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As far as I've worked with blender and GIMP(which are the ones I mostly worked with). I've only been concerned about resolutions of the images in dimensions such as 1080 X 1920. But, in some employment sites I have found that images need to be in high resolution and the standard is 300dpi. Even in blender I have never seen this option anywhere in setting up the export. I was aware that DPIs means quality of the images(didn't know it was resolution).How can we ensure that our exports are of a certain DPIs of resolution?

BumbleBee
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    Blender doesn't set the DPI on rendered images, that is determined by whatever is displaying or printing it. – PGmath Sep 11 '16 at 14:39
  • @PGmath, when a client(someone) tells I need a high resolution atleast 300dpi image, then what does this mean? And there is a page how to create a 300dpi image using photoshop in here: http://proshooter.com/article_whatisa300dpiJPeg.htm , so I don't really understand. – BumbleBee Sep 11 '16 at 14:53
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    Simplified explanation: If the client says he needs 300 DPI, he or she most likely uses DPI to mean PPI. What you need to ask then, is at what physical size it's going to be printed. Simply setting the pixel density in an image file doesn't do anything at all for the overall quality. It simply tells the printer driver how densely to pack the pixels, i.e. how large the print will be. If the image is going to be used only on monitors, the DPI and PPI are irrelevant, but if it makes the client happy, simply open the image in e.g. Gimp and change the resolution settings. –  Sep 11 '16 at 15:29
  • However, for high quality prints, you should use 16 bits/colour channel, which the current stable release of Gimp isn't capable of. If you're limited to non-commercial software, you can try ImageMagick instead. To change the pixel density, run something like convert <input_image.png> -density 300 -units PixelsPerInch <output_image.png>. –  Sep 11 '16 at 15:41
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    related links: http://blender.stackexchange.com/questions/13936/output-for-print and http://blender.stackexchange.com/questions/5082/setting-the-dpi-value-in-rendered-images –  Sep 11 '16 at 18:12
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    @BumbleBee Take your print dimensions and merely multiply them by the requested density. Also remember bleed when determining render density and composition. That will be in the brief or can be estimated. Quarter to half inch is common. Typically all of this will be delivered within a PDF, and the PDF should communicate much of the relevant information via metadata. – troy_s Sep 11 '16 at 18:54

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Edit: As Duane points out in the comment below, DPI is often incorrectly used and PPI is more appropriate. It is very likely, however, that DPI and PPI are being used interchangeably in the context of OP's question.

DPI PPI means "Dots Pixels Per Inch." It is an expression of resolution over length and is most commonly used in printing. It is used to tell other software how far to spread out the pixels for printing.

For this to be meaningful in Blender, you'll need to know how large the final reproduction of the graphic will be on a page (or poster, or whatever you're making). The math is pretty straightforward:

Render X Resolution = Print Width * DPI PPI

Render Y Resolution = Print Height * DPI PPI

For example, if you were rendering for a 4 x 3 print, the render resolution would be 4*300 x 3*300 ==> 1200 x 900.

Be aware that the rendered file will carry a default DPI PPI definition of 72 or 96, but that only tells the Inkscape or Illustrator how large to make the picture on the page at import. Once you have your render, you can change the file's DPI PPI definition in GIMP or Photoshop without changing the actual pixel count. (Be aware that other softwares may be using DPI and PPI interchangeably.)

Ron Proctor
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    DPI actually refers to the individual dots making up each pixel. A printer needs to make several dots to produce a wide range of colours from only 4-6 inks or toners. DPI refers to those dots, or sub-pixels if you will. Unfortunately many software vendors and even professional printing service use DPI and PPI interchangably. PPI vs. DPI: what’s the difference?. –  Sep 11 '16 at 14:27
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    @Ron Proctor, I actually found it on an employment site, so I don't know if the term was used interchangably. How exactly can we ensure that we create a 300 dpi image? Just changing the render dimensions of the camera wouldn't do it, would it? – BumbleBee Sep 11 '16 at 14:59
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    I realise that the OP probably mixed up DPI and PPI. However, this wrong use of the two interchangably is the source of much confusion, and I feel it's important to straighten it out. I wasn't trying to criticise you. –  Sep 11 '16 at 15:17
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    No prob Duane. It's cool. I'm glad you pointed it out. – Ron Proctor Sep 12 '16 at 01:01
  • @BumbleBee: We can verify the PPI of the image in Photoshop or Gimp. – Ron Proctor Sep 12 '16 at 01:06
  • @Ron Proctor, how can we verify this? – BumbleBee Sep 12 '16 at 02:15
  • Here's info on checking DPI in Gimp: http://www.gimpusers.com/forums/gimp-user/1660-dpi-from-75-300 And here's Photoshop: https://www.techwalla.com/articles/how-to-set-the-dpi-in-photoshop – Ron Proctor Sep 12 '16 at 02:57
  • @Ron Proctor, so changing the PPI and print size of the image both to big such as a 20X20 inches to print image with a 300 DPI value would mean to drastically scale up the image's actual size(in pixel)? How can we ensure that no quality is lost in the process, would we have to re render the whole image so big? – BumbleBee Sep 12 '16 at 11:30
  • Gimp/Photoshop will probably try to upscale your image when you change the PPI (or DPI if the software says that). Simply input 300 PPI/DPI and change the pixel dimensions back to match the pixel counts you rendered and it won't upscale. The only difference is how big the pixels will print on a page. – Ron Proctor Sep 13 '16 at 06:36
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Dpi stands out for "dots per inch".300 dpi can be both good and both bad resolution.It depends.300 dpi in an A0 paper would be great resolution compared to 300 dpi in an a5 paper....This is because the A5 paper has much smaller surface than the A0 paper.If you had 300 dpi in a 1x1 inch paper this papers resolution would be 300x300 dots.If you had a 2x4 paper with 300 dpi then you would have totally 600x1200=720000 dots (note that 600 is how many dots are in the x axis and 1200 in the y axis) and so on...Hope this helps!

Extra info:https://en.wikipedia.org/wiki/Dots_per_inch

xlxs
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    300dpi both in A0 and A5 is the same quality. The density of the dots is the same. – Jaroslav Jerryno Novotny Sep 11 '16 at 14:30
  • @Jerryno yes but the you got way more pixels in A0 than in A5 thus the level of detail is higher in A0. So I think quality is higher in A0. – xlxs Sep 11 '16 at 14:33
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    In printing the quality is not defined with how much dots it was printed with, but the meaning of quality refers to the dot's density. Here it is in a picture: http://i.stack.imgur.com/Pg7nU.png. The first two A5s have the same dpi, the first is a crop from A0, the second is the whole A0 but with same dpi. The last one is the whole A0 with all of it's dots - and because of that with higher dpi. This is how the whole world understands it. – Jaroslav Jerryno Novotny Sep 11 '16 at 16:41