About DPI, digital imaging (and Repper)

August 8, 2011 @ 08:46 in Tutorial by Studio Ludens

Every so often we get a message from a customer asking us how to save patterns in Repper Pro at a higher DPI than 72.

The short answer is: you can’t.
The relevant answer is: DPI does not affect image quality, it is just some metadata in an image file. All that matters are these two things:

  • 1) the resolution of your file (= the amount of pixels)
  • 2) compression quality (in case of JPEG)

Read on for the long but satisfying answer that includes info about what DPI really is, how it relates to digital images, to what extend it matters and how to change the DPI of an image (if you really need to).

1. What DPI really means

DPI is an acronym for “Dots Per Inch” and usually refers to the number of ink dots per inch as produced by a printer. As such, it is a measurement of quality (i.e. resolution per unit) of the physically printed image. On computers this measurement is also sometimes called PPI (Pixels Per Inch), referring to the number of pixels that will be printed per inch. Computers themselves do not really care about the DPI/PPI of an image, because the image is digital and is only stored as pixels. The image doesn’t have an explicit physical size to the computer, it’s just pixels.

Therefore, there is no difference between these two images (for a computer):

  • 1) a 1″x1″ image at 300 DPI (= 300×300 pixels on hard-drive)
  • 2) a 10″x10″ image at 30 DPI (=300×300 pixels on hard-drive)

2. What DPI affects (and what it doesn’t)

Graphical software like Photoshop deals with turning digital images into physical ones and so here we get to the use of DPI. Because most graphic designers and printing companies were used to working in DPI before computers came around, software engineers used the same term. If you set a file in Photoshop to 300 DPI, it basically suggests your printer to print it at 300 DPI. This does not mean the printer has to print it at 300 DPI. If you change the dimensions in your printer dialog, it will happily print your image at a lower or higher DPI. Note that changing the DPI or changing the print dimensions does not change the quality of your original digital file, only of your physical print.

Let’s do a similar comparison between two files:
* An image of 3000×3000 pixels printed at 10″x10″ will be 300 DPI (3000/10 = 300)
* An image of 3000×3000 pixels printed at 50″x50″ will be 60 DPI (3000/50 = 60)

You will notice that the term “high resolution” or “high quality” image is always relative, depending on the intended physical size. The aforementioned image printed at 10″x10″ could be considered high quality, while the exact same image printed at 50″x50″ could be considered low quality (or more accurate, low DPI). To make it even more relative, the perceived quality depends on the purpose and material of the print. A large paper poster at 100 DPI might be perfectly fine and many fabric prints are around 180 DPI, much below the standards for paper printing.

3. Why DPI is still used

So now we know that DPI refers to the physical print and does not tell anything about the quality of the digital image. But why is it still used in graphical software? Well, it’s a great way to let the computer calculate the amount of pixels you need for a 10″x10″ print at 300 DPI (see the “Image Size” dialog in Photoshop) and it’s a good way for designers to suggest at what size an image should be printed (e.g. when included in a magazine, though don’t expect many mags to pay attention to it!). For compatibility between different software programs, this DPI setting is usually saved as metadata in the image file.

Again, this DPI in the metadata of an image is only a suggestion to other software. For example, imagine you open an image of 1000×1000 pixels at 72 DPI into Photoshop and copy its content. Then you create a new image with the same dimensions (1000×1000 pixels), but at 300 DPI, and copy the original image into it. Even though it has a much higher DPI, the original image will cover then whole image, because they are both 1000×1000. They have the same resolution.

4. How Repper fits in all this

You may have noticed that Repper only saves images at 72 DPI. This is currently a technical restriction of Flash –the framework on which Repper is built– and we cannot do much about it. By now, you know that this does not make the images that come out of Repper “low quality”. Repper saves your patterns pixel-for-pixel from your source image by default and saves pattern tiles and images up to 2800×2800 pixels. Copying Repper’s 72 DPI images into other files that are at 300 DPI should not result in any loss of quality (or by now, you would probably say “resolution”).

Some questions you might still have:

a. A printing service I use demands the correct DPI (usually 300 DPI). What should I do?

If you encounter a printing service complaining about the DPI of a file you sent them, you could tell to just print it at the size you requested them and trust you that the file has enough resolution (because you know how to calculate that: resolution / print size in inch = DPI). If they are stubborn, you can convert the DPI yourself as described in the next section.

b. I need an image set to the right DPI for a specific software tool or printer driver (or stubborn printing service). What should I do?

If you really have to set the DPI in the metadata of your file to make things work, you can do so with Photoshop. There are also free software tools for Windows and MacOSX that allow you to do the same thing. You can find more information about the software and instructions at this webpage about changing DPI with various (free) tools.

c. When will Repper support control over the DPI in the metadata of the images?

There is of yet no official way to control the DPI of an image in the Flash framework and we are also not aware of any “less official” workaround. When we do, we will definitely update the software to support this. For now, if you really need to set the DPI you can use other software as described above.

Even more information about DPI and common misconceptions about DPI & digital images:
The Myth of DPI
DPI on Wikipedia