The quality of digital images is generally designed to look good on screens. However, when it comes to printing these images, there are some basic principles that one needs to understand to optimise their quality, file size and resolution. This post will discuss some of the basics of resampling digital image sizes to be used for print.
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Producing digital images
Digital images, e.g. photos, 3D renders, and scanned images, can be created in multiple ways. Depending on the software used, e.g. a phone’s camera software, Photoshop, Blender 3D, these images are, by default, saved in certain ways w.r.t. their image size, quality and resolution (see later). These are most often set to cater to the most likely way they will be used. Although default saving criteria often include decent quality for general printing purposes, optimisation for larger, or specific, sized prints is often not perfect.
It’s all about the pixel dimensions
The print quality of an image is determined by the image quality. Image quality is mainly expressed by the amount of pixels it is wide and the amount of pixels is high (easier visualised as the pixel dimensions). Some might also refer to this as the image resolution. Just like a PC screen is a certain size (e.g. 15 inches) and can display a certain amount of pixels (i.e. 1920 x 1080 px), printers work in similar ways. When creating images to be printed, the printer’s image resolution (e.g. 300 dpi – also see later) and print size (e.g. A4, 148 x 210 mm / 5.8 x 8.3″, etc.) are used to calculate the total amount of pixels the digital image should be.
Let’s say a digital image for high-quality printing needs to be created. High-quality printers print at 300 dpi. This needs to be printed on a canvas that is A3 (11.69 x 16.53″).
If each inch is to have 300 pixels (300 dpi), the image quality needs to be at least (300 x 11.69) = 3507 px wide and (300 x 16.53) = 4959 pixels high.
After the image is created, the resolution can either be changed locally using photo editing software such as Adobe Photoshop or GIMP, or using the printer software (see later).
Image file size, quality and resolution
In order to understand how to resample images for printing, basic knowledge about image file size, quality and resolution will be required. This will also give a better understanding of how these variables interact with each other.
Image file size
The image file size is the physical space (in KB, MB and nowadays sometimes in GB) the image will use to be saved on a storage device. These days this value is probably more important when using the internet (e.g. email or cloud storage) to transfer files from one device to another.
The image’s resolution and quality are more often directly proportional to its file size. In other words, the higher the resolution and quality, the larger the file size.
Very often images that are intended for print needs to be transferred to an external device, in which case its file size often matters.
Image quality (pixel dimensions)
A digital (and printed) image is basically a combination of colour dots that is seen as a whole. On digital devices, these dots are referred to as pixels (abbreviated px). Image quality is mainly determined by the amount and the level of accuracy of these pixels.
A good example where we are often confronted with image quality is with camera phones. With mobile phones, their camera quality (hence the image quality they can produce) is expressed in megapixels. One megapixel is a million pixels. The more pixels, the higher the quality of the image.
Apart from referring to the number of pixels in an image, pixels are also used to express the image (quality) size (or better expressed as pixel dimensions). A horizontal image that is 1024 px wide by 768 px high has a total of 786 432 pixels (calculated by simply multiplying the height and width values). One can appreciate that an image that is, for example, 640 px by 480 px, has fewer pixels than an image that is 1024 px by 768 px simply because there is less “volume” for pixels.
When looking at the size of a digital image file, the data of that file is stored by saving, amongst other things, the colour and location of the pixels. The first pixel might, for example, be saved as red, the second as blue, the third as black, etc. This is why an image with more pixels will be larger in file size and vice versa.
The resolution of an image is the fineness of detail. It is basically a simple math calculation to express how much of something is in a certain amount of space. Resolution is typically expressed as pixels per inch or ppi.
Note that although the principle remains the same, dpi refers to dots per inch and is more correctly used for printed images. dpi is not the same as ppi. The same goes for dpc (dots per cm) and dpf (dots per freckle).
The dpi of an image means the number of pixels that are packed into each inch of that image. Generally, an image with a higher resolution produces a better print quality, but takes up more space.
Sidenote: 300 ppi = 762 dpi and 300 dpi = 118.11 ppi
Typical resolutions for printers
When it comes to printing digital images, the resolution value that is generally recommended is 300 dpi. Although this will produce a high-quality print in almost all cases, some images might not have enough pixels and it is also expensive in terms of space requirements.
The typical resolutions for printers are as follows:
- 133 – 150 dpi is typically reserved for draft prints. The print will give you an idea of what it will look like, but won’t be very smooth.
- 200 – 220 dpi will produce a good quality print
- 267 – 300 dpi is the more typical standard for high-quality printing
- 360 dpi is the standard for high-quality inkjet printing
Setting image resolutions for printing
After a digital image is created for print, it might need to be converted to be understood by the printer and/or printer software. Although the pixel dimensions might be enough to achieve a certain sized print at a certain resolution, the header of the image might need to be updated. This can be done by resampling the image’s resolution for print and saving it under a different name.
The A3 example used earlier will be illustrated using Adobe Photoshop and GIMP. Inches and dots are used.
When using the Image Size function in Photoshop (Image -> Image Size…), the image resolution might not necessarily show up as 300 Pixels/Inch (in this case it was 72 dpi). Because the image resolution was lower (and the pixel dimension = dpi x inches), the Document Size (in inches) in this example is much higher than the 11.69 x 16.53 “.
To set the correct resolution and print size, deactivate Resample Image and dial in the correct Resolution. The Document Size will be automatically updated to the original calculated ones.
To set the print resolution in GIMP, the Set Image Print Resolution function needs to be used (Image -> Print Size…). In this example, the image resolution will show up as 72 (and not 300) pixels/in.
To set the correct resolution using GIMP, the correct X and Y Resolution can be inserted. By changing the Print Size measurement dropdown from pt to inches, the correct original calculated measurements will show up.
When printing images, the edges of the printed area can either be set to have an empty border or can extend to the edges of the print medium (e.g. paper).
When the image is to extend to the edges, bleed pixels need to be added. This is commonly done when the print medium edges are to be trimmed (e.g. on a press) and the medium needs to be trimmed down to size. If bleed pixels are not inserted, any misalignment while cutting will result in the image not running to the edge of the medium. Typically 1/8″ is used as bleed pixels.
To continue with the example used earlier, where the image size is to be A3 (11.69 x 16.53 “) and the resolution 300 dpi: the original calculated pixel dimensions were 3507 px wide (300 x 11.69) and 4959 pixels high (300 x 16.53). To add about 1/8″ of additional bleed pixels, the calculation is as follows:
0.125 x 300 = 37.5 for the width plus 0.125 x 300 = 37.5 for the length equals 75.
So instead of using 300 dpi, 375 dpi is used, which brings the total pixel dimensions to 3484 px wide by 6198 pixels high.