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Image Size, File Size, and Image Quality ... A Discussion

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I've been wanting to do this for a while and today I have a little time, so here it goes.


I am constantly bombarded by statements about how reducing image weight reduces the image quality.  For the purpose of this discussion, "weight" is determined by the "quality" setting dialogue box when you save a jpeg.


First, it is important to understand the difference between image size and file size.  Image size is simply the length of the image times the height of the image times the resolution in pixels per inch (ppi).  File size is the amount of physical space the entire file of the image (including metadata, etc.) on the hard drive.  File size is often dependent on the internal algorithms within the imaging software being used.


When you press the V key, MSFS captures the screen image and saves it as a bitmap (bmp) image.  Usually, the user will change the image to a jpeg.  When saving as a jpeg, a dialogue box will appear with the choice (a slider) to adjust the image quality.  Obviously, the higher the setting, the more space the file will take up on your hard drive, or a server.  In an effort to reduce the space taken up by forum images on the AVSIM servers, the size limit has been set to 1600 pixels and 400Kb IF you save your image to the AVSIM servers.  If you save your image on a third party image hosting site, obviously that space is not used by an AVSIM server, so the limitations are lifted.


A monitor screen has a resolution of 72 pixels per inch (ppi).  That's the highest resolution it can handle.  It doesn't matter if you call up a 100 megabyte image with a resolution of 300 pixels per inch, the monitor will only display that image at 72 pixels per inch.  Images with resolutions of 300 ppi are used for printed outputs.


My monitor is a 27" ASUS.  When I hit the "V" key, the image size is 1920 x 1017 pixels, at 72 ppi.  This makes a 100% image approximately 26.7 x 14.125 inches on the screen (if my screen was that large).  The algorithms of MSFS and the screen capture vs. monitor size is beyond the scope of this discussion, and my understanding.  Suffice to say, the image saved, on my computer, is larger than the actual screen size at 72 ppi. 


If I save that image as a jpeg without adjusting the size, the result is a 1.43 Megabyte file.  If I reduce the image size to 1600 pixels max, and image quality at max (12), the file is 1.07 Megabytes.  This is still too large to save on the AVSIM servers.  To make the file "legal" you must either reduce the size of the image or the "quality" or both.


Below is four images.  The first image is the original size saved at the highest quality. (1.43 Mb)  The second image is reduced to 1600 pixels at highest quality. (1.07 Mb)  Still to large for AVSIM servers.  The third image is a 1600 pixel image at quality medium 8. (325 Kb).  Quite a change in file size and now we are "legal".  The fourth image is the original saved at 1600 pixels and quality setting low 4.  The image file size is now only 179 Kb.  I challenge you to tell a difference in visual quality.



Original screen capture saved as a jpeg, quality max (12).  1.43Mb





Original image reduced to 1600 pixels max, image quality max (12), 1.07Mb





Original image reduced to 1600 pixels max, quality level medium (8), 325 Kb





Original image reduced to 1600 pixels max, image quality low (4), 179Kb







I hope this clears up a few concepts about image size, file size, and image quality.  Please feel free to comment or ask questions.

Dennis Trawick


Screen Shot Forum Rules



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I use Fraps to capture the image to a PNG.  My resolution is 2560 x 1600 and the PNG size is around 2.6MB.  I use Photoshop CC to save the PNG to JPEG with 60% quality setting (called High in Photoshop CC) with final file size around 360KB.  This images I host on my own web site and just link to.  For images that I'll "attach" in AVSIM I down size the to 1024 x 640 @ 80KB file size, sometimes I'll do 1280 x 800 @ 115KB file size.


But another factor that will change how the image looks on the "end users" computer is the Browser they use (Chrome will look different from IE10).  Also to further complicate matters, if you download the picture to your local hard drive and then use Windows "Preview" (meaning just double click on the image that has a file extension associate with the default OS viewer) you will see an image of lesser quality (thanks to Microsoft - ugh).  I recommend you don't use the built in Windows "Viewer" to look at any downloaded images.


My 2 cents, Rob.

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Here's my take on this...


Since most forums are programmed to display images at a width of between 800 and 1000 pixels, it's pretty much useless to upload them in higher resolution, unless someone makes a request.


First, I do not use the V key, but the 'print screen' key, which saves pictures to fsx root, it's a preference thing. Then, I take all my shots and using an 'action' I made in photoshop, it will re-size them to 1440x900 and add a watermark. :)


By doing that I cut the file size of shot by a lot. Then, I save the shots @ quality level '12' and upload my shots to flickr, earlier acquired by Yahoo. With that purchase, flickr had a redesign (which I hate) and also expanded the free user capacity to 1tb WITH FULL RESOLUTION / ORIGINAL access as well. I'm yet to come across a free image host (that is reliable) that matches this. When uploading pictures to flickr, it will auto batch each shot into couple of sizes (also very convenient) and @ maximum quality so your shots are not compressed / lose quality.

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