got a problem when rendering a model with material in model space, using autocad 2012, in earlyer versions of autocad when the rendering is saved as png format, the background is transparant, now I got the image with the colour of the working space. Does somebody know what I do wrong ? 

Are you saving it as 32 bits (24 bits + Alpha)?If you don't tell Autocad to include the alpha channel then you won't get the transparent background.

Great, worked for me, thanks