Clear Operations

Where ``draw-'' commands make pixels black, ``clear-'' commands make them white:

•   ((clear-viewport viewport))
  clear-viewport takes a viewport descriptor. It returns a function that whitens the entire contents of viewport.
•   ((clear-pixel viewport) p)
  clear-pixel takes a viewport descriptor. It returns a function that clears a pixel in viewport at the specified position.
•   ((clear-line viewport) p1 p2)
  clear-line takes a viewport descriptor. It returns a function that clears a line in viewport connecting positions p1 and p2.
•   ((clear-rectangle viewport) pos width height color)
  clear-rectangle takes a viewport descriptor. It returns a function that clears a rectangle border in the viewport with the top-left of the rectangle at the position pos and with sides width across and height tall. The optional color argument defaults to black.
•   ((clear-solid-rectangle viewport) pos width height color)
  clear-solid-rectangle takes a viewport descriptor. It returns a function that erases a solid rectangle in the viewport with the top-left of the rectangle at the position pos and with sides width across and height tall. The optional color argument defaults to black.
•   ((clear-ellipse viewport) pos width height color)
  clear-ellipse takes a viewport descriptor. It returns a function that clears an ellipse border in the viewport. The pos, width, and height arguments are as in clear-rectangle; the ellispse is inscribed within the specified rectangle. The optional color argument defaults to black.
•   ((clear-solid-ellipse viewport) pos width height color)
  clear-solid-ellipse takes a viewport descriptor. It returns a function that erases a solid ellipse in the viewport. The pos, width, and height arguments are as in clear-rectangle; the ellispse is be inscribed within the specified rectangle. The optional color argument defaults to black.
•   ((clear-string viewport) p string)
  clear-string takes a viewport descriptor. It returns a function that clears a string at a specified location in viewport. The lower left of the string begins at p.
•   ((clear-pixmap viewport) filename p)
  clear-pixmap clears an X bitmap into viewport with its upper left corner at p.