I made an observation yesterday in my daybook, while I was working on the next drawing. There is a definite difference in the creation of space (on paper) between Line drawings and Rendering. The lines of an orthographic drawing define the space, interior and exterior, with a single stroke. The line divides one area of the page from another, just as a wall would do in the built world. It is a straightforward way to create the spaces that the designer is looking for.
With a rendering, however, it is more complicated. You can't create that boundary in the same way. Rendering is the illustration of how light interacts with the different components in a design. (ie. walls, windows, furniture, etc). Instead of giving a bounding line, rendering needs to fill the space with light and shadows to give it definition.
1 month ago