Well, I have a little question about renderings resolution and I hope samone can help me…
I have to produce an image to be printed on paper for a poster in size, let’s say, 20" x 30"; the costumer asked me to make the image at 300 dpi.
And now the question: how must I set the rendering resolution in the Blender rendering window to obtain an image of 20" x 30" at 300 dpi?
Thank you in advance for any suggestion.
Env
You haven’t checked your PMs on www.blender.org eh? 
200 dbi means dots per inch… henche if you want a 20" x 30"
image a 300 dpi you need to set blender to produce a
6000 x 9000 image (If I recall well blender’s limit is 10000x10000)
you might want to render it in parts (Xparts, Yparts, set them > 1, don’t use panorama)
you might also need a lot of patience and a lot of disk…
happy blending
Stefano
Ciao Stefano! I must talk in italian…
Ehm… ho qualche problema con Blender.org, evidentemente, perchè l’ho settacciato per giorni per vedere se arrivava la tua risposta… 
e lo vista solo adesso… grazie.
Non ho proprio tutta l’esperienza che potrebbe sembrare; cosa sarebbe il “rendering in più parti” (parrebbe molto interessante)?
ENGLISH VERSION
Hi Stefano!
Ehm… maybe I’ve some problem with Blender.org, because I’ve cecked it for days to find your answer… I saw just it now… thank you.
I’m not so experencied as it could seems; what would be the “render in parts” (looks interesting)?
Env
For Posters 300 dpi is often too much. That is,because Posters are usually viewed from greater distance than eg an image in a magazine
It depends on how the Poster is printed (Offset, Inkjet…)
I think 100-200 dpi should be enough. Ask your Printing-Office.
Daniel
Yes, I know about the 100 dpi for poster, thank you DanielP. I have to contact the printer office this Monday.
I’ve found the render in parts; it looks interesting but I’m wondering if it actually let me have renders bigger than 10000 dpi. I tried a render in 4 parts setting the x= 10000, but the final image risulted in 6200 dpi only…
UPDATED
I discovered the gap… I had to unclick the 25% button in the render window… gh!
Env
300 dpi is surely the right thing (for good quality).
There are three values that matter: the Lines Per Inch, the Dots Per Inch and the resolution of the image (Pixel Per Inch ).
If the image resolution is half the dpi of the printing machine, the quality is best (more would be ridiculous by any means) the dpi/ lpi ratio should be at least 10 to 1, 20 to one gives realy good quality.
example: A normal magazine has 150 lines per inch. Then the resolution of the raster image processor (the printing machine) should be 3000 dpi and the images should have 1500 pixel per inch.
Of course you can go smaller, but the quality will not be as high.
Thank you haunt_house!
So, let me see if I understand perfectly…
I have to produce a poster of 62,2" x 20,8" (it’s a space landscape); doing the math, for 300 dpi I’d have to set the Blender render at 18,600 x 6,240 pixels, and this is a huge work for my poor computer…
If the Printing Office will say me that they print at 300 dpi, I have to obtain an image at 150 ppi, setting the Blender render at 9.331 x 3.130, and this is possible to do.
Is it correct?
Env
yes and no
What printing machine do they use?
Inkjet or something more serious?
I talked about a raster image processor, which is used to do the big style printing (offset and so on)
The problem is that a normal printing machine can only do four colours:
cyan
magenta
yellow
and black
It cannot print let’s say 50%black. So what it does is putting larger points together by using many of its smallest availabe points.
example (not realistic, but more clear):
You want to print with 150 lines per inch. That means 150x150 points per square inch.
If the printer had a max resolution of 150 dpi, it could only print cyan or cyan and yellow or such combinations by printing the rather huge dots over eachother. not much of a colour variety.
lets say we double the printer’s dpi. So it can print 2x2 dots of every colour in one of your points. Now the printer can choose to set one or two or three or four dots in the area that one of our points. So it could choose to fill one point with half magenta and half yellow. Much more freedom.
If you have a ratio of 10 to 1, you have 10x10 dots for one point and you can choose theoretically from 0 to 100.
Really good quality is a ratio of 20 to 1, so you have 400 dots to assemble one point of your printed work.
But why can one not say 1 pixel of the picture for one point in the printed work?
Because the grids of the four colours are not horizontally aligned.
One version is:
C 75 degree
m 15 degree
Y 0 degree
K 45 degree
So the more pixels it can use to build the printing grid out of, the better.
So 10 times more ppi than lpi and 20 times more dpi than lpi is good.
You can see what I mean if you look at a cd cover with a magnifying glass or a binocular turned upside down or a good webcam or a good scanner
But if your customer wants it to be 300 dpi and 20x30 inch, Stefano is quite correct.
The problem is two things.
cmyk and ppi.
As far as I recall, you cannot change the dpi in blender. And blender can only give you RGB, suitable for screen.
So save as targa (NOT JPG)
Go into an image processor (photoshop is best for print)
change the format to cmyk
and make sure the ppi of the image is set to 300 without changing the amount of pixels.
and save as TIF. EPS is even better, but a bit tricky for its versions.
HH
I would say that you let the printing office make the conversation to cmyk (separation), or at least ask for the right settings, or an ICC! This is quit important, because Photoshops cmyk-presets are not that good.
If you want a good result, the cmyk settings have to be optimized for the printing machine.
greetings
Daniel
quite right 
You also have to take into account dot gain, wet ink spreads a little when it hits paper, Photoshop will let you set dot gain so that your poster will look clean and not all blurry from dot gain. You need to ask what the dot gain is on the the presses at your print shop.
Wow! That’s a lot of answers I didn’t expect… thank you very much to everybody: I’m sure the Blender Comunity is one of the more friendly and kind on the web.
This afternoon I tried to do a rendering at 9331 x 3130: it tooks half an hour, but this is not a problem. The problem shown when I examined the rendering in Gimp at 1:1; a lot of artifacts, invisible in an average sized rendering…
Well, at this point it looks like I have to ask to the Printing Office of the customer for more technical details.
Thank you all
Env
I am now rendering it in 2.23 and it is still doing it. i have almost doubled the # of snowflakes. i might be able to post it for someone else to render it.
If someone wants to, leave one.
If someone can answer the question, please do.
BMD
did you save as targa?
haunt_house wrote:
did you save as targa?
Yes, I saved it as targa then opened in Gimp at 150 dpi and converted in jpg (just to have a try; for the final render I will try to save in tiff LZW compressed, if I can install the plugin in Gimp).
Env
For anyone who could be interested, I talked with the Printing Office: they asked me to save as psd RGB (with logos on different layers) at 150 ppi for the biggest posters, and at least 200-250 ppi for the smaller.
Thanks to everybody for the suggestions!
Env
Sounds about right.