Getting the best results from your design
On the whole designing a document for printing on an HP Indigo digital press is no different from any other printer. However, as with all devices, by understanding the process and its limitations better results can be obtained. Indeed, by gaining a greater appreciation for the capabilities of the device you may find even more inspiration for your designs.
A question of resolution
If the image resolution is too high for the printer, the file size, transfer time and RIP times will be unnecessarily long; if the resolution is too low then the quality of the printed image will be poor. As a rough rule of thumb the resolution of the image should be twice the screen ruling used to print the image. Ideally, images should be at a resolution of 300 dpi for printing on the HP Indigo digital press. However, you will find that at resolutions between 250 dpi and 300 dpi, there is little appreciable difference in the quality of the resultant printed image. The amount of detail in an image (resolution) is defined not only by the number of pixels, but also by the dimensions of the file, hence the unit of measure is dots per inch (dpi). For example, a typical image may start off with 1600x1200 pixels, which at 72 dpi produces an image with the physical dimensions of 22"x16.5", But this image can be otherwise converted to a 300 dpi 5"x4" image without any loss in detail, i.e., it still has 1600x1200 pixels. So if we were to create a 5"x4" picture box in our page layout and either import the 300 dpi image at 100% or scale the 72 dpi image to fit we should get the same quality image produced.
In practice it is always better to convert the image to the correct resolution first and then import it. It is also good practice to also create the image at the size you intend to use it since scaling images adds significantly to processing times and can give poor results. On the whole you will see a loss in quality of the image if it is scaled up by more than 200% unless you have compensated for it in the resolution. If necessary, resolution can be "invented" by using re-sampling or interpolation of the image. Interpolation is the process of multiplying the number of pixels contained in an image using different algorithms to increase its resolution or to prepare it for printing at a larger size, while maintaining the same resolution.
Interpolation is particularly useful to compensate for insufficient sampling. In other words, it can help to compensate for too low a density of dots in relation to the print size. Linework should be scanned at a minimum of 812 dpi for output on an HP Indigo digital press.