Chapter 24 | The graphics library

The `graphics` library provides a set of portable drawing primitives. Drawing takes place in a separate window that is created when `open``_``graph` is called.
Unix:

> This library is implemented under the X11 windows system. Programs that use the `graphics` library must be linked as follows:

>             ocamlc other options graphics.cma other files

> For interactive use of the `graphics` library, do:

>             ocamlmktop -o mytop graphics.cma
>             ./mytop

> Here are the graphics mode specifications supported by `open``_``graph` on the X11 implementation of this library: the argument to `open``_``graph` has the format `"`display-name geometry`"`, where display-name is the name of the X-windows display to connect to, and geometry is a standard X-windows geometry specification. The two components are separated by a space. Either can be omitted, or both. Examples:
> `open``_``graph "foo:0"`
>   connects to the display `foo:0` and creates a window with the default geometry
> `open``_``graph "foo:0 300x100+50-0"`
>   connects to the display `foo:0` and creates a window 300 pixels wide by 100 pixels tall, at location (50,0)
> `open``_``graph " 300x100+50-0"`
>   connects to the default display and creates a window 300 pixels wide by 100 pixels tall, at location (50,0)
> `open``_``graph ""`
>   connects to the default display and creates a window with the default geometry.

Windows:

> This library is available only under the toplevel application `ocamlwin.exe`. Before using it, the Caml part of this library must be loaded in-core, either by typing

>             #load "graphics.cmo";;

> in the input windows or by using the ``Load'' entry of the ``File'' menu.

The screen coordinates are interpreted as shown in the figure below. Notice that the coordinate system used is the same as in mathematics: y increases from the bottom of the screen to the top of the screen, and angles are measured counterclockwise (in degrees). Drawing is clipped to the screen.

24.1 | Module `Graphics`: machine-independent graphics primitives

    exception Graphic_failure of string

> Raised by the functions below when they encounter an error.

Initializations

    val open_graph: string -> unit

> Show the graphics window or switch the screen to graphic mode. The graphics window is cleared and the current point is set to (0, 0). The string argument is used to pass optional information on the desired graphics mode, the graphics window size, and so on. Its interpretation is implementation-dependent. If the empty string is given, a sensible default is selected.

    val close_graph: unit -> unit

> Delete the graphics window or switch the screen back to text mode.

    val set_window_title : string -> unit

> Set the title of the graphics window.

    val clear_graph : unit -> unit

> Erase the graphics window.

    val size_x : unit -> int
    val size_y : unit -> int

> Return the size of the graphics window. Coordinates of the screen pixels range over `0 .. size_x()-1` and `0 .. size_y()-1`. Drawings outside of this rectangle are clipped, without causing an error. The origin (0,0) is at the lower left corner.

Colors

    type color = int

> A color is specified by its R, G, B components. Each component is in the range `0..255`. The three components are packed in an `int`: `0xRRGGBB`, where `RR` are the two hexadecimal digits for the red component, `GG` for the green component, `BB` for the blue component.

    val rgb: int -> int -> int -> color

> `rgb r g b` returns the integer encoding the color with red component `r`, green component `g`, and blue component `b`. `r`, `g` and `b` are in the range `0..255`.

    val set_color : color -> unit

> Set the current drawing color.

    val black : color
    val white : color
    val red : color
    val green : color
    val blue : color
    val yellow : color
    val cyan : color
    val magenta : color

> Some predefined colors.

    val background: color
    val foreground: color

> Default background and foreground colors (usually, either black foreground on a white background or white foreground on a black background). `clear_graph` fills the screen with the `background` color. The initial drawing color is `foreground`.

Point and line drawing

    val plot : x:int -> y:int -> unit

> Plot the given point with the current drawing color.

    val plots : (int * int) array -> unit

> Plot the given points with the current drawing color.

    val point_color : x:int -> y:int -> color

> Return the color of the given point in the backing store (see "Double buffering" below).

    val moveto : x:int -> y:int -> unit

> Position the current point.

    val rmoveto : dx:int -> dy:int -> unit

> `rmoveto dx dy` translates the current point by the given vector.

    val current_x : unit -> int

> Return the abscissa of the current point.

    val current_y : unit -> int

> Return the ordinate of the current point.

    val current_point : unit -> int * int

> Return the position of the current point.

    val lineto : x:int -> y:int -> unit

> Draw a line with endpoints the current point and the given point, and move the current point to the given point.

    val rlineto : dx:int -> dy:int -> unit

> Draw a line with endpoints the current point and the current point translated of the given vector, and move the current point to this point.

    val curveto : int * int -> int * int -> int * int -> unit

> `curveto b c d` draws a cubic Bezier curve starting from the current point to point `d`, with control points `b` and `c`, and moves the current point to `d`.

    val draw_rect : x:int -> y:int -> w:int -> h:int -> unit

> `draw_rect x y w h` draws the rectangle with lower left corner at `x,y`, width `w` and height `h`. The current point is unchanged.

    val draw_poly_line : (int * int) array -> unit

> `draw_poly_line points` draws the line that joins the points given by the array argument. The array contains the coordinates of the vertices of the polygonal line, which need not be closed. The current point is unchanged.

    val draw_poly : (int * int) array -> unit

> `draw_poly polygon` draws the given polygon. The array contains the coordinates of the vertices of the polygon. The current point is unchanged.

    val draw_segments : (int * int * int * int) array -> unit

> `draw_segments segments` draws the segments given in the array argument. Each segment is specified as a quadruple `(x0, y0, x1, y1)` where `(x0, y0)` and `(x1, y1)` are the coordinates of the end points of the segment. The current point is unchanged.

    val draw_arc :
            x:int -> y:int -> rx:int -> ry:int -> start:int -> stop:int -> unit

> `draw_arc x y rx ry a1 a2` draws an elliptical arc with center `x,y`, horizontal radius `rx`, vertical radius `ry`, from angle `a1` to angle `a2` (in degrees). The current point is unchanged.

    val draw_ellipse : x:int -> y:int -> rx:int -> ry:int -> unit

> `draw_ellipse x y rx ry` draws an ellipse with center `x,y`, horizontal radius `rx` and vertical radius `ry`. The current point is unchanged.

    val draw_circle : x:int -> y:int -> r:int -> unit

> `draw_circle x y r` draws a circle with center `x,y` and radius `r`. The current point is unchanged.

    val set_line_width : int -> unit

> Set the width of points and lines drawn with the functions above. Under X Windows, `set_line_width 0` selects a width of 1 pixel and a faster, but less precise drawing algorithm than the one used when `set_line_width 1` is specified.

Text drawing

    val draw_char : char -> unit
    val draw_string : string -> unit

> Draw a character or a character string with lower left corner at current position. After drawing, the current position is set to the lower right corner of the text drawn.

    val set_font : string -> unit
    val set_text_size : int -> unit

> Set the font and character size used for drawing text. The interpretation of the arguments to `set_font` and `set_text_size` is implementation-dependent.

    val text_size : string -> int * int

> Return the dimensions of the given text, if it were drawn with the current font and size.

Filling

    val fill_rect : x:int -> y:int -> w:int -> h:int -> unit

> `fill_rect x y w h` fills the rectangle with lower left corner at `x,y`, width `w` and height `h`, with the current color.

    val fill_poly : (int * int) array -> unit

> Fill the given polygon with the current color. The array contains the coordinates of the vertices of the polygon.

    val fill_arc :
            x:int -> y:int -> rx:int -> ry:int -> start:int -> stop:int -> unit

> Fill an elliptical pie slice with the current color. The parameters are the same as for `draw_arc`.

    val fill_ellipse : x:int -> y:int -> rx:int -> ry:int -> unit

> Fill an ellipse with the current color. The parameters are the same as for `draw_ellipse`.

    val fill_circle : x:int -> y:int -> r:int -> unit

> Fill a circle with the current color. The parameters are the same as for `draw_circle`.

Images

    type image

> The abstract type for images, in internal representation. Externally, images are represented as matrices of colors.

    val transp : color

> In matrices of colors, this color represent a ``transparent'' point: when drawing the corresponding image, all pixels on the screen corresponding to a transparent pixel in the image will not be modified, while other points will be set to the color of the corresponding point in the image. This allows superimposing an image over an existing background.

    val make_image : color array array -> image

> Convert the given color matrix to an image. Each sub-array represents one horizontal line. All sub-arrays must have the same length; otherwise, exception `Graphic_failure` is raised.

    val dump_image : image -> color array array

> Convert an image to a color matrix.

    val draw_image : image -> x:int -> y:int -> unit

> Draw the given image with lower left corner at the given point.

    val get_image : x:int -> y:int -> w:int -> h:int -> image

> Capture the contents of a rectangle on the screen as an image. The parameters are the same as for `fill_rect`.

    val create_image : w:int -> h:int -> image

> `create_image w h` returns a new image `w` pixels wide and `h` pixels tall, to be used in conjunction with `blit_image`. The initial image contents are random, except that no point is transparent.

    val blit_image : image -> x:int -> y:int -> unit

> `blit_image img x y` copies screen pixels into the image `img`, modifying `img` in-place. The pixels copied are those inside the rectangle with lower left corner at `x,y`, and width and height equal to those of the image. Pixels that were transparent in `img` are left unchanged.

Mouse and keyboard events

    type status =
      { mouse_x : int;              (* X coordinate of the mouse *)
        mouse_y : int;              (* Y coordinate of the mouse *)
        button : bool;              (* true if a mouse button is pressed *)
        keypressed : bool;          (* true if a key has been pressed *)
        key : char }                (* the character for the key pressed *)

> To report events.

    type event =
        Button_down                 (* A mouse button is pressed *)
      | Button_up                   (* A mouse button is released *)
      | Key_pressed                 (* A key is pressed *)
      | Mouse_motion                (* The mouse is moved *)
      | Poll                        (* Don't wait; return immediately *)

> To specify events to wait for.

    val wait_next_event : event list -> status

> Wait until one of the events specified in the given event list occurs, and return the status of the mouse and keyboard at that time. If `Poll` is given in the event list, return immediately with the current status. If the mouse cursor is outside of the graphics window, the `mouse_x` and `mouse_y` fields of the event are outside the range `0..size_x()-1, 0..size_y()-1`. Keypresses are queued, and dequeued one by one when the `Key_pressed` event is specified.

Mouse and keyboard polling

    val mouse_pos : unit -> int * int

> Return the position of the mouse cursor, relative to the graphics window. If the mouse cursor is outside of the graphics window, `mouse_pos()` returns a point outside of the range `0..size_x()-1, 0..size_y()-1`.

    val button_down : unit -> bool

> Return `true` if the mouse button is pressed, `false` otherwise.

    val read_key : unit -> char

> Wait for a key to be pressed, and return the corresponding character. Keypresses are queued.

    val key_pressed : unit -> bool

> Return `true` if a keypress is available; that is, if `read_key` would not block.

Sound

    val sound : freq:int -> ms:int -> unit

> `sound freq dur` plays a sound at frequency `freq` (in hertz) for a duration `dur` (in milliseconds).

Double buffering

    val auto_synchronize : bool -> unit

> By default, drawing takes place both on the window displayed on screen, and in a memory area (the ``backing store''). The backing store image is used to re-paint the on-screen window when necessary.
> To avoid flicker during animations, it is possible to turn off on-screen drawing, perform a number of drawing operations in the backing store only, then refresh the on-screen window explicitly.
> `auto_synchronize false` turns on-screen drawing off. All subsequent drawing commands are performed on the backing store only.
> `auto_synchronize true` refreshes the on-screen window from the backing store (as per `synchronize`), then turns on-screen drawing back on. All subsequent drawing commands are performed both on screen and in the backing store.
> The default drawing mode corresponds to `auto_synchronize true`.

    val synchronize : unit -> unit

> Synchronize the backing store and the on-screen window, by copying the contents of the backing store onto the graphics window.

    val display_mode : bool -> unit

> Set display mode on or off. When turned on, drawings are done in the graphics window; when turned off, drawings do not affect the graphics window. This occurs independently of drawing into the backing store (see the function `remember_mode` below). Default display mode is on.

    val remember_mode : bool -> unit

> Set remember mode on or off. When turned on, drawings are done in the backing store; when turned off, the backing store is unaffected by drawings. This occurs independently of drawing onto the graphics window (see the function `display_mode` above). Default remember mode is on.