On screen, the same line may look “smooth” or ragged, depending on several factors, like the size of the canvas, angle of the line (aspect ratio), and thickness of the line. That’s because it is drawn as a raster image; to make it always smooth it has to be a vector. If you save it as a ps, eps or pdf, it will look smooth; if you save it as gif, png or jpg, it won’t. Compare the png and PDF versions of this:
root [0] TCanvas *c=new TCanvas("c","c",400,400)
root [1] TF1 *f = new TF1("f","x",0,5)
root [2] f->SetLineWidth(3)
root [3] f->Draw()
c.pdf (14.0 KB)