Title: STUDY MATERIALS - Requirements for printing PIXEL AND VECTOR GRAPHICS
Course: Pre-Press Procedure / e-design > Section: Part II – Pre-press procedure in practice, planning and analysis of the work

After the online lectures, the study materials along with the map of new concepts will be uploaded to Moodle.

Requirements for printing PIXEL AND VECTOR GRAPHICS

Requirements for PIXEL AND VECTOR GRAPHICS in printing

In computer graphics and digital preparation of prints, we see two different types of graphics: vector graphics and pixel graphics.

Pixel graphics is sometimes erroneously referred to as raster graphics, but this is not quite accurate because the rasterisation of a digital image takes place only during the final stage of prepress, during colour separation in a RIP (Raster Image Processor). Although we see both types of graphics in the form of pixel graphics on a computer monitor, the structure and generation of a digital image differ significantly and are meant to carry different information.

Pixel graphics

Pixel graphics

Resolution and bitmap density

Resolution and raster density

Resolution is the pixel density per unit and is generally expressed as dots per inch (DPI). In order to prepare a photo suitable for printing and to determine the optimal resolution, we need to know the ability of the material/paper to absorb ink, i.e. at what raster density it is possible to print. Paper parameters are measured by line per inch (LPI). This determines directly what should be the pixel density of the photo.

To calculate the optimal resolution, we have a simple formula to use:

raster density x 2 = resolution.

Thus, if the printing resolution of the newsprint is 95-115 lines per inch, then according to a simple calculation the image resolution must be 190-230 DPI.

Below is a table showing generally suitable photo resolutions according to the paper type.

Newsprint – 95-115 LPI = 190-230 DPI

Magazine paper – 130 LPI = 260 DPI

Uncoated sheet offset paper – 135-150 LPI = 270-300 DPI

Coated sheet offset paper – 150-175 LPI = 300-350 DPI

Multi-coated paper (cast coated paper) – 200 LPI = 400 DPI

In addition to considering the difference in colour space, it is important to evaluate the photo resolution and correct it if necessary.

Pixel graphics is a type of graphics depending on resolution, so if we increase or decrease the resolution of an image, it results in a loss of quality.

Theoretically, it is possible to leave an image with a sufficient (higher than necessary) resolution unchanged during prepress, but then the image will be downsampled either during the generation of PDF or later in the RIP and will be adjusted according to the paper. This approach can result in loss of details because a large number of pixels are removed at once. It would be appropriate to reduce the resolution of the photos yourself in a photo editing program. When converting Photoshop pixels according to the algorithm, the correct reduction step is considered to be 5-7%. The same requirement applies to increasing the resolution of images. Although there is no more pixel information coming in from anywhere, it is possible to change the image by about 30% by gradually increasing the resolution so that the quality loss is not noticeable.

Bit depth

Bit depth

Wherever we are dealing with a digital image, we face bit depth that depends on the architecture of the computer.

The smallest amount of information in a computer is called a bit, and each bit has 2 values. The bits are grouped by eight and these form a byte. The byte can represent 2 to the 8th power or 256 different values. In essence, 256 values are brightness levels that a computer is able to transmit per channel. By channel, we mean the colour tone that the video card and monitor are capable of displaying from maximum saturation to minimum brightness. Although a human eye is able to distinguish about 5,000 brightness levels, the limitation of computer architecture and the PostScript language based on it shows the human brain's ability to adapt and cope with such a limited amount of information. The figure shows the possibilities of pixel graphics.

In the case of a 1-bit image, as shown in the example, the colour may have 2 values, i.e. whether there is colour or not.

In the case of an 8-bit image, an image is formed of 256 light-darkness levels. In the case of a 24-bit image, we can get 256 to the 3rd power different values = approximately 16.7 million colour shade qualities.

In the case of a CMYK image, which has no longer 3 but 4 channels, the values still remain at the level of a 24-bit image because the black (K) added to the CMY model does not add new colours. In professional literature, both a 24-bit CMYK image and a 32-bit image can be seen – they both mean the same thing.

The advantages of pixel graphics are

  - the good ability to convey smooth colour transitions because each pixel carries individual colour information.
  - light printing, as for digital printers based on PostScript, the digital image is made easily “edible”.

The biggest disadvantages are caused by dependence on resolution and the significantly more complex construction compared to vector graphics.

Vector graphics

Vector graphics

Vector graphics, as the name implies, consist of vectors or lines, which in turn are used to create surfaces. Vector graphics are used on objects with simple, well-defined surfaces, such as logos, illustrations, typographic elements, drawings, etc. The biggest difference compared to pixel graphics is the construction of objects: if pixel graphics need a large number of pixels to form a smooth circle, then in vector graphics only the centre point needs to be determined and only four anchor points are enough to draw a circle. The curves formed between the anchor points are also called Bezier curves (vector calculation method). Also, if you need to create a straight line in raster graphics, the exact length of the line is created from the necessary pixels, but in vector graphics it is enough if we know the start and end coordinates of the line.

As a result, a file with vector graphics is usually smaller in size than a file with the same image in pixel graphics. Another important difference from pixel graphics is their independence from resolution. We can enlarge or reduce a vector object indefinitely without quality loss – the coordinates of the vectors are constantly recalculated. Theoretically, a photorealistic image can also be achieved with vector graphics, but the more detailed the images become, the more anchor points and vectors are needed to describe it.

At one point, in addition to the significantly larger file size, there will be major problems when printing a large image.  As digital images are rasterised when sent to an output device, errors can easily occur when rasterising complex vector graphics.

To avoid this, it is recommended to rasterise complex objects during prepress. This is especially true for raster effects (3D, transparency, etc.) possible in Adobe Illustrator and Corel.

Unlike pixel graphics, a digital image is easy to edit in vector graphics and it does not depend on resolution, which has some advantages, but the disadvantage is the real inability to create a photorealistic impression.

While the quality criteria for pixel graphics in prepress were mainly the correct resolution and colour space chosen for the printing technology and paper, some of the following aspects should be considered when designing a print in vector graphics.

Line thickness

In general, the optimal line thickness for sheet-fed offset printing is 0.12 pt if this line is printed in 1 full colour (100%).

If for design purposes it is necessary to mix the colour of the line using 2 colours, it is reasonable to choose 0.25 pt for the minimum line thickness, in which case we minimise the possibility of the line becoming a “staircase”, because not all raster dots may be eventually printed successfully on paper. Of course, much depends on the technology and paper used. In web offset printing, such as newspaper printing, we have to take into account a significantly larger dot and thus also a bigger minimum line thickness.

Font size

The criteria for selecting the typeface and size are, of course, again based on the technology and paper used for printing. The general recommendation is not to use a font size smaller than 4 pt, for a negative letter 7-8 pt. Serif fonts, italics or thin fonts (light, thin, etc.) are also not recommended for negative letters, as small details of the letter structure may be lost during printing.

Overprint

Overprint

Overprint is a method of hiding material stretching or offset errors. Overprint is always added to the objects that are 100% black in the print shop and repro shop. In essence, an overprint means that the colour to which it is assigned is printed on the colour below it.

If, as a general rule, a knock-out of an object is made on the colour image below during prepress. In special cases overprint is applied, i.e., the colour objects are stacked on top of each other, thereby synthesising mixed colours.

Automatic overprinting is applied to 100% black objects primarily because the text used in the design of the print is usually black.

Automatic overprinting is applied to 100% black objects primarily because the text used in the design of the publication is usually black.

.... otherwise, the result may be as follows.

The colour of the image under the text, as well as the knock-out made, may start to flicker under the text as a result of stretching or offsetting the paper and make reading much more difficult.

However, for large black surfaces, overprinting is not recommended, as the image below it (such as a photo) may become visible through it and to avoid an unwanted result a colour recipe 1, 1, 1, 100 (CMYK) should be used on the object, then automatic overprint will not apply.

Quite often, however, such black surfaces may appear thin and grey. To solve this problem, the term rich black is used in printing, which may vary in composition, but common recipes can be either

            - 40, 20, 20, 100 (deep black) or
            - 60, 40, 0, 100 (dark black)

It is also possible that overprint has been applied to some coloured objects during design. To eliminate it some printers use automatic overprinting of coloured objects in addition to automatic overprinting to 100% black. The biggest mistake that can happen is when an overprint happens to get on an object that is white (i.e. of the colour of the paper). Obviously, the image so defined does not print because white is not used in traditional offset printing.

It is recommended to use design programs in the overprint preview mode. In this way it is possible to immediately notice unwanted processes.

Work in overprint mode so you can spot any errors right away

Interesting effects can be achieved by applying overprinting on purpose

Trap

Trap

Trap is essentially a method of an overprint, where objects are not completely printed on top of each other, but in which case edges are overlapped. Trapping is done automatically for coloured objects, but there may be situations where you need to set the trap yourself.

This is mainly necessary in the case of a negative letter, where the background is enriched black. In order to prevent the text used from becoming unreadable due to offset or excessive stretching of the paper, it is advisable to add to the white text for example 0.2 pt outline with recipe 1, 1, 1, 100, in which case this line thickness remains as a buffer in case of deviations in registration.

 https://youtu.be/xJPrGyAXxiM

Gradient

Gradient

The preferred application of today's very common so-called poetic designs is a gradient, which at some point can cause some printing problems. When we are dealing with a gradient where one colour goes over to white (the paper colour), we should take into account the fact that paper does not tolerate it and offset technology generally does not allow printing a raster dot smaller than 3-4%, so in this area of the gradient colour information can end suddenly and the impression of a smooth transition can be spoiled.

In order to prevent this, it is advisable to leave 3-4% colour even in the lightest part of the gradient – to the human eye, such a sparse raster still looks white, but we minimise the possibility of an unexpected result.

Colour information ends before the object is completely covered

If we leave a small percent of the colour at the lightest end, there is a better chance to achieve a smooth gradient.