I need add invisible watermark for user to render data of OpenGL ES 2.0. And if user will do screenshot by home+lock, then watermark will be slightly visible.
How I can do it? Or it's not impossible?
Maybe with blending?
Home+lock takes a copy of exactly what's on screen when pressed; it doesn't modify anything and you can't change your views programmatically. You therefore can't have a watermark that isn't visible to the user but becomes visible when they take a screenshot.
With blending you could add a watermark that's always visible. With a suitable shader and some basic cryptography you could add a watermark that's invisible to humans but could be detected by software, with varying levels of complexity and robustness.