Reply With QuoteThanks Bryan. That's very helpful, particularly the last part about paper stock.Originally Posted by Midgardsormr
It's pretty easy if you remember what "dpi" actually means: "Dots per inch." Dots = pixels (sorta, but for these purposes it's close enough). So if you have A4, which is 8.3" x 11.7", you need to know how many "dots" (pixels) fit into that area. 300 dots per 8.3" is 300 x 8.3 = 2490. That's only if you can print all the way to the edge, though, so probably plan on losing at least half an inch in each direction, making it more like 7.8 x 11.2.
7.8 x 300 = 2340
11.2 x 300 = 3360
Those would be your maximum pixel dimensions for A4 with a quarter-inch margin printed at 300dpi. You do have to make sure the printer driver honors that resolution, though. It may not be carried along in the metadata of the image for one reason or another, or the printer may ignore it. If the print resolution is not read from the file and you use some default, it may print at 100 dpi or even as low as 72 if the printer assumes an ancient Macintosh screen standard, resulting in a much larger print than you expected. If you force the printer to 300dpi mode, though, it should print at the expected size.
Just make sure that the paper you're using can handle that ink load. Ordinary copy paper will be saturated at that resolution; colors will run, and the paper will warp.
Bryan Ray, visual effects artist
http://www.bryanray.name
Thanks Bryan. That's very helpful, particularly the last part about paper stock.
But how does it translate into real size? what is 2340 pixels in cm? If dpi scales the actual width and height size, what is the reason for choosing it anyway, if i can just make default 72 dpi and make dimensions larger putting what i want in pixels.
If you choose 72 dpi, then 2340 pixels is 2340 / 72 = 32.5 inches, which is approximately 85.8 cm. (2.54 cm per inch, if I recall correctly). At 300 dpi, then 2340 pixels is 2340 / 300 = 7.8 inches, or 19.8 cm.
Changing the dpi changes the visual quality and size of the image when it's printed. The closer the dots are to one another, the better the colors blend together and the sharper the image can appear (to a point—your paper must be able to bear the load of the ink). Choose the print resolution based on the distance you anticipate the viewer being from the page. If it's something they're going to hold in their hands, 300 dpi is a good starting point. 600 is good for gallery quality prints. 120 is pretty good for a poster. A billboard might be as low as 30 dpi because it's meant to be viewed from very far away.
But in summation:
pixels / dpi = physical size
pixels / size = dpi
dpi * size = pixels
Bryan Ray, visual effects artist
http://www.bryanray.name
Midgard has it spot on.
Regarding the bit of text thats confusing you from post #4 "A4 which is 8.3 x 11.7 then at 300dpi that means images much larger than 2500x3500 are a bit wasted" then what I was saying is that A4 is exactly 210x297mm which is approx 8.26 x 11.69 inches. If you use 300 dots per inch then thats 300 x 8.26 dots wide or 2478. For the height its 11.69 x 300 which is 3507. So if you hold a piece of (good quality) paper in front of you without getting right up close it to or using a magnifier, then any image with more than (2478,3507) pixels cannot be printed with any more visual detail. If you plan on getting right up close to it as if you were printing a bit of card for a model that you might look at really close and your paper can cope (which it wont) then maybe you could use 600dpi in which case the image could be up to 5000 by 7000 pixels. Or you could print an A2 image with that same 5000 by 7000 pixel image at 300 dpi.
You can note that if you only ever plan on printing A4 photos and never hold them up to your nose then any digital camera with much more than 8.75 mega pixels is just burning hard drive space for no reason. Theres other reasons why very large megapixel consumer digital cameras are a dumb idea but thats another topic.
Also note, when your printer says 1440 dpi or 2800 dpi or something equally more than 300 dpi that has an impact on dithering more than the colour dpi of an image. When we say 300 dpi we mean 300 dots of any colour per inch. When your printers says 1440 dpi it means 1440 dots of one of the ink colours which is usually one of 4 or one of 6 only and it has to dither it to get a full colour pixel at MUCH less than 300 dpi. You probably wont be able to print at 300 dpi on any normal ink jet printer you can buy but its the setting of the 300 dpi param in the photocopier / printer driver that you need to get to grip with in order for it not to print 4 sheets of A4.
0 cm or a googleplexx of cm'swhat is 2340 pixels in cm?
the pixels have not "changed size" in the two images below
the are the exact same pixels from the same image
now the dimensions are 4096x2048
and that can be printed on a 0.00000001 mm plate
or on a 10000000000000000000000000000000000000000000000 kilometer piece of whatever
physical size is a meaningless concept in digital data
a bit better " visual add" would be
How many lcd screens is it ?
that is dependent on the RESOLUTION it is set at
the default is normally 1920x1080
so that 4096x2048 px map is approx 4 screens ( 2 x 2 )
but still that is a meaningless concept
the res can be set to 800x600
than it is a 5x4 grid of screens
Last edited by johnvanvliet; 02-02-2017 at 04:10 PM.
--- 90 seconds to Midnight ---
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--- Penguin power!!! ---
An interesting post was made to one of the tech news sites and it has some more detail about dithering and shows 11 algorithms and the results to a test image. So if your interested in more detail about error diffusion dithering and how each of the named methods spread the error then this is the post for you:
https://tannerhelland.com/2012/12/28...urce-code.html
I am putting it here so that I can refer back it this myself if I ever need to. Might code up and add two row Sierra in my little image processing app. I dont have any dither in it at present and this one looks better than Floyd Steinberg.
Last edited by Redrobes; 02-24-2021 at 06:41 AM.
http://www.ipol.im/pub/art/2011/g_lmii/ might be of some interest to someone, somewhere...
ipol is a cool site , been using
http://www.ipol.im/pub/art/2017/189/
to inpaint polar areas .It works a bit better than resysenthizer in gimp
--- 90 seconds to Midnight ---
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--- Penguin power!!! ---
An interesting site I came across today that has some in depth explaination of the gamma issue I posted about in #12 of which, as I mentioned, I was not completely familiar with the reasons for it. The author mentions that you can use the gamma adjustment pre and post scaling to sort of fix the issue but it seems that this is a bit of a hack and to do it properly requires some more coding on the scale. So maybe one day ill add some gamma corrected scaling to my utilities.
But if your really interested to know then this page is really extensive. Its certainly cleared up why it happens although a lot of the examples in the pages are said to be incorrect but look correct on my monitor. It could be that my monitor is not performing the right gamma adjustments or has been set so that it linearizes the brightnesses and therefore breaks the assumption of the gamma adjustment. Well, you may get different milage from this to me !
http://www.ericbrasseur.org/gamma.html
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