2 Position Operations

A position is a pixel location within a viewport. The upper-left corner is pixel (0, 0), and positions increase to the left and down.

(struct   posn   (x y))

x : real?

y : real?

Represents a positions.

((get-pixel viewport) p) → (one-of/c 0 1)

viewport : viewport?

p : posn?

Returns the color of the pixel at position p in viewport; 0 denotes white and 1 denotes not white.

((get-color-pixel viewport) p) → rgb?

viewport : viewport?

p : posn?

Returns an rgb value for color of the pixel at position p in viewport.

((test-pixel viewport) color) → rgb?

viewport : viewport?

color   :   (or/c (integer-in 0 299)       string?       rgb?)

Returns the color that will actually be used if color is used to draw.