Ok, I modified Texture::convert_to_pnmimage to read like this:
bool Texture::
convert_to_pnmimage(PNMImage &pnmimage, int x_size, int y_size,
CPTA_uchar image, size_t page_size, int z) const {
if ( get_component_type() == T_unsigned_short )
pnmimage.clear(x_size, y_size, _num_components, 65535 );
else
pnmimage.clear(x_size, y_size, _num_components );
And I increased my texture size, now it’s working much better!