DPI?

[Exar Kun]

I've never really understood the concept of DPI (in the graphic design sense). I know what it stands for but how do you know what the final resolution of something needs to be without knowing the size of the page it'll be printed on?

My problem is this - a graphic designer who I did a couple of renders for a while back wants them re-done "really big" at 300 DPI so he can put them in a book he's doing. Now, they were made for use on the net originally so were only 2000 pixels across - how many pixels across would 300 DPI work out to be?

[DexterPunk]

yeah for printing they usually ask for around 240-300 dpi. I may be wrong here, but as far as i know, the target size of your image should be the same no matter what the dpi... which i mean the actual physical print size. I can scan a 35mm film neg/tranny and set the output at 100%.. so the actual image when outputted is the size of a 35mm piece of film, but i can scan with a res of like 1200dpi, and on screen it can be reasonably large (but still a little image). Or i can scan that same film, at 400% and at a dpi of 300.

oh and with those calculations, photoshop etc should handle it for you.. just tell it what size you want the image to be (physically) and what dpi, the actual pixels it needs to use is kinda irrelevant. There is fairly simple calculations you can do (which ive forgotten now) where you can work out all that stuff. But 2000 pixels across at 300dpi is going to be a reasonably small image on paper. It can be hard to upscale these things to a decent size if they werent created that way. Photoshop will have to interpolate the extra information.

[Exar Kun]

Well that's it - I have no idea what the print size will be so how do I know what res 300 DPI is? I need to be able to render to a set output of x times y sort of thing - I can't set the renderer to a DPI.

[DexterPunk]

What size do they want the image to be when outputted?

if its 10 x 8in

10 x 300 = 3000
8 x 300 = 2400

3000 x 2400 px

if they specify in cm, then you can convert it.. but photoshop does all of it for you anyway.. file => New then change px to cm inches mm what ever.. and your DPI to what you want, and change back to your pixels again it will tell you what it is.

[Exar Kun]

Thanks dude, I'll use photoshop to check what sizes work - but they haven't told me what size they need it to be. They just said "really big" @ 300 DPI.

[DexterPunk]

lol that really doesnt help you very much.. 'really big' could be 32 x 20" or 32 x 20m

[DexterPunk]

id give them an uncompressed TIF as well... jpg's print noticeably worse.

[Exar Kun]

id give them an uncompressed TIF as well... jpg's print noticeably worse.

ta

[ysu]

I'd say if they meant to print it in a book, it won't need to be bigger than about 200mm
However if they want a poster size then it's going to be gigantic

2.5m = ~100in
@ 300 dot/in => 300x100 = 30,000 (pixels)

So yeah, it's good to know what they want to do with it.

An uncompressed image 30 thousand pixels wide and with a normal aspect ratio may not fit on your average usb drive either

[DexterPunk]

[Exar Kun]

I'd only need about 96gb of RAM to render an image 30000 pixels across too! As it is, I need to optimize my file to get 4000 across to render without crashing

[wobblysauce]

I'd only need about 96gb of RAM to render an image 30000 pixels across too! As it is, I need to optimize my file to get 4000 across to render without crashing

time to upgrade again ?

[KNAPPO]

Thanks dude, I'll use photoshop to check what sizes work - but they haven't told me what size they need it to be. They just said "really big" @ 300 DPI.

Yeah Dex is about right but they will need to let you know the physical size for you to be able to work it out properly in Photoshop.

One quick note that can make this all come unstuck is a simple setting in Photoshop called "resample Image" when you are doing resizing. You dont really want this on.
Basically you dont want photoshop to to add any pixels as this can blur the image. Ive ballsed this up before with a $2000 digital print which was rather embarrassing.

My eyes are poping out of my head at the moment and my brain isnt quite working properly so im struggling to explain this properly but this is from the PS help.

Resampling is changing the amount of image data as you change either the pixel dimensions or the resolution of an image. When you downsample (decrease the number of pixels), information is deleted from the image. When you resample up (increase the number of pixels, or upsample), new pixels are added. You specify an interpolation method to determine how pixels are added or deleted.
Keep in mind that resampling can result in poorer image quality. For example, when you resample an image to larger pixel dimensions, the image loses some detail and sharpness. Applying the Unsharp Mask filter to a resampled image can help refocus the image details.

You can avoid the need for resampling by scanning or creating the image at a sufficiently high resolution.

If you want to preview the effects of changing pixel dimensions on‑screen or to print proofs at different resolutions, resample a duplicate of your file.
Photoshop resamples images using an interpolation method to assign color values to any new pixels based on the color values of existing pixels. You can choose which method to use in the Image Size dialog box.

Nearest Neighbor A fast but less precise method that replicates the pixels in an image. This method is for use with illustrations containing edges that are not anti-aliased, to preserve hard edges and produce a smaller file. However, this method can produce jagged effects, which become apparent when you distort or scale an image or perform multiple manipulations on a selection.

Bilinear A method that adds pixels by averaging the color values of surrounding pixels. It produces medium-quality results.

Bicubic A slower but more precise method based on an examination of the values of surrounding pixels. Using more complex calculations, Bicubic produces smoother tonal gradations than Nearest Neighbor or Bilinear.

Bicubic Smoother A good method for enlarging images based on Bicubic interpolation but designed to produce smoother results.

Bicubic Sharper A good method for reducing the size of an image based on Bicubic interpolation with enhanced sharpening. This method maintains the detail in a resampled image. If Bicubic Sharper oversharpens some areas of an image, try using Bicubic.

I NEVER resize an image with "resample Image" turned on if the job will be going to print. Ive Learnt this lesson the hard way

When you know the minimum width that your client needs the render at I can give you a hand with working it all out.
What dpi does you rendering prog out put at? Ive done some of this style stuff with Lightwave which outputs at 72dpi so i had to make my renders quite wide in pixels then down size em with resample off till i reached 300dpi. and the width i was after.

Do some tests with a blanc doc in PS and resize it before you run the render as this will give you an idea on how wide the render will have to be. Hope this makes sense, my eyes are popping out of my head.

[DexterPunk]

cheers Knappo, theres a lot i still dont know about, and im glad you weighed in to help him out.

[Exar Kun]

Thanks heaps for that Knappo. I have no idea what DPI max renders out as - you set a resolution x by y and that's it. I'll get back to the guy tomorrow to ask him - that should give me some time to get the thing to render without crashing!

[AstrO]

What's the deal with people still using TIFF and not PNG. Can TIFF do something I'm unaware of?

[KNAPPO]

No idea Astro, i just know that most of the time print houses prefer TIFF over anything else and they can crack a spaz if you supply em .jpeg's ect.

EDIT: It seems PNG is traditionally used for the web or on-screen use where as TIFF seems to be a bit more robust.

TIFF format supports CMYK, RGB, Lab, Indexed Color, and Grayscale images with alpha channels and Bitmap mode images without alpha channels. Photoshop can save layers in a TIFF file; however, if you open the file in another application, only the flattened image is visible. Photoshop can also save annotations, transparency, and multiresolution pyramid data in TIFF format.

In Photoshop, TIFF image files have a bit depth of 8, 16, or 32 bits per channel. You can save high dynamic range images as 32‑bits-per-channel TIFF files.

Exar, open up one of you high quality renders in PS and checks its dpi, that'll tell you. I dont know if Max has an option for dpi on output which can be changed but i know a frew render progs are limited to 72dpi as this is the standard screen res. So you just make em wider/bigger and downsize em to bring up the dpi.

[DexterPunk]

are PNG un compressed??

yeah pegasus light based printers deal with TIF... jpg is horrible for any larger size prints.

[KNAPPO]

Yeah I think they are.

Developed as a patent-free alternative to GIF, Portable Network Graphics (PNG) format is used for lossless compression and for display of images on the web. Unlike GIF, PNG supports 24‑bit images and produces background transparency without jagged edges; however, some web browsers do not support PNG images. PNG format supports RGB, Indexed Color, Grayscale, and Bitmap mode images without alpha channels. PNG preserves transparency in grayscale and RGB images.

It looks like PNG only supports RGB so that it the biggest factor when comparing it to TIFF. You NEED a CMYK image for press printing, RGB is useless.

[DexterPunk]

im not sure about that, most of the pro labs use Adobe 98 RGB colour space as far as i know.... even though the actual prints are done with subtractive CMYK.

[KNAPPO]

We have to convert all of our images when outputting to CMYK as the press printing process is 4 colour minimum. We might be talking about different things tho Dex.
The art stays RGB whilst editing and converted at the output stage to CMYK for press printing, digital is another ball game. RGB is fine for digital output.

[DexterPunk]

ahhhhh yeah

[norbs]

http://x-equals.com/blog/?p=2653


that might help.

[Exar Kun]

Thanks norbs. So it turns out the image only has to be "at least 21cm" which is good as I should be able to get away with something a fair bit smaller than what I've been trying to push my PC to do.