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Drawing text

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Após entender como aplicar estilos e cores no capítulo anterior, nós veremos agora como desenhar texto dentro do contexto de uma canvas.

Desenhando texto

O context de renderização da canvas fornece dois métodos para renderização textual: 

fillText(text, x, y [, maxWidth])
Preenche com um determinado texto as cordenadas (x,y) recebidas. Opcionalmente com uma largura máxima para o desenho.
strokeText(text, x, y [, maxWidth])
Traçeja um determinado texto nas cordenadas (x,y) recebidas. Opcionalmente com uma largura máxima para o desenho.

Um exemplo com fillText

O texto a seguir é rederizado utilizando fillStyle.

function draw() {
  var ctx = document.getElementById('canvas').getContext('2d');
  ctx.font = '48px serif';
  ctx.fillText('Hello world', 10, 50);
}

A strokeText example

The text is filled using the current strokeStyle.

function draw() {
  var ctx = document.getElementById('canvas').getContext('2d');
  ctx.font = '48px serif';
  ctx.strokeText('Hello world', 10, 50);
}

Styling text

In the examples above we are already making use of the font property to make the text a bit larger than the default size. There are some more properties which let you adjust the way the text gets displayed on the canvas:

font = value
The current text style being used when drawing text. This string uses the same syntax as the CSS font property. The default font is 10px sans-serif.
textAlign = value
Text alignment setting. Possible values: start, end, left, right or center. The default value is start.
textBaseline = value
Baseline alignment setting. Possible values: top, hanging, middle, alphabetic, ideographic, bottom. The default value is alphabetic.
direction = value
Directionality. Possible values: ltr, rtl, inherit. The default value is inherit.

These properties might be familiar to you, if you have worked with CSS before.

The following diagram from the WHATWG demonstrates the various baselines supported by the textBaseline property.The top of the em square is
roughly at the top of the glyphs in a font, the hanging baseline is
where some glyphs like आ are anchored, the middle is half-way
between the top of the em square and the bottom of the em square,
the alphabetic baseline is where characters like Á, ÿ,
f, and Ω are anchored, the ideographic baseline is
where glyphs like 私 and 達 are anchored, and the bottom
of the em square is roughly at the bottom of the glyphs in a
font. The top and bottom of the bounding box can be far from these
baselines, due to glyphs extending far outside the em square.

A textBaseline example

Edit the code below and see your changes update live in the canvas:

ctx.font = '48px serif';
ctx.textBaseline = 'hanging';
ctx.strokeText('Hello world', 0, 100);

Advanced text measurements

In the case you need to obtain more details about the text, the following method allows you to measure it.

measureText()
Returns a TextMetrics object containing the width, in pixels, that the specified text will be when drawn in the current text style.

The following code snippet shows how you can measure a text and get its width.

function draw() {
  var ctx = document.getElementById('canvas').getContext('2d');
  var text = ctx.measureText('foo'); // TextMetrics object
  text.width; // 16;
}

Gecko-specific notes

In Gecko (the rendering engine of Firefox, Firefox OS and other Mozilla based applications), some prefixed APIs were implemented in earlier versions to draw text on a canvas. These are now deprecated and removed, and are no longer guaranteed to work.

Etiquetas do documento e colaboradores

 Colaboradores desta página: netoguimaraes
 Última atualização por: netoguimaraes,