Can I set transparent pixels with the function to warp image perspective?

cvWarpPerspective( src, dst, map_matrix, CV_INTER_LINEAR+CV_WARP_FILL_OUTLIERS, cvScalarAll(0) );

the last parameter cvScalarAll(0) set the background pixels (usually the corners of the output image, since they remains empty once the image is warped).

At the moment is 0. I would like those pixels being transparent (I'm saving the file as png).

thanks