Innovatoria Print

Pixels and resolution
By dividing the number of pixels on the height and width of the raster files you can define the physical size of the image. For example an image with 300 ppi, which is 900 pixels wide and  600 pixels high has size 3 x 2 inches (one inch equals approximately 25,4 mm.)
900 pixels ? 300 ppi = 3 inches width
600 pixels ? 300 ppi = 2 inches width



Classification of the images
Monochrome images (1 bit images). The resolution of this type of images must be 900-1200 ppi, in order to be suitable for print. Theoretically the resolution of 1 bit images should be equal to the resolution of the device. The resolution of CTP (computer-to-plate) devices is usually 2400 ppi. Despite this the recommended resolution is between 900 and 1200 ppi.

The optimal resolution for the colored images is 300 ppi. This resolution is possible when the images are scanned in the same size, in which they will be printed afterwards.  For example the normal size of the image is 3 1/3 inches (around 8,5 cm.) for standard images in one column, which will illustrate a certain publication.

The resolution for combined colored images must be between 500 and 900 ppi. Because of the presence of text and/or graphic elements the high resolution is necessary in order to avoid blurring the image. When the image has too high resolution, it increases the physical file size  and makes it very difficult to process. In order to avoid this the recommended resolution for combined colored images is not too high – it must preserve the good quality of the image and at the same time not to make the size of the image very big.

Resolution and restart
During the restart process series of pixels turn into halftones with different size and position. Better results can be achieved when the density of a certain point includes more than one pixel. So the coefficient for quality can be used to identify the recommended resolution for the colored images.

For colored images, which will be rastered for example at 133 lpi (lines per inch) the applied coefficient is 1,5х, i.e. you need an image with 200 ppi. For images, which include geometrical objects like lines, patterns and other forms, the recommended coefficient is 2.0х (around 266 ppi) in order to achieve good results after rastering the image.

The common standard for the colored and halftone grey images, intended for print is 300 ppi. Those images, which are designed or modified for Internet usually have resolution of 72 ppi, which is not enough for a good quality print. The strict application of the quality factor and the recommended resolution ensures the perfect balance between the quality and the size of the file. It is a common practice to use images, downloaded from Internet with low resolution of 72 ppi and try to make them suitable for print. They don’t have enough resolution and have very low quality after printing.

Standards for image resolution.
Raster images are classified as monochrome, tonal and combined. You must take into consideration the minimum resolution for each of these three types.  It is important to avoid the use of poor quality images in the print files. They are among the main reasons for cancelation and delay of graphic projects.

The monochrome (1 bit) images usually are scanned from graphics or printed text. As every bit can accept only two values (0 and 1), the pixels in the monochrome image have only two values – black and white. Those types of images require higher resolution than the tonal images, in order to avoid blurring and rough edges of the diagonal lines.  The recommended resolution is 900 – 1200 ppi.

The tonal images are usually extracted from pictures/photographs. They are described with 8-bit information, displayed as 256 levels of grey for black and white images. CMYK images have 32-bit information, i.e. they have 256 levels for each of the basic colors (cyan, magenta, yellow, black). The recommended minimum resolution for this type of images is 300 ppi.
The combined images have tonal and text and/or graphic elements at the same time. Because of that they also have 8-bit information, describing every pixel in every color channel. The combined images can be saved in CMYK, as well as in Grayscale color regime.  Because of the combination of images, text and graphic elements the recommended resolution for this type is higher. It is between 900 and 500 ppi.

Effective resolution
Since raster images have a specific resolution (number of pixels per inch), the change of the image size changes also the distribution of the pixels in the designated space. The resolution, which is a result from the scaling, is called effective resolution.

If an image is enlarged it will have lower resolution, i.e. fewer pixels per inch (unless additional pixels have been added by the so called interpolation). So if an image is reduced, it will have higher resolution (unless the existing pixels have been discarded). This means that scaling reduction generally is less problematic regarding visible defects. However it is very important to know that unnecessary resolution can contribute to excessive physical file size. Cropping the scanned image to the necessary size intended for print will benefit toward the reduction of physical file size.

When placing and scaling images, you are changing what is known as the image effective resolution. It is a calculation of the actual resolution factored for the scaling performed in the page layout application. If you reduce the scale of an image in the page layout, you increase the effective resolution. As you increase the amount of scaling, you will decrease the effective resolution. It defines the quality of a certain image and has some specific criteria, applicable toward it.