Why are our files printing out too light?

 

This one is kind of a hard on to always answer because there are so many variables involved especially when you are trying to compare output from two different printers or systems.  The real answer to the question is why are you clients creating PDF files with such thin lines to begin with.

 

Most PDF files are a combination of vector objects and graphics. 

 

 

 

The majority of the files we see where customers are complaining that they are not printing dark enough is because they were created with a very thin lines say 0.06 pts in thickness. 

 

You divide the points by 72 to get how many inches wide the lines are supposed to be so that works out to 0.06 / 72 = 0.0008333 inches wide.  If we assume a typical monochrome printer at 600 dpi we take the width and multiply it by 600 to get 0.0008333 * 600 = 0.5 dot (pixels) wide.  That's right, your customer created a file that isn't even a full pixel wide.  Luckily this gets rounded up to 1 pixel wide but even at that it's going to print out very thin and look light to the eye.

 

But you say it looks fine on the screen.  Sure, because your screen is only 1900 pixels wide and most PDF viewers will display it as at least 1 pixel regardless of the zoom level.  If you look at displaying a full D Size drawing in a PDF Viewer on your screen you are probably zoomed out to 25% of full size.  Since the DPI of your screen is probably about 96 dpi we take 0.06 * 0.25 / 72 * 96 = 0.02 pixels wide.  So in reality that line should really only be displayed as 1/50th of a pixel in width on the screen.  Even at 100% zoon factor it should only be displayed as 0.08 pixels wide on the screen.

 

 

 

If it is a case where the lines were too thin the best thing to do is to teach your clients to create decent files in the future.  Assuming that they may be printed out on a 400 dpi plotter and you always want at least 2 pixels wide you would want a minimum line thickness of 2 / 400 = 0.005 inches or 0.36 pts.

 

In our newer versions we have a Minimum Line Thickness setting in the interface.  You could increase that to 2, 3, or 4 to make up for the bad way that they created the PDF file.  For most PDF files a setting higher than 1 won't hurt but there are some PDF files where it may create different colors (or shades of gray), or gaps in solid fills, or extra lines in solid fills.  The PDF spec is extremely complex and there are times where identical colors overlapping each other can actually produce a different color and when you artificially make lines thicker things like this can happen.  Or hatch patterns made up of alternating lines 1 pixel wide could turn into a solid fill.  But again for the majority of files a minimum line thickness of 2 or 3 won't hurt.

 

 

 

Again as an Engineer or CAD User we don't always think about the downstream importance of things.  CAD Users typically design  on a black background where colors like Yellow for centerlines look great.  Or on a white background on a computer monitor a light gray looks great also.  But the reality of printing on a monochrome printer is that these colors just do not work.

 

First let's look at the case where a 50% black (gray) line has to be printed out to a monochrome printer.  Since they printer only prints in black the only way to make a 50% gray color is to drop out half of the pixels being printed.  When converted to a monochrome tiff most software including our AcroPlot Repro will use a dither matrix to calculate which pixels get dropped out.  Most dither matrixes will be at least a 10x10 matrix but to make things simple let's look at just a simple 4x4 matrix.  In the matrix each pixel is assigned a percentage black that must be met for that pixel to be printed.

 

 

So if we look at the 50% gray line that is 2 pixels wide these pixels would be printed as black.

 

 

But if we look at the 50% gray line that is only 1 pixels wide then due to the matrix only 1 pixel is going to be printed resulting in it looking like a 25% gray line.

 

 

The color yellow converts to about an 11% black line or a very light gray color.  Basically 90% of the pixels are going to have to be dropped out to create a yellow line on a monochrome printer.

 

 

 

Again the best solution is to teach your clients or users that light grays and light colors just are not going to print well to a monochrome printer.  In reality anything lighter than 50% for lines or text is probably not going to print out well.

 

In AcroPlot Repro you can reduce the Brightness in our interface.  The Brightness ignores pure black and pure white but all other colors and shaded of gray are adjusted based on a calculation.  If you set the Brightness to -50 it will make the colored lines darker.  It does not increase the thickness but it will result in more pixels being printed.  Our AcroPlot Repro Enhanced Dither 2 pattern is also optimized to drop out fewer pixels in thin lines and text.

 

 

 

Especially when you are trying to compare the output from 2 different printers you really have to look at what DPI you are converting the TIFF to as well as the native DPI of the printer.  Most programs like Reprodesk you will define the output DPI of the conversion to TIFF independent of the native DPI of the printer.

 

So if you convert to a 400 dpi tiff and the printer is 400 dpi then everything is going to print out as it was converted.

 

But if you convert to a 400 dpi tiff and the printer is 600 dpi then everything has to be scaled up 150%.  This is where things start to get tricky.  A 1 pixel width line at the 400 dpi will scale up to 1.5 pixels so will likely end up being 2 pixels in the 600 dpi image.  Where a 2 pixel wide line will scale up to 3 pixels so it's not twice as wide as the 1 pixel line was before the printer had to scale it up.  It of coarse is far more complicated than this because in the scaling up you also have to look at the other pixels around each one and things like that.

 

Try converting at the native DPI of the printer or at half the native DPI of the printer so things are at least a 200% scaled up instead of a 150% scaled up.