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: Syed Zaeem Hosain <Syed -dot- Hosain -at- aeris -dot- net>
To: "Wright, Lynne" <Lynne -dot- Wright -at- Kronos -dot- com>, Monique Semp <monique -dot- semp -at- earthlink -dot- net>, TechWR-L <techwr-l -at- lists -dot- techwr-l -dot- com>
Date: Sat, 8 Apr 2017 06:22:27 +0000

> 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.

As I stated below, your observation applies when resizing a bitmap image ... but that is not what I am describing. I am talking about zooming in within application programs that are converting vector data to pixels (rasterizing) for display.

> 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?

Hmmm ... you are missing a fundamental point. If I zoom in on the _original_ [vector format] data for a drawing or text that uses a vector font (true today), etc., the application program that is converting (rasterizing) the vector data "in real-time" for the specific display _will_ render with more pixels - this is a fundamental graphics concept.

Maybe the following links and documents will explain this concept better than I am doing: https://en.wikipedia.org/wiki/Line_drawing_algorithm and https://en.wikipedia.org/wiki/Rasterisation.

Also look at page 7 of this: http://groups.csail.mit.edu/graphics/classes/6.837/F02/lectures/6.837-7_Line.pdf

Basically, when you zoom in into such vector images at the point of display, the _number_ of pixels used to show a given line segment or text from a vector font, etc., is much larger.

Hope this helps,

Z

-----Original Message-----
From: Syed Zaeem Hosain [mailto:Syed -dot- Hosain -at- aeris -dot- net]
Sent: April-07-17 2:50 PM
To: Wright, Lynne <Lynne -dot- Wright -at- Kronos -dot- com>; Monique Semp <monique -dot- semp -at- earthlink -dot- net>; TechWR-L <techwr-l -at- lists -dot- techwr-l -dot- com>
Subject: RE: SOLVED - blurry screen captures - Snagit for Mac (4.1.2) -- DOES ANYBODY REALLY UNDERSTAND PNGs?

Sorry ... another long-winded post from me.

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.

****

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!

****

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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References:
RE: SOLVED - blurry screen captures - Snagit for Mac (4.1.2) -- DOES ANYBODY REALLY UNDERSTAND PNGs?: From: Wright, Lynne
RE: SOLVED - blurry screen captures - Snagit for Mac (4.1.2) -- DOES ANYBODY REALLY UNDERSTAND PNGs?: From: Syed Zaeem Hosain
RE: SOLVED - blurry screen captures - Snagit for Mac (4.1.2) -- DOES ANYBODY REALLY UNDERSTAND PNGs?: From: Wright, Lynne

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