I have been trying to get a nice, smooth diagonal line using Adobe Photoshop, but to no avail. It's something that I've tried to perfect, and more times than not I end up getting some horrid looking pixelated mess of a diagonal line.
I have tried using the line tool, creating a path with it, and then stroking the path with a pencil or a brush, but that doesn't seem to work well. It seems to me the brush would work better but it's just all faded. I've also tried the pen tool, but I get the same results as the line tool.
The problem here is that it's essentially a grid pattern your working on. Any horizontal or vertical line you draw will appear smooth because it flows naturally with the grid. However, any diagonals would appear like steps when viewed closely.
The solution here is to have more pixels per square inch. Another solution is to draw your images big, then resize the image to the desired dimensions.