Reprographers (like Woodie Rush of Plan Express) using Océ Repro Desk products have reported that printing DWF files at half size results in shaded areas being darker than expected. To address what is happening here, let's look at the problem outside the context of DWF and then apply the situation to DWF.
Here is an image that is 400 pixels by 400 pixels. It consists of randomly placed black dots. I created it using the simplest of programs - Microsoft Paint:
If I use Microsoft Paint to reduce the image by 50%, I get the following image:
If you look closely, you will see that the smaller image appears darker. Microsoft Paint is providing some gray-scale antialiasing, which printers do not do, so the problem does not appear as bad in this example as it does in reprographics practice, but you can still see the effect and get the idea.
When Microsoft Paint reduces the image, it does so by averaging the pixels to determine when to mark a pixel as black or white in the new, smaller image.
When a reprographics customer or reprographer uses Océ Publisher or makeldf.exe wired into Océ Repro Desk Server to create an LDF from a DWF, the vector-based DWF file is converted to a Tiff image. The Tiff image is stored inside the LDF file. When Océ Repro Desk extracts the resulting Tiff from the LDF and prints it at full size - all is well. When the resulting Tiff is reduced to half size, as an image format, decisions regarding which pixels to make black versus which ones to make white are required. Like Microsoft Paint, Océ software averages the pixel colors; however, the problem is compounded by the fact that when there is a tie, the Océ software errs on the side of making a pixel black. This is done for a very good reason. No one wants a dashed line to look like a dotted line when printed at half size. As any drawing legend can show you, dashed lines and dotted lines have very different meanings; however, this strategy does result in a darkening effect on shaded areas.
I had a thought that processing a DWF to one LDF at full size and again to another LDF at half size would solve the problem. My hope was that each LDF would contain a Tiff image matching the desired paper size. Each LDF could have been printed at full size for the different paper sizes. I would have had two LDF files instead of one, but at least the second one would yield a half-sized print without darkening. Alas Océ Publisher and makeldf.exe create one large Tiff - independent of page size - to provide the highest line fidelity and flexibility for various paper sizes. A small Tiff image that needs to be scaled up to a larger paper size would result in poor image quality. So Océ creates the largest possible Tiff image to be safe. The LDF has the greatest flexibility in being used for any size paper. As a result, this approach produced the same result.
Océ is aware of this issue. While Océ investigates the issue, some alternatives include:
- Use the Pen Set Editor of Océ Publisher to change the pen setting from 100% black to 50% black before processing the DWF into LDF. This tip was suggested by Océ Software Development Manager Rob Newsom.
- Experiment with dithering methods other than Error Diffusion (the default). Methods like Simple Dither result in "fewer black pixels in full size" which in turn leads to "less of a darkening effect when reduced to half size." According to Océ Technical Writer, John Taniel, as documented in his help file for Océ Publisher:
- Not Dithered means everything (geometry and images) that is not white is rendered black.
- Simple Dither and Error Diffusion result in an LDF that approximates the colors in the source file with varying degrees of black, white, and gray; results of either option vary based on the source data.
- All Black Geometry renders all geometry in black; images are not affected.
Alternatives such as PLP PlotWorks and CADzation AcroPlot Repro create images for devices with the desired page size taken into account. KIP Powerprint keeps the DWF file as a DWF until the image is ready to be committed to paper, so it does not have this issue. In addition, the problem does not occur if Autodesk DWF Viewer or Autodesk Design Review are used to print to a device as a Windows system printer.