When creating a vector file, how should I determine my vector file canvas size given the png sizes I need to export to?
For example, if I need to export a 32x32 png and a 64x64 png, what size should my control vector file be?
The main reason I ask this is because I recently did an iPhone app icon and I needed to export to 57x57, 114x114, 29x29, 58x58, 72x72, 50x50 and 512x512. I used a vector file that was 512x512. All the exported pngs came out looking good except for the 29x29. It was a circular icon and the circle border on the 29x29 came out fuzzy. Does this usually happen when you size a big vector file down to a small png? How would one handle that situation?
Originally I thought that it didn't matter because it's a vector file.
This seems to be a common theme right now. When creating any graphic, you should be considering your target resolution - whether it be in pixels, inches, hot-dogs, whatever. Vectors are not magical fairy dust that can be sprinkled on a design to make it look good at any resolution.
Consider the following image. Using the same vectors in a 100 x 100pt bounding box (in Illustrator), I created 3 images. The first is the target size of 100 x 100px, the second was created at 50 x 50px, the third at 10 x 10px. I then scaled the results up to 100px so you can see the level of image detail in each.
The smallest image simply doesn't hold as much image information as the larger images. This is limited by mathematics and has nothing to do with source format, only with the final rasterized output.
Once you rasterize your image, you are limited by the raw number of pixels (and bits per pixel). A 100px x 100px image has 10,000 pixels. A 10x10 image has only 100 pixels. It is impossible for 100 pixels to hold as much information as 10,000 pixels. In your case, scaling an image from target size of 512x512 down to output size of 29x29 means that the final raster only holds 3/10ths of a percent of the image data of the target image.
Oftentimes you will see a small icon that appears to have as much detail as a larger version of the same icon. This is generally not the result of simply resizing the icon, but of a graphic designer taking the same concept and manually tweaking it for the different resolutions. Take a look at Apple's icon guidelines and you'll see more evidence of this. If you take the larger Settings icon and scale it down, you will see that it is not as crisp as the smaller icon, and the border is smaller, and the shadows are slightly different.
As others have noted, this is almost certainly a pixel alignment issue.
Here's a simple example: a 29 × 29 pixel rendering of a circle and a square, first with pixel-aligned edges and second with the whole image shifted 0.4 pixels right and 0.2 pixels up:
To show more clearly what's happening, here are the same images scaled up by a factor of four:
Here, even the shifted version doesn't really look so bad, because the shapes are so simple that they look OK even with blurred edges. But if you have a lot of small details in your image, bad pixel alignment can easily turn them all into so much mush.
Fonts solve this problem with hinting, but unfortunately there's no way that I know of to hint an SVG image. (Well, I guess you could do it with scripting, in principle... let's say, no easy way that I know of.) Thus, you'll have to resort to other solutions. Here are a few tips I've found useful.
This is sort of a no-brainer, but if you're making your drawing with a particular output size in mind, make that the page size from the beginning. That makes setting up grids and such much easier, and also lets you easily preview the drawing at the target size by pressing 1 in Inkscape.
Create pixel grids for your document. In Inkscape, you can do this in the Grids tab under File → Document Properties.... Having a grid that shows where the pixel boundaries are makes it much easier to align the edges properly, especially with careful use of the snap to grid feature.
Note that you can have multiple grids in a single drawing, and toggle their visibility individually. Thus, you can create a separate grid for each of your target sizes (or at least for the ones where you expect problems). For this, it's most convenient to set your nominal page size to some number of pixels that is divisible by all of your target sizes: that way, all the grid spacings will be integer numbers of pixels.
If you just can't get your drawing to look good at all sizes, you may need to create multiple versions of it. For this, you can either create multiple copies of the entire document, or, if it's only part of the drawing that's causing problems, use Inkscape's layers feature. Just put the problematic parts of the image into a separate layer (or layers) and duplicate it, then tweak the different copies to work at the different target sizes. Then, when exporting a given target size, make the layers for that size visible and hide the rest. Don't forget to give the layers suitable names so that you'll remember which is which!
Unfortunately, as far as I know, Inkscape doesn't have any simple way to scale a page and its contents simultaneously, like GIMP's Scale Image tool. Of course, it's pretty easy if your drawing exactly fills the page (just resize the content and then auto-resize the page to match), but things get trickier if you want to keep some margin around the edges.
One thing you can do is create a "document rectangle": a simple white or transparent rectangle that exactly covers the page. (Set it to fill only, no stroke, and then adjust the dimensions to match the page.) It may be most convenient to put this rectangle in its own layer, since that way it stays out of the way, and you can easily select it even if it's fully transparent by going to that layer and using Edit → Select All (Ctrl+A).
Then, when you want to change the size of the drawing, you can select all the objects in it (using Edit → Select All in All Layers or Ctrl+Alt+A), scale them to the desired size using the transform tool controls bar (or Object → Transform...), and then resize the page to match by using Resize page to content from the Page Properties dialog.