It’s not that hard to understand. It shouldn’t be this hard to convert a phonecam image to monochrome and then introduce some subtle toning to selective parts of it. But I’ve yet to see a photo app for Android or, to a lesser extent, iOS that gets it right and offers any real control over the process.

First of all, go read the wikipedia entry for Photographic print toning. Then look at some toned photos. Lots of toned photos. Look at sepia ones. Selenium ones. Platinum ones. Platinum-Palladium ones. Cyanotypes. And especially split-toned pictures. Just google them. Learn some history. And taste.

Note the following:

• Toning does not mean the even application of an overall colour cast. Otherwise it would be called ‘ruining’, not toning.
• ‘Sepia’ toning does not usually call for deep brown/orange applied as a wash to the whole image.
• Yes, cyanotypes are typically blue. No, not that blue. Jesus, what kind of nightmares are you trying to conjure up?
• Frequently, the shadows and highlights are toned differently. Sometimes even the midtones too. And the differentiation between these three areas may vary.
• A duotone is not a colour cast either. More like a tint really.
• Toning was used: to increase the longevity of the print; to correct unwanted colour renditions of particular papers and chemicals; to enhance local contrast without altering overall contrast; to add a pleasing effect appropriate to the subject (for example, ‘warming’ the tones of a portrait). Note that “to make the viewer vomit copiously and weep at the visual desecration committed” is not one of these aims.
• A popular school of thought says that if the toning effect is obvious, it’s too strong. Practitioners write of “warming” (red shift) or “cooling” (blue shift) particular tones, not “paintbrushing” them. Hint, hint.
• “Selective colouring”, “colorization” (sic), “spot colour” - none of these have anything to do with toning. Whatsoever. Look under “dubious gimmick long past its sell-by date” instead.

Now go back and redesign your app. Yeah, the one with the “300+ cool FX and filters, including retro and vintage looks!!!” Starting at the top:

• Provide a way to convert the image to monochrome. (If this is all your app does, it isn’t finished.)
• Provide a way to control the contrast and brightness of that conversion, and ideally the tonal mapping by filtering particular colours from the underlying image (RGB channels).
• Add a colour picker for the base colour..
• Add a way to gradually apply that colour by a variable degree to either the shadow or highlight areas of the image, preferably without affecting the overall luminance values too much.
• Now repeat that with a different colour to the opposing areas.
• Bonus points: provide a way for the user to define or vary what the ‘shadow’ and ‘highlight’ areas consist of and the rate of fall-off from each. Add separate control of the remaining midtones as well and you’re buzzin’.
• Provide some sane preset examples, illustrating subtle use of the above controls. As a minimum: sepia; selenium; split tone (cool shadows, warm highlights); platinum. (Ideally, enable the user to add their own presets and perhaps make them stackable, so they could combine “cyano shadows” with “sepia highlights”.)

Heck, I’d even settle for the last one alone if it was done well.

Look, I don’t mind a bit of fun: faux ‘Olga-like and Retro-Cam apps, wacky effects, pictures overprinted with lolcats. But we’ve got a surfeit of those and there’s not much to choose between them; can’t at least one developer who knows a little about actual photography give us a serious alternative? Guess I’ll just have to wait for Snapseed on the new Nexus 7…

(Note: I’ve made suggestions to the Vignette team about better support for toning, but I’m fairly lenient on their app because it’s very good at what it does so far anyway and it’s not claiming to be a complete editor.)