Greyscaling an image in GLES 1.x

[Lucretia]

Hi,

I have a need to make parts of an image dynamically greyscale.

I was thinking about using a small quad with a grey texture or just grey vertex colours and then blending that with the colour image in some way, but I don't know which blend function to use for this, could someone point me in the right direction?

Thanks,
Luke.

[uruk]

A simple method is (R,G,B) dot3 (0.3,0.3,0.3).

[Lucretia]

aparently, that doesn't work under Android...I have had a play and it made no difference.

[BionicBytes]

Can you not modulate the texture with a grey-scale texture?

Failing that there are options depending what's supported:
glTexEnv
You may be able to use DOT3 if you have support for it.