PDF technology: transparency in the PDF format

From the very beginning, PDF technology has lived on constant further developments and adaptations, which then also affect media designers, designers or the printing industry. This is a factor that should not be underestimated, especially nowadays. One example of such developments is the work with transparencies:

Today we are used to being able to create partial transparency effects everywhere and it is standard to be able to work with overlapping graphics or drop shadows. As a rule, today’s PDF viewers support all transparency functions and these are simply part of working with PDF documents and other portable documents. But that was not always the case.

Adobe got the ball rolling…

Introduction of PDF 1.4 – transparent imaging model

The development towards this self-evident model began about 20 years ago in 2001 with an introduction by Adobe. Here, a technical innovation was introduced that helped shape and change the way transparencies are handled. This introduction set in motion a development that then also had a clear influence on the way PDF files are used and developed.

This refers to the Adobe PDF 1.4 specification regarding partial transparency – through this specification, the opaque imaging model of earlier PDF versions was extended by the transparent imaging model. Adobe PDF 1.4 thus then offered functions that went beyond alpha compositing. It was possible from then on for authors to create artistic and realistic effects with a minimal number of objects, while avoiding the need to rasterise their designs when exporting to PDF.

This new possibility laid the foundation 20 years ago for graphic designers and artists to work with much more scope for creativity and flexibility. In older PDF versions, it was only possible to create the appearance of partial transparency by overprinting, i.e. by applying ink over previous colour applications. But now it was possible to use functions such as drop shadows, soft edges, blurring and others through transparency.

Today, it is considered standard for illustration and office applications to offer the possibility of applying transparent effects to any object, and the basis for this was laid by Adobe in 2001. With earlier PDF versions, it was initially only possible to apply full transparency, i.e. to make an object or part of an object completely invisible, for example. Examples of this are stencil masks (this was possible with Level 1 PostScript as well as with early PDF versions).

This is to be distinguished from partial transparency, which is a much more complex procedure. Here, only a certain amount of both the foreground object and the underlying object (the background) should be blended together (alpha compositing or alpha blending). However, with the introduction of PDF 1.4 and the support of the transparent image model, new possibilities were then introduced. These included “constant alpha, soft masks, blend modes, matte, shape and opacity, and transparency groups“.

Working with transparencies – The PDF/X file format

The changes that PDF 2.0 (ISO 32000-2:2020) brought with it should also be emphasised. Here, new features were developed with regard to print production and accessibility (Tagged PDF). In addition, there were improvements of an editorial and technical nature, such as the fact that the processing of transparent objects was specified more precisely.

On the subject of transparencies, the PDF/X format should not go unmentioned here. PDF/X is the standard for the printing industry. The requirements here differ from the pure display on the monitor or specifications that are more important for archiving or accessibility. The aim is to optimise the materials for the printing sector. The PDF/X standard is established in the printing industry and is supported by many software solutions. The PDF/X-4 format is often used in the print area. PDF/X-4 makes it possible to use the same colour spaces as PDF/X-3, but also transparencies and layers.

Here, too, a development has taken place regarding the possibility of transparencies. While PDF/X-1a only allowed CMYK and spot colours and did not allow transparencies and layers, PDF/X-3 made it possible to use RGB, LAB and ICC-based colours in addition to CMYK and spot colours. Transparencies and layers were automatically reduced. With PDF/X-4, it was then possible to use transparencies and layers. When using PDF/X-4, however, it must be said that the creator of the PDF files has more personal responsibility and the correctness of the data must be ensured even more. Note: PDF/X-6 has been the ISO standard for digital artwork since 2020: https://www.webpdf.de/blog/en/the-new-pdf-standards-2020/

More on the topic of transparency reduction: