Inkscape

Are there specs for print size and resolution?

Asked by Martin Andersen

I note a number of templates for real-world print sizes, such as A4, DL Envelope, CD covers, etc, and I can extrapolate that Inkscape assumes 90pixels per inch for physical measurements.
But is there any specification for print resolution? Eg you place an image 600px wide at 600dpi, it should measure 1inch across. What determines how large it is on the page? Or rather, how all the objects relate to the page border?
A quick test with Illustrator seems to indicate they use 72pixels per inch (a 1-inch square opens in Inkscape as 72pixels across), the traditionally-agreed screen resolution, though monitors vary widely. Certainly sizes between Inkscape and Illustrator do not match, and I suspect the same is true for other programs.
I see there's an SVG Print proposal, but no specifics on how this is determined:
http://www.w3.org/TR/2003/WD-SVGPrint-20030715/
http://www.w3.org/TR/2007/WD-SVGPrintPrimer12-20071221/

In the absence of any standard on pixel resolution, is there a specific reason for 90pixels/inch?

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Best prkos (prkos) said : 		#1

Im not sure I understand exactly your question but Ill try to throw in some thoughts to start a discussion as Im also interested in this topic.

As far I know the 90dpi is the default dpi in the SVG specs, thats why Inkscape uses it as default (i couldn't find where exactly its stated in the spec). Ive heard it said that SVG is primarily web oriented spec meaning onscreen display has priority over print display, thats why dpi is closer to screen res (printing requires min 300dpi but for screen graphics 72dpi is enough.)

A similar question was asked on the inkscapeforum and microUgly gave an explanation I think is appropriate to quote here:

"Exporting at 300 dpi should be more than adequate for printing. But you need to make sure you're printing at the size you expect for the resolution. For example, if you create a 300 dpi image that is 300 x 300 pixels then print it so it's 2 x 2 inches on the page, then the resulting page resolution is only 150 dpi. The dpi in a file isn't a hard-set number - what ever software you are printing with can ignore it or override it. Pixels is the only true way to measure your image, so double check the maths and make sure the pixel size of the image is high enough to print at the size and resolution you want.

i.e. a 2 x 2 inch print at 300 dpi will require the image to be 600 x 600 pixels (2 x 300 = 600)."

From this I conclude:
If you plan to print your Inkscape drawing you need to calculate the drawing size yourself (multiply the inches by the desired dpi to get document pixels)

You cannot rely on Inkscape templates (A4, letter, etc) for printing at those paper sizes as they use 90dpi, and you need at least 300dpi for a good quality print

If you make a A4 Inkscape document and want to print it to A4 size paper, you need to export the page at 300dpi and print the exported bitmap (to get 100% accurate number you also need to take printing margins into account)

If you import an image that is 600dpi and 600px wide, it will appear very big in Inkscape (4000px), heres a detailed explanation: http://wiki.inkscape.org/wiki/index.php/FAQ#Why_do_images_.27grow.27_when_imported_into_Inkscape.3F

Printing software can stretch and contract your drawing (for example often software for printing photos has options to print a different number of photos on the same paper size, 1 photo stretched on A4, or 2 photos on A4, or 16 photos on A4....) Again if you want to make sure that the quality of the printout is satisfactory you need to calculate yourself if the size of the drawing in pixels is enough to get minimum 300dpi on paper.

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Martin Andersen (msandersen) said : 		#2

I didn't see mention of a specific physical resolution, so was unsure if it was in a spec somewhere. I came across 90dpi used as an example in the SVG spec, but then i also saw 96dpi elsewhere i think. Incidentally I understand this is what Corel Draw uses for its SVGs. As no SVG-capable software seems to agree on resolution, it is likely it is undefined. I thought maybe there was something in SVG-Print being developed. Rather it seems everything is relative to Pixels, whose size is flexible.
It does not surprise me if there are none, as SVG is a Web vector format, a world of pixels.
I realise physical resolution with vector graphics is relative, as it is resolution-independent, but it would be nice if there was an agreed-upon spec, so you can draw an accurate scale. Having physical measurements in Inkscape like Inches or Millimetres is nonsensical without an agreed pixel-resolution, hence my query.
I do understand Inkscape will never be a competitor to illustrator, despite what some think, as SVG just isn't aimed at print, only RGB output. Coming from Publishing and Printing, I understand resolution well (in printing, you deal with ppi, dpi, and lpi, 3 different types of resolution).

I'll take it then there are none and call it Problem Solved.

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Martin Andersen (msandersen) said : 		#3

Thanks prkos, that solved my question.