Threshold is a filter on Photoshop, it turns any image into just Black and White, useful for punchy graphic photo images. I always find though, its tricky to get the balance right, either too much dark or too much light with the important detail disappearing, ok for creating faux rubber stamps, which I do often. But this Mobits Cam app Threshold filter always seems to deliver the goods. Great fine detail, and a good balance of B&W. 
It appears to use tones of grey (which  Photoshop doesn't ) making for a fuller image,  very flat and graphic. More like a monochrome posterization . Its also a bit  like the Silhouettes cool app only for iPad (which I no have). You could take the image into PS, or a similar prog if you have, and put a two tone gradient over it, nice. I love how you can view the effect as you search and compose your subject. These photos, in my garden
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