Modern DTP applications allow designers to vary the degree of the transparency of objects from 100 per cent opacity (opaque) to zero per cent opacity (transparent). When an object's opacity is decreased the underlying artwork becomes visible through the object.
Transparency is not really a new option, but in the early days of desktop publishing, such special effects as transparent overlays and soft drop shadows required software such as Adobe PhotoShop to export flattened layers and transparency to EPS or TIFF files.
Transparency is introduced into a layout when an object's opacity is changed; when a drop shadow is introduced; when feathering is is used; when blending modes are employed; or when any of the effects in the transparency palette are introduced.
Transparent content in files can either be live or flattened. Files that contain live transparency can be opened in the originating (native) software and the transparent objects can be modified. Native software that supports live transparency includes PhotoShop, native Illustrator, Illustrator EPS, native InDesign, Quark XPress (version 7) and Adobe PDF 1.4, PDF 1.5, PDF 1.6 and PDF 1.7.
Flattened transparency occurs when a file is exported to a non-native format. Flattened file formats include PostScript, EPS, DCS, Adobe PDF 1.3, GIF, JPEG, BMP, and TIFF images created with PhotoShop prior to version 6. Files created using Acrobat Distiller do not contain live transparency regardless of the PDF version selected. Transparency is flattened when a PostScript file is generated. Note that the Illustrator EPS format could contain live transparency as is actually comprises of two files - one native Illustrator format that only Illustrator can open and the other in an EPS format that other applications can import/place in which the transparency is flattened.
At the time of writing (May 2007) most installed RIPs are PostScript devices. As discussed above postScript does not support live transparency. Therefore, flattened transparency is required. Some RIPs flatten live transparency prior to RIPping, others require only flattened files to be presented. This year the Adobe Print Engine was introduced upon which many future workflows and RIPs will be based - but for now most PDF specifications will call for PDF 1.3.
Headley Brothers PDF specification recommends that pages are printed to PostScript and distilled to PDF using Acrobat Distiller to PDF 1.3. However, designers often prefer to export to PDF. Either way, the PDF 1.3 specification requires transparency to be flattened. The flattener settings can be saved in a transparency flattener preset (Edit>Transparency Flattener Presets). You can then apply these settings when you print or export to PDF 1.3 (Acrobat 4) or even EPS formats.
During flattening, areas where transparent objects overlap other objects are analysed to determine if they can be represented in vector format or if they must be rasterised to achieve the expected transparency effect. Rasterisation is the process of changing vector graphics, fonts, gradients, gradient meshes into bitmap images for display and printing. Adequate rasterisation resolution is critical to the quality of the printed output.
Headley Brothers RIPs (Raster Image Processor) rasterise vector elements at 2540 dpi. Vector content rasterised by the flattening process is set to 2400 dpi in the recommended flattener settings.
When overlapping transparent objects are flattened each shape that results is called an atomic region. The shape of an atomic region will follow the lines, curves and shapes of the objects. Areas where multiple transparency effects overlap are called complexity regions. Rather than being flattened as many small atomic regions the objects within a complexity region are combined into a single rasterised shape during flattening. When the flattener settings are set to the highest fidelity (as the recommended preset) no complexity regions are produced. This produces the best output but may slow the process.
Identifying if transparency effects are present
Pages that contain objects with transparency will display the the Pages palette with a chequerboard pattern. Use the Flattener Preview palette (Window>Output Preview>Flattener) to identify which objects have transparency effects and which objects are affected by them. The Flattener Preview palette highlights transparent objects/images, as well as areas affected by transparency.
When All Affected Objects is selected in the Flattener Preview palette areas that are affected by flattening are displayed in pink, while areas that are not affected by flattening are displayed in grey.
If you place a graphic on to a page which is in a file format that could retain live transparency you will add that live transparency to your layout. Once placed the graphic will interact with other objects on the page and you can use the graphic-manipulation features of the software to modify the appearance of the graphic.
File formats that you could place into your layout which might contain live transparency include:
- Native Illustrator (.ai)
- Native PhotoShop (.psd)
- Adobe PDF 1.4
- Adobe PDF 1.5
- Adobe PDF 1.6
- Adobe PDF 1.7
If your DTP appication does not understand live transparency (Quark XPress 6.x and earlier) you should use flattened artwork for placement of transparent content. Most advertisements supplied for inclusion into magazine pages are now supplied as PDF, and your PDF specification should reflect the DTP software to which it will be imported/placed.
As you increase the number of overlapping transparent objects on a page the complexity of the transparency information also increases. For instance, placing text with a drop shadow in front of the blank page background is less complex than placing it in front of an imported graphic where the drop shadow must be combined with the underlying graphic for display and printing.
Use transparency effects sensibly. For example, it is possible to feather the edge of really small, fine-serifed type, but the resulting text will probably be impossible to read.
Object stacking and order and transparency
When a new object is created on a page it is placed on top of all existing objects. A stack of objects is created, one on top of the other, called the stacking order. (For layouts with multiple layers new objects are placed at the top of the selected layer.) The stacking order becomes more complicated when working with a group or a layer, each of which has its own stacking order and can have transparency applied differently to an object, a group of objects, or a layer of objects. The stacking order is changed by moving an object forward or backward on the layer or page. Layers, and objects on layers are also organised in a stacking order. The layers at the top of the list in the layers palette are in front of the layers at the bottom of the list.
Objects with transparency effects applied do not have to be overlapping other objects but they often do. You could apply an opacity value of less than 100 per cent to lighten an object whether it is overlapping other objects or not. However when you apply a lower opacity value you not only make it lighter you also make it semi-opaque, which will cause it to blend with any overlapping objects.
If you apply transparency to objects that overlap, changing the stacking order can change the appearance of the areas where the objects overlap. Ensure that the stacking order produces the desired results.
In general, type should be on top of all other objects (unless it needs to interact with transparency). Placing text frames on top of the stacking order will help prevent them from being involved in any transparency flattening.
If your pages are to be printed in the four process colours and are not intended to print with any extra colours (spot colours) you must be really careful not to inadvertently include any spot colours on your pages where transparency is used.
The flattening process handles spot colours in a special way and with overprints, and if a flattened file which includes spot colour is presented to a CMYK workflow, which will convert the flattened spot colour to CMYK it will probably not reproduce as expected. Only incorporate spot colours in your layout if you know they will be used on the press.
Be careful when mixing overprinting and spot colours with transparency. To view the printed results of overprinting on-screen, enable Overprint Preview.
