Snagit for Mac (4.1.2) -- DOES ANYBODY REALLY UNDERSTAND PNGs? From:"Sweet, Gregory P (HEALTH)" <gregory -dot- sweet -at- health -dot- ny -dot- gov> To:"Wright, Lynne" <Lynne -dot- Wright -at- Kronos -dot- com>, TechWR-L <techwr-l -at- lists -dot- techwr-l -dot- com> Date:Mon, 10 Apr 2017 14:02:22 +0000
I think where you are getting hung up is in the difference between difference between raster (each pixel gets data) and vector (lines and shapes mathematically defined) images, and how a display unlike an established image file can contain both raster and vector elements.
Vector images/elements donât care about individual pixels. They simply define the space to be filled with pixels for give size so clarity is maintained at almost all sizes. The image can become small enough that you canât fit in enough pixels to maintain clarity.
Raster images store information about each pixel.
When I have a 96 ppi raster image displayed on a 96ppi screen at 100% zoom, itâs pixel size will be 1:1, each pixel of the image will take exactly one device pixel to display.
Zoom in on that image to say 200% then the display must use more physical pixels to render the image. In this case four device pixels will be used to render each image pixel (two in each direction). Since the computer has to interpret what the three extra pixels look like, things can start to look blocky and pixelated.
Zoom out to 50% the display must use fewer physical pixels, or one device pixel for every four image pixels. Since the computer has to decide which three pixels not to show, the image can lose clarity.
This is almost identical to resampling an image in an image editor.
Zooming in makes a difference when you take a screen capture because when you take a screen capture you record the state of the device pixels as they exist at that point in time, regardless of what caused those pixels to be displayed (raster or vector sources) and the pixel data is saved to a new raster image file. If the source elements are vector everything should be nice and clear, if the source was a raster youâll capture whatever the zoom resample did to the image. And just so Z wonât get mad at me â technically you are capturing the pixels as defined by the graphics card so imperfections from your monitor are not captured.
Cheers!
Greg
On 4/7/17, 4:29 PM, "techwr-l-bounces+gregory -dot- sweet=health -dot- ny -dot- gov -at- lists -dot- techwr-l -dot- com on behalf of Wright, Lynne" <techwr-l-bounces+gregory -dot- sweet=health -dot- ny -dot- gov -at- lists -dot- techwr-l -dot- com on behalf of Lynne -dot- Wright -at- Kronos -dot- com> wrote:
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mmmm...i'm skeptical: I've read several online articles trying to sort this out, and in all of them, the rule of thumb is that resizing an image changes pixel sizes (but the number of pixels stays the same); resampling changes pixel count; and that the number of pixels in an on-screen image is determined by the screen resolution, not the zoom level.
You seem to be saying that if you zoom in on an on-screen image (which is basically resizing it, right?), in effect the image is resampled ... but not always... it depends on the source image? Aside from the fact that that contradicts the rule of thumb, couldn't you achieve the same results by taking the shot at whatever zoom level, then apply resampling when you output the image+callouts graphic?
Lynne Wright wrote:
> I'm confused about why zooming in before taking the screen shot would make a difference. As you said, the number of pixels that makes up an image is fixed; and the amount of visual information in each pixel is fixed. So no matter what the zoom level of the object that you're taking a screenshot of, the image is going to contain the same number of pixels; its just that the larger the image, the larger the pixels within it are.
Um, no, not quite. Pixels do _not_ become larger as you zoom in, but the _number_ of pixels in the capture increase. As long as the specific _section_ of what I am capturing is the same, of course!
But I should have clarified my comment a bit for one thing: It depends on the source of that displayed image that I am capturing - i.e., whether it is from a bitmap (like a picture) or from a vector format (PDF with text) to begin with.
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For example, let's say my capture is of a section of a page from a PDF that contains text (from Word or FrameMaker or ... whatever) in vector form, or a line drawing from some other application program.
When I "zoom in", that text is rendered ("rasterized" in older jargon - from CRT monitor days) in higher resolution - the conversion of the font (in vector format today!) to the number of _specific_ screen pixels depends on how large the text is shown.
So, the section of the image I am capturing actually gets more and more total "pixels" in it as I zoom in to such vector sources - the pixels do not get larger.
If I use a decent high-resolution monitor (mine are 1920 x 1200 - laptop is 1920 x 1080), then enlarging the specific portion I am capturing, results in more total pixels in each dimension.
If I had a 4k monitor or Retina monitor, then the total pixels in each dimension would also be higher ... numerically.
(Importantly, Snagit - like _all_ screen capture programs - does not "read" the _physical_ pixels on your monitor, it "reads" the graphics memory of the graphics cards/chip that is connected to that display.)
So, if you have dead pixels on your monitor, or the color is somehow screwed up (Green not showing for example), the captured output from Snagit will still be fine! The capture will still display badly on that broken monitor, of course, but will be fine elsewhere!
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Now, what if what you are capturing is a displayed bitmap to begin with (like a photograph from a web site)?
This does _not_ necessarily benefit from zooming in. It depends on the number of absolute pixels in that bitmap (photograph) to begin with, and the ratio of "image pixels" to "screen pixels" shown during the capture.
To understand this, let's imagine a photograph with absolute 1920 x 1080 pixels in the X and Y dimensions. When viewed using a 1:1 map of "image pixels" to "screen pixels", this would fill the screen of a 1920 x 1080 monitor, leave a black band above/below a 1920 x 1200 monitor, and be a small portion of a 4k monitor. And be too large to show 1:1 on a 1280 x 1024 monitor!
To get a good capture of a bitmap image, it is ideal to try for a 1:1 mapping of image pixels to screen pixels.
Zooming a bitmap image _up_ means that you are capturing the ability/quality of the display program that "adds" interpolated information and do a good job.
Zooming a bitmap image _down_ means that you are dropping original source information. Depending on the ratio of that map, some display programs don't do a good job of showing that bitmap.
If an image is too large to show with 1:1 mapping on a display, I try to zoom out such that the ratio is integral (like 2:1 or 3:1) ... this allows the display program to simply drop/average pixels rather than trying to resample unusual ratios decently.
I have also used the "scroll" ability of Snagit for good results too.
> So isn't it the resolution of the screen that makes a difference, not the zoom level? Ie. higher res screen image = higher density of pixels = higher res png capture?
Well, yes, resolution of the monitor makes a difference in a way (better said, the resolution being used by the graphics card makes the difference).
Rendering a vector original source to a higher resolution monitor (really: more graphics memory in the graphics card) provides a higher total number of pixels and higher capture.
Note that a 21" monitor with 1920 x 1080 native resolution has smaller _physical_ pixels than a 27" monitor with the same 1920 x 1080 native resolution.
But a screen capture on _either_ of the above monitors will be identical (as long as the "zoom" on both monitors is set the same for a given image being captured).
I.e., the physical size of the monitor does not matter.
And, yes, capturing a vector source information rendered on a 4k monitor will give you more total pixels in the capture than a 1920 x 1200 monitor too!
This is because the underlying application program that converted that vector source to the monitor (graphics card) pixels had more to work with!
Again, ultimately, it is the memory of the graphics card that is captured, not the physical pixels on the monitor.
Hope this helps explain matters a bit better.
Z
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