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I am requested to provide an image file that should be supplied at 300 dpi resolution for .jpeg and .tiff or as .png files. I already have a png file here. When I check "file" in cmd I got:
Figure 1-1.png: PNG image, 560 x 747, 8-bit colormap, non-interlaced
From this information, how do I know that the file satisfies the requirement?
As I've previously said here and here...
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...
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.
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.
To directly answer the question...
Checking your file's meta data won't help.
You need to find the physical size requirements (in inches) and multiply those by 300 to get your pixel size requirements. If your image is that size (or bigger) then it is OK.
The PNG-standard specifies an optional header with the DPI. In the PNG file two 4-byte numbers gives the pixels per meter og the width and height, which can be converted to DPI by multiplying with 0.0254 meters/inch.
You can see the DPI of the image by using, for example, GIMP by using image -> image properties. It will be shown by the header "Resolution".
Images don't have a DPI until you print them.*
All they have are dimensions, in pixels.
Printed at 300 dpi your image would come out at 47 × 63 mm
*Some page layout programs use the value to interpret how an image will fit to a page, but this is actually just an interpolation/interpretation of the final printed image.
Simply : 10 inch x 10 inch print @ 600 dpi = 6000 x 6000px. It's that simple.
Here is a useful calculator: [https://www.pixelcalculator.com/][1]
All you need to know is that the printer on which your graphic will be printed can manage to squeeze in certain number of dots into a one inch square. To fill this space correctly, you need to produce enough pixels to ideally place one pixel into one dot.
At the time of writing 300dpi is about right for many print applications, but i would encourage you to look to 600dpi, for future proofing your work.
Where I think people get confused is about hardware device PPI. PPI can refer to the "density" of pixels in a device. I.e How closely packed are the pixels in a monitor, or a phone for example. This will affect how the device displays your graphic. If the pixels are dense, then the image will appear much smaller, exactly as with a high dpi printer, printing at high dpi setting, the image will appear much smaller.
Tesujin is correct: "Images don't have a DPI until you print them." They have pixel file sizes only. You need only worry that you have a suitable amount of pixels in your working environment for the print destination device.
I hope that helps.
To not give you a long-winded answer, simply divide the number of pixels you have by 300dpi. You have 560 x 747 pixels. So take each dimension and divide it by 300dpi (so 560 divided by 300 and then 747 divided by 300). You are left with 1.87 inches by 2.49 inches - that's the size your image can print at in 300dpi.
I get asked many times by people who don't have professional graphic programs "what size" something can print as, and I offer them this simple explanation.
Pixels Per Inch (PPI) or Dots Per Inch (DPI) translate pixel dimensions to physical dimensions. This can be used when printing or viewing on a screen in order to render the image at the desired dimensions.
Your image does indeed have a DPI encoded in it or implied by omission.
On a Mac, open your image in Preview. Hit CMD+I for the Inspector and it will list the image DPI. The default DPI is 72 if you have not encoded any DPI (or dots per meter) into the PNG file pHYs chunk.
To change the DPI in Preview, select Tools -> Adjust Size and change the "Resolution" value.
