I'm trying to make some textures, but I have this problem with the trans: the border showns "diffuse". How can I obtain a "linear" borders?

Thanks in advance to all

I am unsure of your use of the words "diffuse" and "linear" here....but the only thing I can see with regard to your posted image is your jagged edges on the transmapping, which could be solved by using a larger map size, thus providing a cleaner resolution and cleaner line for your transparency.

I think the word miedes' is looking for is 'aliased' or 'antialiased'. While a larger resolution should help, you also need to pay attention to how you are creating the lines in your graphics program - there will be controls there that control the antialiasing. While it may seem backward to antialias an edge you want to be sharp, this really should with your lines.