Dibujando formas con canvas

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La cuadrícula

Antes de que podamos empezar a dibujar, necesitamos hablar sobre la cuadrícula del canvas o el espacio de coordenadas. La plantilla HTML de la página anterior tenía un elemento canvas con un 'height' y un 'width' de 150 píxeles. A la derecha, puedes ver este canvas con la cuadrícula por defecto superpuesta. Normalmente una unidad en la cuadrícula corresponde a un píxel en el elemento canvas. El origen de esta cuadrícula está posicionado en la esquina superior izquierda (coordenada (0,0)). Todos los elementos estan posicionados de manera relativa a este punto, así que la posición de la esquina superior izquierda del cuadrado azul es de 'x' pixeles desde la izquierda y 'y' pixeles desde arriba (coordenada (x,y)). Mas tarde en este tutorial veremos como trasladar el punto de origen a una posicion diferente, girar la cuadrícula e incluso darle una escala diferente. Por ahora nos dedicaremos a lo mas común.

Dibujando rectángulos

A diferencia de SVG, <canvas> solo soporta una forma primitiva: rectangulos. Todas las otras formas deben ser creadas por la combinación de uno o más trazos, listas de puntos conectados por líneas. Afortunadamente, tenemos una variedad de funciones para dibujar trazos  que hacen posible componer formas muy complejas.

Primero veamos el rectángulo. Aquí hay tres funciones que podemos usar en el canvas para dibujarlos:

fillRect(x, y, width, height)
Dibuja un rectángulo relleno.
strokeRect(x, y, width, height)
Dibuja el contorno de un rectángulo.
clearRect(x, y, width, height)
Borra un área rectangular especificada, dejándola totalmente transparente.

Cada una de estas tres funciones toma los mismos parámetros. X e Y especifican la posición del canvas (en relación con el origen) desde la esquina superior izquierda del rectángulo. Tambien especifica los parámetros de anchura y altura que proporcionan el tamaño del rectángulo.

A continuación se muestra la función draw() de la página anterior, pero ahora haciendo uso de estas tres funciones.

Ejemplo de forma rectangular

function draw() {
  var canvas = document.getElementById('canvas');
  if (canvas.getContext) {
    var ctx = canvas.getContext('2d');

    ctx.fillRect(25,25,100,100);
    ctx.clearRect(45,45,60,60);
    ctx.strokeRect(50,50,50,50);
  }
}

El resultado de este ejemplo se muestra a continuación.

ScreenshotLive sample

La función fillRect() dibuja un cuadrado grande negro de 100 píxeles en cada lado. La función clearRect() luego borra un cuadrado de 60x60 píxeles del centro, y luego strokeRect() es llamado para crear un contorno rectangular de 50x50 píxeles dentro del cuadrado borrado.

En las próximas páginas veremos dos métodos alternativos para clearRect(), y también veremos cómo cambiar el color y el trazo de diferentes formas.

A diferencia de las funciones de trazo que veremos en la próxima sección, las tres funciones del rectángulo dibujan inmediatamente en el canvas.

Dibujando trazos

Crear formas mediante trazos requiere algunos pasos adicionales.

  1. Primero, se crea el trazo.
  2. A continuación, se usan comandos de dibujo para dibujar dentro del trazo.
  3. Después, se cierra el trazo.
  4. Una vez el trazo ha sido creado, se le puede dar contorno o relleno para renderizarlo.

Estas son las funciones que se usan para llevar a cabo estos pasos:

beginPath()
Creates a new path. Once created, future drawing commands are directed into the path and used to build the path up.
closePath()
Closes the path so that future drawing commands are once again directed to the context.
stroke()
Draws the shape by stroking its outline.
fill()
Draws a solid shape by filling the path's content area.

The first step to create a path is to call the beginPath(). Internally, paths are stored as a list of sub-paths (lines, arcs, etc) which together form a shape. Every time this method is called, the list is reset and we can start drawing new shapes.

Note: When the current path is empty, such as immediately after calling beginPath(), or on a newly created canvas, the first path construction command is always treated as a moveTo(), regardless of what it actually is. For that reason, you will almost always want to specifically set your starting position after resetting a path.

The second step is calling the methods that actually specify the paths to be drawn. We'll see these shortly.

The third, and an optional step, is to call closePath(). This method tries to close the shape by drawing a straight line from the current point to the start. If the shape has already been closed or there's only one point in the list, this function does nothing.

Note: When you call fill(), any open shapes are closed automatically, so you don't have to call closePath(). This is not the case when you call stroke().

Drawing a triangle

For example, the code for drawing a triangle would look something like this:

function draw() {
  var canvas = document.getElementById('canvas');
  if (canvas.getContext){
    var ctx = canvas.getContext('2d');

    ctx.beginPath();
    ctx.moveTo(75,50);
    ctx.lineTo(100,75);
    ctx.lineTo(100,25);
    ctx.fill();
  }
}

The result looks like this:

Moving the pen

One very useful function, which doesn't actually draw anything but becomes part of the path list described above, is the moveTo() function. You can probably best think of this as lifting a pen or pencil from one spot on a piece of paper and placing it on the next.

moveTo(x, y)
Moves the pen to the coordinates specified by x and y.

When the canvas is initialized or beginPath() is called, you typically will want to use the moveTo() function to place the starting point somewhere else. We could also use moveTo() to draw unconnected paths. Take a look at the smiley face below. I've marked the places where I used the moveTo() method (the red lines).

To try this for yourself, you can use the code snippet below. Just paste it into the draw() function we saw earlier.

function draw() {
  var canvas = document.getElementById('canvas');
  if (canvas.getContext){
    var ctx = canvas.getContext('2d');

    ctx.beginPath();
    ctx.arc(75,75,50,0,Math.PI*2,true); // Outer circle
    ctx.moveTo(110,75);
    ctx.arc(75,75,35,0,Math.PI,false);   // Mouth (clockwise)
    ctx.moveTo(65,65);
    ctx.arc(60,65,5,0,Math.PI*2,true);  // Left eye
    ctx.moveTo(95,65);
    ctx.arc(90,65,5,0,Math.PI*2,true);  // Right eye
    ctx.stroke();
  }
}

The result looks like this:

ScreenshotLive sample

If you'd like to see the connecting lines, you can remove the lines that call moveTo().

Note: To learn more about the arc() function, see the Arcs below.

Lines

For drawing straight lines, use the lineTo() method.

lineTo(x, y)
Draws a line from the current drawing position to the position specified by x and y.

This method takes two arguments, x and y, which are the coordinates of the line's end point. The starting point is dependent on previously drawn paths, where the end point of the previous path is the starting point for the following, etc. The starting point can also be changed by using the moveTo() method.

The example below draws two triangles, one filled and one outlined.

function draw() {
  var canvas = document.getElementById('canvas');
  if (canvas.getContext){
    var ctx = canvas.getContext('2d');

    // Filled triangle
    ctx.beginPath();
    ctx.moveTo(25,25);
    ctx.lineTo(105,25);
    ctx.lineTo(25,105);
    ctx.fill();

    // Stroked triangle
    ctx.beginPath();
    ctx.moveTo(125,125);
    ctx.lineTo(125,45);
    ctx.lineTo(45,125);
    ctx.closePath();
    ctx.stroke();
  }
}

This starts by calling beginPath() to start a new shape path. We then use the moveTo() method to move the starting point to the desired position. Below this, two lines are drawn which make up two sides of the triangle.

ScreenshotLive sample

You'll notice the difference between the filled and stroked triangle. This is, as mentioned above, because shapes are automatically closed when a path is filled, but not when they are stroked. If we left out the closePath() for the stroked triangle, only two lines would have been drawn, not a complete triangle.

Arcs

To draw arcs or circles, we use the arc() method. You can also use arcTo(), but its implementations are somewhat less reliable, so we won't cover it here.

arc(x, y, radius, startAngle, endAngle, anticlockwise)
Draws an arc.

This method takes five parameters: x and y are the coordinates of the center of the circle on which the arc should be drawn. radius is self-explanatory. The startAngle and endAngle parameters define the start and end points of the arc in radians, along the curve of the circle. These are measured from the x axis. The anticlockwise parameter is a Boolean value which, when true, draws the arc anticlockwise; otherwise, the arc is drawn clockwise.

Note: Angles in the arcfunction are measured in radians, not degrees. To convert degrees to radians you can use the following JavaScript expression: radians = (Math.PI/180)*degrees.

The following example is a little more complex than the ones we've seen above. It draws 12 different arcs all with different angles and fills.

The two for loops are for looping through the rows and columns of arcs. For each arc, we start a new path by calling beginPath(). In the code, each of the parameters for the arc is in a variable for clarity, but you wouldn't necessarily do that in real life.

The x and y coordinates should be clear enough. radius and startAngle are fixed. The endAngle starts at 180 degrees (half a circle) in the first column and is increased by steps of 90 degrees, culminating in a complete circle in the last column.

The statement for the clockwise parameter results in the first and third row being drawn as clockwise arcs and the second and fourth row as counterclockwise arcs. Finally, the if statement makes the top half stroked arcs and the bottom half filled arcs.

function draw() {
  var canvas = document.getElementById('canvas');
  if (canvas.getContext){
    var ctx = canvas.getContext('2d');

    for(var i=0;i<4;i++){
      for(var j=0;j<3;j++){
        ctx.beginPath();
        var x              = 25+j*50;               // x coordinate
        var y              = 25+i*50;               // y coordinate
        var radius         = 20;                    // Arc radius
        var startAngle     = 0;                     // Starting point on circle
        var endAngle       = Math.PI+(Math.PI*j)/2; // End point on circle
        var anticlockwise  = i%2==0 ? false : true; // clockwise or anticlockwise

        ctx.arc(x, y, radius, startAngle, endAngle, anticlockwise);

        if (i>1){
          ctx.fill();
        } else {
          ctx.stroke();
        }
      }
    }
  }
}
ScreenshotLive sample

Bezier and quadratic curves

The next type of paths available are Bézier curves, available in both cubic and quadratic varieties. These are generally used to draw complex organic shapes.

quadraticCurveTo(cp1x, cp1y, x, y)
Draws a quadratic Bézier curve from the current pen position to the end point specified by x and y, using the control point specified by cp1x and cp1y.
bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)
Draws a cubic Bézier curve from the current pen position to the end point specified by x and y, using the control points specified by (cp1x, cp1y) and (cp2x, cp2y).

The difference between these can best be described using the image on the right. A quadratic Bézier curve has a start and an end point (blue dots) and just one control point (indicated by the red dot) while a cubic Bézier curve uses two control points.

The x and y parameters in both of these methods are the coordinates of the end point. cp1x and cp1y are the coordinates of the first control point, and cp2x and cp2y are the coordinates of the second control point.

Using quadratic and cubic Bézier curves can be quite challenging, because unlike vector drawing software like Adobe Illustrator, we don't have direct visual feedback as to what we're doing. This makes it pretty hard to draw complex shapes. In the following example, we'll be drawing some simple organic shapes, but if you have the time and, most of all, the patience, much more complex shapes can be created.

There's nothing very difficult in these examples. In both cases we see a succession of curves being drawn which finally result in a complete shape.

Quadratic Bezier curves

This example uses multiple quadratic Bézier curves to render a speech balloon.

function draw() {
  var canvas = document.getElementById('canvas');
  if (canvas.getContext) {
    var ctx = canvas.getContext('2d');

    // Quadratric curves example
    ctx.beginPath();
    ctx.moveTo(75,25);
    ctx.quadraticCurveTo(25,25,25,62.5);
    ctx.quadraticCurveTo(25,100,50,100);
    ctx.quadraticCurveTo(50,120,30,125);
    ctx.quadraticCurveTo(60,120,65,100);
    ctx.quadraticCurveTo(125,100,125,62.5);
    ctx.quadraticCurveTo(125,25,75,25);
    ctx.stroke();
  }
}

ScreenshotLive sample

Cubic Bezier curves

This example draws a heart using cubic Bézier curves.

function draw() {
  var canvas = document.getElementById('canvas');
  if (canvas.getContext){
    var ctx = canvas.getContext('2d');

    // Quadratric curves example
    ctx.beginPath();
    ctx.moveTo(75,40);
    ctx.bezierCurveTo(75,37,70,25,50,25);
    ctx.bezierCurveTo(20,25,20,62.5,20,62.5);
    ctx.bezierCurveTo(20,80,40,102,75,120);
    ctx.bezierCurveTo(110,102,130,80,130,62.5);
    ctx.bezierCurveTo(130,62.5,130,25,100,25);
    ctx.bezierCurveTo(85,25,75,37,75,40);
    ctx.fill();
  }
}

ScreenshotLive sample

Rectangles

In addition to the three methods we saw in Drawing rectangles, which draw rectangular shapes directly to the canvas, there's also the rect() method, which adds a rectangular path to a currently open path.

rect(x, y, width, height)
Draws a rectangle whose top-left corner is specified by (x, y) with the specified width and height.

When this method is executed, the moveTo() method is automatically called with the parameters (0,0). In other words, the current pen position is automatically reset to the default coordinates.

Making combinations

So far, each example on this page has used only one type of path function per shape. However, there's no limitation to the number or types of paths you can use to create a shape. So in this final example, let's combine all of the path functions to make a set of very famous game characters.

function draw() {
  var canvas = document.getElementById('canvas');
  if (canvas.getContext){
    var ctx = canvas.getContext('2d');

    roundedRect(ctx,12,12,150,150,15);
    roundedRect(ctx,19,19,150,150,9);
    roundedRect(ctx,53,53,49,33,10);
    roundedRect(ctx,53,119,49,16,6);
    roundedRect(ctx,135,53,49,33,10);
    roundedRect(ctx,135,119,25,49,10);

    ctx.beginPath();
    ctx.arc(37,37,13,Math.PI/7,-Math.PI/7,false);
    ctx.lineTo(31,37);
    ctx.fill();

    for(var i=0;i<8;i++){
      ctx.fillRect(51+i*16,35,4,4);
    }

    for(i=0;i<6;i++){
      ctx.fillRect(115,51+i*16,4,4);
    }

    for(i=0;i<8;i++){
      ctx.fillRect(51+i*16,99,4,4);
    }

    ctx.beginPath();
    ctx.moveTo(83,116);
    ctx.lineTo(83,102);
    ctx.bezierCurveTo(83,94,89,88,97,88);
    ctx.bezierCurveTo(105,88,111,94,111,102);
    ctx.lineTo(111,116);
    ctx.lineTo(106.333,111.333);
    ctx.lineTo(101.666,116);
    ctx.lineTo(97,111.333);
    ctx.lineTo(92.333,116);
    ctx.lineTo(87.666,111.333);
    ctx.lineTo(83,116);
    ctx.fill();

    ctx.fillStyle = "white";
    ctx.beginPath();
    ctx.moveTo(91,96);
    ctx.bezierCurveTo(88,96,87,99,87,101);
    ctx.bezierCurveTo(87,103,88,106,91,106);
    ctx.bezierCurveTo(94,106,95,103,95,101);
    ctx.bezierCurveTo(95,99,94,96,91,96);
    ctx.moveTo(103,96);
    ctx.bezierCurveTo(100,96,99,99,99,101);
    ctx.bezierCurveTo(99,103,100,106,103,106);
    ctx.bezierCurveTo(106,106,107,103,107,101);
    ctx.bezierCurveTo(107,99,106,96,103,96);
    ctx.fill();

    ctx.fillStyle = "black";
    ctx.beginPath();
    ctx.arc(101,102,2,0,Math.PI*2,true);
    ctx.fill();

    ctx.beginPath();
    ctx.arc(89,102,2,0,Math.PI*2,true);
    ctx.fill();
  }
}

// A utility function to draw a rectangle with rounded corners.

function roundedRect(ctx,x,y,width,height,radius){
  ctx.beginPath();
  ctx.moveTo(x,y+radius);
  ctx.lineTo(x,y+height-radius);
  ctx.quadraticCurveTo(x,y+height,x+radius,y+height);
  ctx.lineTo(x+width-radius,y+height);
  ctx.quadraticCurveTo(x+width,y+height,x+width,y+height-radius);
  ctx.lineTo(x+width,y+radius);
  ctx.quadraticCurveTo(x+width,y,x+width-radius,y);
  ctx.lineTo(x+radius,y);
  ctx.quadraticCurveTo(x,y,x,y+radius);
  ctx.stroke();
}

The resulting image looks like this:

We won't go over this in detail, since it's actually surprisingly simple. The most important things to note are the use of the fillStyle property on the drawing context, and the use of a utility function (in this case roundedRect()). Using utility functions for bits of drawing you do often can be very helpful and reduce the amount of code you need, as well as its complexity.

We'll take another look at fillStyle, in more detail, later in this tutorial. Here, all we're doing is using it to change the fill color for paths from the default color of black to white, and then back again.

 

Etiquetas y colaboradores del documento

 Colaboradores en esta página: alkaithil, faqndo, martinzaraterafael, gabriel15, Marezelej
 Última actualización por: alkaithil,