I'm having trouble getting my head around the relationship between the size of a vector object, the size of an exported bitmap, and the resolution of the exported file. It's not that I don't understand what happens, it's just completely backwards from how it should be intuitively, to me anyway.

Say I have a 500 px by 300 px object, and I export a 50 px by 30 px bitmap. It ends up with a resolution of 8.91 DPI, very low. If the original object is 50 px by 30 px, exporting the same size bitmap gives a resolution of 89.83 DPI.

So the smaller my drawing is, the higher the resolution I can get when exporting it? If I'm going to need both 500x300 and 50x30 bitmaps, I should make my original 50x30, or maybe even smaller, in case I need even smaller output.

That's inside out to me. Intuitively, a larger drawing is higher resolution, has more information, and should allow higher-res bitmap output. It's clear how it actually works, and I can live with that, but I was wondering if someone can explain it to me in such a way that it doesn't seem upside down any more.

Thanks,
- e

I'm not sure if this is an appropriate comparison. But part of my college education involved reading and using topographic maps. The higher the map resolution, the smaller area it covers (and the lower the resolution, the more area it covers). I think the concept of resolution IS backwards in that way.

But I also get confused about dpi resolution, and image dimensions. And the only reason I haven't figured it out, is because I've never needed to. But I remember a topic where someone explained it. It didn't really sink in, because again, I didn't need to use the info. But you might try searching all the forums here. Or if we're lucky, whoever it was might see this topic and explain it again

We may not need to know the theory behind it, but there are big implications for what we do every day.

Do you create your vector images at very *small* sizes, to allow them to be converted to high quality bitmaps? If we were talking about bitmaps (like pictures), it'd be the other way around. You'd want your original to be huge, so it could be high-res, lots of pixels (bits worth of information), and smaller images with less total info in them would be derived from that.

So this does actually matter. What do you actually do? Or am I unaware of some way to export a high-res bitmap with smaller dimensions than the original vector drawing?

I also wonder if this is a limitation in Inkscape, or something inherent in the difference between vector and bitmap graphics. I'm not a professional digital artist (programmer actually), but I thought that if you knew you were going to need many sizes at high resolution, vector graphics was the way to go. You could resize the work to any required dimensions with no loss of quality. High-res glossy publication, even wall or billboard size, you're good. That's why (I thought) commercial logos are vector drawings. Or is that only true when you're targeting a medium that can use the svg directly? Which the web is not, yet, for instance.

- e

There is a difference between DPI and resolution. Resolution is a measure of how many pixels an image has say 100px by 100px. You can display this at varying sizes 1cm by 1cm Or 1m by 1m. The number of pixels hasn't changed; so its quality has not but the DPI has changed a lot...

See http://www.rideau-info.com/photos/mythdpi.html for more

Oh yay, thanks tomh!

My advice is to use Inkscape to export the bitmap dimensions you want, i.e. a 500px X 300px image, and then use another program to specify the printing resolution (DPI) of the image. I use Faststone Image Viewer, unless I already have the GIMP open. Of course you can do it in Inkscape, and the export bitmap dialog does the calculation for you, but only in pixels. The others allow inch and metric input and I find that useful.

I get all messed up on DPI vs resolution as well. What I've ended up doing is figuring out the size that wherever I'm uploading to can handle, then when exporting, I reduce the bitmap size by about 33% (which is 1/3 of 100%), and UP the DPI to 300% manually. That gets me both the size I need and the good resolution. It works is all I know.

Also, if it is for screen display, the only thing that really maters is the size (i.e. 500px by 300px).

-Rob A>

Thanks folks. I am in fact only concerned with screen (web) display.

What got me thinking about this at all was that the bitmaps I was getting were slightly distorted. Not horrible, but slightly glitchy-looking circles, etc.. Led me to wonder about the stated dpi being so low; other than that, I would have left well enough alone.

And that turns out to be a combination of the reality of bitmap life, but also a slight mismatch in the width and height. The document was square originally, set to integer width and height, but at some point I'd changed the size of the actual drawing enough that I reset the document size to match, forgetting to make sure it was still exactly square. When exported and viewed with css that made it square, it was slightly messed, completely my fault.

To some degree, bitmaps will never be as clean or high-res as the vector original, but fixing that got me a lot closer.

Thanks again,
- e

Hi.

I find Inkscape usefull when placing overlaps on maps, circuits or other types of imported images.

When units is pixels, then exporting an image at 90dpi ensures that the image file output in pixels dimensions matches the Inkscape canvas perfectly.

And that turns out to be a combination of the reality of bitmap life, but also a slight mismatch in the width and height. The document was square originally, set to integer width and height, but at some point I'd changed the size of the actual drawing enough that I reset the document size to match, forgetting to make sure it was still exactly square. When exported and viewed with css that made it square, it was slightly messed, completely my fault.

I'm not sure about this, but there's an extension called PixelSnap, which I think is meant to help with some of the distortion (Extensions > Modify Path > PixelSnap).