DPI Vs. PPI Vs. Image dimension, what should be taken to consideration?

Question

I've been really facing this problem and I have found a similar link, that does not really address the problem. First of all, I'd like to mention that I use blender and GIMP for most graphics that I design and please read all the way through.

Blender has export option that are only available in dimensions (e.g. 1920 X 1860), and no matter how big we set this up and check the PPI in GIMP(or photoshop, knowing GIMP does only handle 8 - bit images), the PPI is always 72.009.

On employment site, we are requested to provide information if we are ready to supply to our clients a high resolution image atleast a 300DPI, and the dimension is not mentioned.

Furthermore, I have visited many websites and confirmed that the terms DPI and PPI are used quite interchangeably.

Even though we can set the image PPI in GIMP from 72.009 to 300 as easily as image -> scale -> 72 to 300, there is no realtime feedback in the screen to know that the image PPI has been changed.

So, the problem is around what should the output would be, would it be a higher dimensioned image, a higher PPI image or a higher DPI image that would really mean HIGH RESOLUTION?
How can we really set it to a standard 300DPI image, on the aforementioned link, I don't really know if it is not to be optimized for printing.

Answer 1

There is no such thing as a 300PPI image

PPI is not an inherent property of an image. There is no such thing as a 300PPI image, or a 72PPI image. PPI is just a useful measurement for determining the print size of an image.

Which means PPI is completely irrelevant unless accompanied by physical dimensions. If someone says "Can we have that image in 300PPI?" they need to tell you a physical size in inches or centimetrs or whatever else, otherwise the question makes no sense.

A 100 × 100 pixel image saved at 300 PPI is exactly the same as a 100 × 100 pixel image saved at 72 PPI, or 10 PPI, or 1 PPI. They are even exactly the same if you print them at the same size.

The only times PPI is a useful measurement are...

1. You have a physical dimension requirement and you need to know how many pixels you need in your image.

   Say you need a 6 × 4 inch image at 300 PPI, that allows you to calculate how big in pixels your image needs to be. 6 × 4 (inches) times 300 (PPI) is 1800 × 1200 — and there is your required size in pixels.

2. You have an image at a certain size in pixels, and you want to know how big you can print that image.

   Say you have a 1800 × 1200 pixel image and you want to print it at 300 PPI. 1800 × 1200 (pixels) divided by 300 (PPI) is 6 × 4 — and that is your print size in inches.

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we are requested to provide information if we are ready to supply to our clients a high resolution image atleast a 300DPI, and the dimension is not mentioned.

As I said, that makes no sense. Explaining this to them is probably going to get you nowhere. Without knowing the specifics, I don't know, but they probably just mean a vague "Are your images high resolution". Assuming you can output your images at a high enough pixel size for what they are being used for—just say "Yes".

Answer 2

The fundamental issue

You want to know what width and height your image has to be, measured in pixels. The PPI value (or DPI, if used interchangeably) can help you determine this. It literally stands for Pixels Per Inch.

The following equation describes the relationship of the image size in pixel, in inches and the pixel-per-inch value that connects both sizes.

(Image size in Pixels) = (Image size in Inches) * (Pixels Per Inch [aka PPI])

You can solve the equation for any of these values if you have the other two, although you may have to transform it. This requires knowledge of algebra, but nothing beyond what you learn in any algebra class. Be not afraid or ashamed to admit if you have no clue about algebra, as this helps people to adjust their answers accordingly.

Adjusting image sizes in the Scale Image dialog

You do not have to use a pocket calculator to calculate image sizes, GIMP (and any other application with a similar dialog) can help. But you have to know about the principles, that is what value you want to calculate, and why.

Changing resolution, size in pixels constant

For example, assume someone gave you an image, which is not supposed to be changed at all, and tells you "set the PPI value of this image to 300 PPI!". You want to do this, and tell this person what the results are. So you do the following:

1. Set the Image Size unit selector to something other than "px" (pixel), e.g. "in" (inches).
2. The values in the Width and Height entries will now indicate the image size in the unit chosen (e.g. mm, and right below this area you'll see the image size in pixels.
3. Now, change the X and Y resolution values to 300. You will notice:
4. The inch values in the size entries change, but the size in pixels remains the same

Then you can tell the person who told you to modify the image: "Task done, the image would be of inches wide and of inches high if we printed it at 300 PPI, for example".

This is expected. Your are telling GIMP to change the image resolution, and this is not supposed to alter your image besides updating the PPI value.

Many image formats are able to store this value (or values, as they can be different for width (X) and height (Y) of an image), and some other applications used them to display images at this intended height.

Now, for the next round:

Changing size in pixels, resolution remains constant

So now we assume a person gave you an image, and told you "this image is supposed to be printed at 300 PPI to be A inches wide and B inches tall, can you adjust it accordingly?".

1. In the Scale dialog, take note of the current image size in pixels.
2. Make sure the Image Size unit is set to something other than "px", e.g. "in".
3. Make sure the resolution is set to the specified PPI value, "300" in this example.
4. Now change the inch value of the Width and Height entries, to whatever values A and B are, respectively.
5. See how the image size in pixels, as shown right below those entries, changes accordingly.

This is expected as well, as you are now telling GIMP that you want the image to be a specific size in inches at a specific PPI value. And you are in the scale image dialog, after all.

You can tell this person: "Ok, this is doable, but the image will now be C pixels wide and D pixels tall - this is a significant change from its original size in pixels, and any scaling will affect the quality. If you had an image that was C pixels wide and D pixels tall as an original, we could use this instead."

As you can see, you may use this to determine what size your image has to be in pixels in order to achieve your task without scaling, provided that you know the expected image size in inches (or millimeters, meters or light-years).

As your image comes from Blender, you may be able to generate it at any size needed, and this would indeed be the best approach.

Changing the PPI

If you want to achieve the opposite, i.e. having the PPI value change according to the size you specify in inches, and the size in pixels to remain the same, you can choose the Print Size dialog to do this.

Addendum

As you have correctly realized, providing just one of the relevant values, the infamous "please send the images at 300 ppi (or dpi)!!!1!" lacks essential information. If this case, it has to be assumed that the person sending the image ought to know the intended image size, for example "[A4] at 300 PPI", or that the person requesting the image has been taught this is sufficient and never questioned why.

TL;DR

Connecting pixels and inches really comes down to one single equation:

(Image size in Pixels) = (Image size in Inches) * (Pixels Per Inch [aka PPI])

People with some knowledge in maths usually can't understand why someone doesn't get this equation or can't transform it, e.g. to get the size in inches if they got size in pixels and the PPI.

Answer 3

It is just a conversion factor and only comes to play if you print your image. Think of it as a post-it note attached to the image saying: "Could you please print this image as x units wide". When no post-it note, ahem metadata, is present image applications revert to 72 PPI. That does not mean the image is not good enough just that nobody bothered to tag the image. This leaves the size up to the whim of whoever is printing, but they are entirely free to do so regardless, its what you call a guideline.

Ok, so armed with this info could we not say that you a 2 by 2 pixel image is high resolution? No, that would be a bit stretching the limits. Asking for a high resolution image usually means meaningfully sized on paper. So there is a understanding that the image is about half to about one A4 sized image. Beyond this size you usually drop yhe LPI so that what prints at A4 at 300 dpi prints more or less at same pixel dimensions on all sizes (although you certainly can A3 in 300 LPI but the angular frequency takes care of the need to do so).

So the question is a slightly underhanded/misguided way of asking do you make images with approximate dimensions of ~3500 x ~2500. Anyway that in itself is by no means enough for optimizing for print you still need to handle color. Also you should test print your stuff until you know how it really works (it is much harder than it seems at first). It is safe to say that unless you know that your work is optimized for print then it is not.

Actionable suggestion, print stuff and experiment with settings.

Answer 4

Image size (in pixels), print size (in inches), and print definition (in PPI) are completely related by a simple formula:

print_size = size_in_pixels / print_definition

This formula can also be expressed as:

size_in_pixels = print_size * print_definition

or

print_definition = size_in_pixels / print_size

But it should be clear that you can only change two terms at the same time, the third term will completely determined by the other two. For instance a 3000x1500 pixels image printed at 300DPI yields a 10"x5" image.

When you create an image in Gimp, the image creation dialog lets you give the image size in inches (just change the units), and specify a print definition. Gimp will compute the required size in pixels for you.

Good quality printing requires at least 300PPI.

Print definition, as associated to the image, is usually only indicative (unless the image has been scanned, in which case it allows you to print the image at its native size). You can change it using Image>Print size. When it is not specified, the universal default is 72DPI.

DPI and PPI are about the same thing on screens and monochrome laser printers. On color printers, a pixel can be implemented with very many dots of different colors, so DPI > PPI (so a 2400DPI inkjet printer may not be able to do better than 600PPI printing)