After some trial and error I've decided on a formula to work out the size of the frame and matte. The frame width is calculated as the width of the narrowest axis (in pixels) * 0.032. I've no real scientific or artistic reason for the choice but it works for me. So, for example, a photograph measuring 4000 * 2000 would produce a frame width of 64 pixels with the matte width being twice that at 128. If I wanted to use a double matte for an edge highlight, then I'd calculate that width using a factor of 0.004.
Bearing in mind that I'm just trying to enhance the on-screen display a bit, I decided to go for a plain-coloured frame, no wood-grain or textured finishes, just a flat colour. After a bit of research that seemed to suggest that the best colour to use would be one that complements the colours in the photograph, I decided to use a site I'd mentioned a few years ago to calculate the best colours - Whats Its Color.
Whats Its Color is an image-color processing utility that'll give you an image's primary and complementary dominant colours, how many visually unique colours there are in it and the top ten visually unique colours used. All very useful when trying to choose frame, matte and highlight colours around an image.
Choosing which colours to use is also quite simple. The frame gets the complimentary colour and I pick a highlight colour from the image for the matte, lightening it a bit if necessary. For a highlight edge, I'd pick the colour closest to the main subject of the image or one of the top-ten colours. As I said before, it's all a matter of personal taste so whatever works, works.
If you're just interested in experimenting with framing without actually editing your photographs, then have a look at the BigHugeLabs Mat tool. It'll let you add a matte and frame border as a percentage of your choice of the original image and has a few extra bells and whistles like bevelled edges and credits.. It's also compatible with Flickr, Photobucket and Facebook.
Of course, if anyone knows of a recognised way to calculate all of the above, I'd be grateful to hear it. Also happy to take comments on any of those that I've framed on my photostream as well.
Related Posts: Whats Its Color