▼ |  | Graphics: Legacy Library

1 | Basic Commands
2 | Position Operations
3 | Color Operations
4 | Draw, Clear, and Flip Operations
5 | World Operations
6 | Miscellaneous Operations
7 | An Example
8 | A More Complicated Example
9 | Protecting Graphics Operations
10 | Mouse Operations
11 | Keyboard Operations
12 | Flushing
13 | Graphics Library as a Unit

On this page:

posn
get-pixel
get-color-pixel
test-pixel

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### 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 right and down.

> > struct

> > (struct |  | posn (x y)

> #:extra-constructor-name make-posn)
> x : real?
> y : real?

Represents a positions.

> > procedure

> > ((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.

> > procedure

> > ((get-color-pixel viewport) p) → rgb?

> viewport : viewport?
> p : posn?

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

> > procedure

> > ((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.
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