Painting From Life vs. From Photos
Drawing can be done directly on a painting surface, but working on paper, and then transferring has advantages. Most obvious is that one can make many drawings and then select the best to transfer to a clean white canvas or panel. Another advantage is that drawing allows for experimentation with picture dimensions, before committing to a particular painting surface.
To transfer a drawing, without enlarging or reducing the size, tracing paper is useful. Tracing paper goes back at least to the 14th century (Cennino Cennini describes three techniques for making it). After the drawing is traced to the paper, it can be transferred to the painting surface in different ways. One is to rub the back of the tracing paper with charcoal, position the paper on a white grounded panel, then go over the lines with a hard pencil or stylus. This is the original carbon paper. Another technique is to prick holes in the tracing paper and then use a pouncing bag with charcoal dust to bring the design onto the painting surface. This is better for canvases, because it does not require the strong local pressure of using a pencil or stylus. Furthermore, it is easy to make a transfer, wipe of the charcoal dust, and make another, to experiment with different positioning of the design on the canvas.
Once the drawing is transferred (either in charcoal lines or dots), it must be fixed, using black ink or paint. Once this is done, the charcoal can be removed, and the drawing developed further before underpainting.