Plot lines: CAD to PDF

I have clients who want definitive plots from PDF, they don’t want DWG they actually want paper and, for them, PDF is paper equivalent. I quickly discovered PDF does its own thing when it comes to line weights, and to make things worse what I see in Acrobat may not be what the client sees when printed at 1:1 on paper. I post here what I have learned from generating 1:50 plots on A0 sheets as PDF.

1. Greyscale is great on screen but fails on paper at A0 sheet size grey lines on a 1:50 plot become near invisible, the printer seems to stretch the grey lines into a dotted or screen effect.

2-tone_01

Using black line or greyscale gives very different effects.

2-tone_02

Using a grey tone (btm) for the light lines improves screen readability but doesn’t help the plotter.

Index-colours_DS2018

Making the difference in line weight visible by use of the stock grey shades is a great way of getting dense detail ( many fine lines close together) to read with out presenting a mass of black but once processed to PDF they can disappear.

2. CAD line weights are not like Rotring pen sizes. In theory CAD sends instruction to the printer to make lines at the widths set in the drawing. This used to work well when a flat bed plotter actually had a physical pen ( although slow and unreliable- I recall leaving plots to run overnight only to find a pen had blocked after 10mins producing a blank plot).

LWT

Laser jet printers make the lines by fusing powder to the media, the width of the line is determined by the digital code sent, it has nothing to do with a mechanical size at all.

Rotring-Pen-1

clicky clicky what a pain these were!

The good news is that (provided we leave grey tones out of it and stick to black) we can have some ridiculously small line weights and no longer have to worry about the performance of ink and nib. 0.05 mm is a common minimum these days- a size of pen that would be impossible to use by hand. 0.13 mm was as fine a line I could reliably plot.

3. Solid objects won’t print unless they are rendered, if the drawing is a hybrid of line (wire) and solid or surface 3D objects they need to be processed to 2D to proceed. Rendering wireframe or linework draws a blank.

4. Display mode is important . This may seem obvious but it came as a surprise to me, if I’m plotting a wire frame with some solid objects in it I would expect the ‘hidden’ view style to get the 3D stuff to work in 2D on my plot. Not so: anything other than a wireframe view will mash arcs and cause parallel lines to ‘step’ or moire with horrible effects.

The AutoCAD plot panel has some extended print options under the ‘more’ arrow:

Display-plot-style_2

DraftSight has similar settings including a maximum quality setting of 4800 dpi which generates huge files :

DS-plot

The ‘shade print’ setting should be set to ‘wireframe’ for best PDF results.

The default ‘as displayed’ is a trap for the unwary, if you are working in anything other than wireframe line quality is compromised.

Bottom is plotted from ‘wireframe’ view, top from ‘Conceptual’ the wireframe view is sharper.

Arcs seem to be particularly damaged by using the wrong view style to plot:

Sharpest results are from the wireframe display style.

5. Not all PDF printers are the same. Depending on the PDF printer the outputs can differ by:

  • file size
  • processing speed
  • layering
  • paper size limit
  • display quality
  • DPI control options

PDF Creator by Source forge, using its default settings at ‘Maximum quality’ is slower than the default but generates smaller file sizes, the display quality of PDFs from PDF creator is better than those from the AutoCAD default, the lines are consistently displayed across the zoom sizes.

Foxit is fast and produces smaller files.

Microsoft’s Print to PDF only goes up to A3 size. Try a bigger plot size and it comes up with:

Ms-wrng

The PDF display used here for comparison is ‘Adobe Acrobat Reader DC’ which is widely bundled with Windows and I consider it a ‘universal’ PDF viewer.

We are looking at this stuff as jpeg. The screen shots here are all jpeg compressed so are not quite as printed in PDF but they are all taken at overscale to give an idea of the effects achieved. Jpeg compression uses grey tones in its image size reduction method so this can give a false impression what happens at full size plot.

B

About billboyheritagesurvey

Heritage worker
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One Response to Plot lines: CAD to PDF

  1. Pieter-Jan says:

    This is a great post Bill, it came just in time, as I was diving into it myself this morning. Thanks!

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