How redraw a part of an image display with a PtImagearea wid

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I’m displaying an image with a PtImagearea , when i change just a small
part of the image (by writting in the buffer PhImage_t->image) , if i
want to redraw the new PhImage_t->image i must do PtsetRessource on the
argument Pt_ARG_IMAGEAREA which redraw all the image (too long) , How
can i redraw just the modify part of the PhImage_t->image.

Futhermore how can i change the color of the grid and the level when it
appear , and on a bitonal image (black and white ; the white is grey) ,
my palette seen no be good?

Thanks for helps