Why do my photos and/or Powerpoint files look ugly when printed or converted to PDF?

A colleague was swearing at our printer more loudly than usual: ‘‘Believe me or not, he said, but those pictures looked just well on the screen. How does the printer dare charging me for this …’’

                                    
[Illustrative pictures. On the left: what he thought he would have; on the right: what he got.]

Well, my answer did not please him: it’s all his fault. Why? would you ask. Easy:

1. MS Powerpoint as well as Adobe Acrobat Reader both include an anti-aliasing renderer*.
2. Your screen resolution is generally around 72 dpi. As a comparison, printers can generally output at 300+ dpi.
3. You are usually not working in the real (paper) scale, instead the workspace is conveniently fitted to the screen: the 3cm×3cm nice picture you see on your screen could become a 25cm×25cm ugly picture as printed!

How is it important?

1. The anti-aliasing filter blurs the picture a bit: it looks nicer on the screen but is therefore less ‘‘detailed’’ (blurring is a lossy conversion).
2. If you look at a printed image at a suitable distance, 300 dpi will generally be enough not to perceive the individual ink dots. This is good enough for most of the purposes. However, it would not be advisable to go far below that. Please also keep in mind that computer screen image quality look decent even at lower resolution because of the underlying displaying technology: a 72 dpi printed picture does not look good.
3. The default behaviour is to import pictures at the screen resolution, not at the printer resolution. This is all wrong if you want to print your document. The default size of the displayed picture is actually calculated to reach screen resolution: it’s rarely the other way around. Typically, folks just ‘‘import’’ a picture into Powerpoint and since it takes half of the workspace and looks good, they don’t change the size and suppose it will look good as printed.

How can I prevent such behaviours?

1. Disable anti-aliasing to look at the pictures ‘‘as they are’’.
2. Whenever possible (graphs, diagram, etc.), use vector graphics such as SVG, WMF, AI and the like: it will make you PDF/PPT lighter and they do not suffer from resizing (there is no resolution per se, the quality remains identical whatever the size).
3. If vector graphics can’t be used (photos), make sure to keep the pictures resolution above 300/600 dpi. Let’s repeat that again: don’t randomly expand your pictures! Look at the image resolution: the best is to fit your printer resolution.

Oh, and by the way, since it was for a poster, MS Powerpoint was not designed to efficiently create posters. That will be the subject of a new post.

*: note that in some PDF visualization, there is no anti-aliasing filters: even in some versions of Adobe Acrobat Reader, this setting is turned off, that can explain apparent loss of quality during PPT to PDF conversion.