This section of the QtWebKit Guide serves as an introduction to the HTML5 Canvas API features of QtWebKit .
The HTML5 Canvas API enables you to draw within a Web page or Web App using JavaScript. After you define a rectangle that serves as your drawing canvas, you can draw straight and curved lines, simple and complex shapes, graphs, and referenced graphic images. You can add text, color, shadows, gradients, and patterns. The canvas API also enables you to save or export the canvas as a .png or .jpeg image file.
To define the drawing area, set the
width
and
height
的
<canvas>
element. For example, the following sets a drawing area with a height of 100 pixels and width of 200 pixels:
<canvas id="mycanvas" width="100" height="200"></canvas>
默认情况下，
canvas
elements are sized 150 pixels high and 300 pixels wide. You can also set the size of the canvas using CSS:
canvas { height : 200px; width : 100px; }
The
canvas
element is transparent and has no visible borders until you
access the 2D rendering context
.
Resetting the width or height of an existing canvas erases its contents and resets all the context properties of the canvas to their default values.
The rendering
context
defines the methods and attributes needed to draw on the canvas.
QtWebKit
currently supports the two-dimensional rendering context. The following assigns the canvas rendering context to a
context
变量：
var context = canvas.getContext("2d")
The 2d context renders the canvas as a coordinate system whose origin (0,0) is at the top left corner, as shown in the figure below. Coordinates increase along the
x
axis from left to right and along the
y
axis from top to bottom of the canvas.
The 2D rendering context supports rectangles, lines, and arcs, which you can combine to build complex shapes and graphic images.
The rectangle is the only geometric shape that is built in to the canvas API. You can draw an outline of a rectangle, a filled rectangle, and a filled rectangle with clear parts. You do not have to create a path to draw a rectangle.
To draw an outline of a rectangle, use the
strokeRect()
方法。
To draw a filled rectangle, use the
fillRect()
method. The default fill color is black.
To clear part of a filled rectangle, use the
clearRect()
方法。
Each method accepts the following series of arguments:
x
is the position on the canvas to the right of the origin (0,0) of the top left corner of the rectangle
y
is the position on the canvas below the origin of the top left corner of the rectangle
width
is the width of the rectangle to be drawn
height
is the height of the rectangle to be drawn
For example, the following code draws concentric rectangles:
var context = canvas.getContext("2d");
canvas.strokeRect(50,50,50,50);
canvas.fillRect(60,60,30,30);
canvas.clearRect(70,70,10,10);
To draw a line, you first have to
"put your pencil down"
on the canvas by creating a path. The
context.beginPath()
method sets a new path in the canvas. Each line that you draw is stored as a sub-path. Sub-paths can be closed to form a shape, or they can be left open. Each time you want to draw a new shape, you have to call the
beginPath()
method to reset the current path.
After calling
beginPath()
, you set your starting position on the canvas by calling the
context.moveTo(x,y)
方法。
moveTo(x,y)
method creates a new subpath on the canvas that begins at the Cartesian point
(x,y)
.
To draw a straight line, call the
context.lineTo(x,y)
method. This adds the point (x,y) to the current subpath and connects it to the previous subpath by a straight line. In other words, (x,y) are the coordinates of the line's endpoint. For example:
context.beginPath();
context.moveTo(10,10);
context.lineTo(30,30);
To get the
pencil
to actually draw on the canvas, first use the
strokeStyle
property to set the color to a value such as black (
#000
):
context.strokeStyle(#000);
(
strokeStyle
property can be a CSS color, a pattern, or a gradient.) Then use the
context.stroke()
method to actually draw the line on the canvas:
context.stroke();
This produces the image below. The numeric coordinates are added for clarity but are not part of the image drawn by the code:
To create a shape, call the
context.closePath()
方法：
context.closePath();
context.moveTo(10,10); // starting point
context.lineTo(30,30); // specify first line
context.moveTo(30,30); // move to end of first line
context.lineTo(60,10); // specify second line
context.moveTo(60,10); // move to end of second line
context.lineTo(10,10); // specify third line to close triangle
context.strokeStyle("#000"); // use black color for lines
context.stroke(); // draws the triangle lines on the canvas
To fill the shape, use the
fillstyle
特性和
fill()
方法：
context.fillStyle("#FF0000"); // use red color for fill
context.fill(); // fill the triangle
The commands, if coded fully, would create the shape below:
注意：
It is not necessary to close the path when calling the
fill()
method. Calling
fill()
closes the path and creates the completed shape.
You can draw lines of various widths, endcap types, and joining options by configuring the following attributes:
lineWidth
sets the thickness of the current line. The value can be any number greater than
0
。例如，
context.lineWidth = 10
sets the line thickness to
10
units. The default value is
1
unit, which is not the same as
1
pixel
. Instead, the line is centered on the current path, with half its thickness on each side of the path.
lineCap
sets the type of endpoint of the current line. The value can be either
butt
,
square
，或
round
. (The default value is
butt
)。
butt
- the ends of the line abutt the line guide.
square
adds a box at both ends of the line.
round
adds a semicircle at both ends of the line.
lineJoin
sets the style with which lines are joined. The value can be either
bevel
,
round
，或
miter
. (The default value is
miter
)。
bevel
flattens the corners at which the lines join
round
rounds the corners at which the lines join
miter
joins the lines at a single point
miterLimit
设置
miter limit ratio
. The value can be any number greater than
0
. The miter limit ratio determines how far the connection point of the outside of the lines can be from the connection point of the inside of the lines. (The default value is
10
)。
To draw an arc, you begin with the same steps your followed to create a line:
context.beginPath()
method to
"put your pencil down"
on the canvas and set a new path.
context.moveTo(x,y)
method to set your starting position on the canvas at the point (x,y).
context.arcTo(x1,y1,x2,y2,radius)
method. This adds an arc with starting point
(x1,y1)
, ending point
(x2,y2)
, and radius
radius
to the current subpath and connects it to the previous subpath by a straight line.
context.arc(x,y,radius,startAngle,endAngle,anticlockwise)
method. This adds an arc to the current subpath that lies on the circumference of the circle whose center is at the point (x,y) and whose radius is
radius
.
Both
startAngle
and
endAngle
are measured from the x axis in units of radians.
A complete circle is
360
degrees, or 2Π radians. A semicircle is
180
degrees, or Π radians. The number of radians is the number of degrees multiplied by Π/180, expressed in JavaScript as:
var radians = (Math.PI/180)*degrees;
anticlockwise
has the value
TRUE
for each arc in the figure above because they are all drawn in the counterclockwise direction.
context.closePath()
method. This marks the current subpath as closed and draws a straight line from the current point to the first point in the path.
context.stroke()
方法。
context.fill()
方法。
strokeStyle
. For example, the code
context.strokeStyle = "#FF0000";
sets the fill color to red.
注意：
It is not necessary to close the path if you are going to call the
fill()
method. The fill closes the path and creates the completed shape.
To create complex shapes, combine lines and arcs:
context.beginPath()
method to
"put your pencil down"
on the canvas and set a new path.
context.moveTo(x,y)
method to set your starting position on the canvas at the point (x,y).
lineTo
,
arcTo
,
arc
,
moveTo
,
closePath
,
stroke
，和
fill
methods and setting the line attributes and fill colors as described above.
You can also create complex shapes by removing portions of the shapes you draw. The
clip()
method creates a clipping path that defines the area along which your "scissor" will cut. Any parts of the shape outside the clipping path are not displayed. To create a complex shape using the
clip()
方法：
context.beginPath()
method to set the clipping path.
lineTo
,
arcTo
,
arc
,
moveTo
，和
closePath
方法。
context.clip()
方法。
The new shape displays. The following shows how a clipping path can modify how an image displays:
You can build complex shapes by drawing shapes on top of each other. It is also possible to draw shapes behind existing shapes and to mask parts of shapes by using
compositing operations
。
globalCompositeOperation
attribute sets the way shapes can be combined.
The first shape drawn on the canvas to which additional shapes are added is called the
destination
shape. The shape drawn on the canvas afterwards to create the composite image is called the
source
shape. The value of the
globalCompositeOperation
attribute must be set to one of the following:
source-over
displays the source (newer) shape over the destination (older) shape unless the source shape is transparent. (This is the default value)
source-in
displays only the portion of the source shape that is opaque and overlaps the destination shape. Everything else is transparent.
source-out
displays only the portion of the source shape that does not overlap the destination shape.
source-atop
displays only the portion of the opaque source shape that overlaps the destination shape and the portion of the destination shape that is not covered by the opaque source shape.
destination-over
displays the destination shape over the source shape. In areas where both shapes are opaque and overlap, the older shape displays.
destination-in
displays only the portion of the destination shape that is opaque and overlaps the source shape. Everything else is transparent. The source (newer) shape is not visible.
destination-out
displays only the portion of the destination shape that does not overlap the source shape. The source shape is not visible.
destination-atop
displays only the portion of the opaque destination shape that overlaps the source shape and the portion of the source shape that is not covered by the opaque destination shape.
lighter
displays both the source and destination shapes. Where the shapes overlap, the their color values are added, producing a lighter color.
copy
displays only the source shape. The destination shape is ignored.
xor
displays both the source and the destination shapes except the areas of overlap, in which both shapes are completely transparent.
The following figure shows the various compositing effects:
You can save or export your canvas drawings as .png or .jpeg image files by calling the
toDataURL()
方法：
canvas.toDataURL([type, ...])
where:
type
is the MIME type to which you want to save or export your canvas. Possible values are:
"image\png"
(Default value)
"image\jpeg"
...
represents additional arguments that depend on the MIME type.
type
is
png
, this argument is
" "
type
is
jpeg
, this argument is the desired quality level of the image. The value is a number in the range 0.0 to 1.0, inclusive.
You can draw text on your canvas by setting the following font attributes on the 2d drawing context:
font
refers to any font, expressed the same way as in CSS properties. This attribute's value can include any font style, variant, weight, size, height, and family. For example:
context.font = "12pt Arial";
默认值为
10px sans-serif
.
If you set the
font
attribute to a relative font size, the browser multiplies it by the computed font size of the
<canvas>
element itself. For example:
context.font = "200%";
textAlign
specifies the alignment of the text. The values can be one of the following:
left
for left-aligned text
right
for right-aligned text
center
for text that is centered within each line
start
(default) - the text is aligned at the beginning of the line. Text is left- or right-justified based on locale-specific writing method: left when text is left-to-right, right when text is right-to-left.
end
- the text is aligned at the end of the line, either left or right depending on locale-specific writing method.
textBaseline
specifies the position at which text is drawn relative to a baseline. The figure below, from
HTML5 Canvas API
, illustrates the possible values for the
textBaseline
属性：
top
is the top of the em square, which approximates the top of the glyphs in a font
hanging
specifies a hanging baseline, where the tops of some glyphs are anchored.
middle
is the mid-point of the em square
alphabetic
(default) is the anchor point of many alphabetic characters
ideographic
is the anchor point of many ideograms, such as the characters used in the writing systems of many Asian languages
bottom
is the bottom of the em square
To draw text on a canvas:
font
attribute on the drawing context. For example:
context.font = "bold 11px arial"
measureText
方法：
TextMetrics measureText("Text to draw");
where
TextMetrics
is the object returned. Its
width
attribute is the width, in pixels, that the
"Text to draw"
would be when drawn with the font specified by the
font
属性。
fillText
draws the text with the font style specified by the
font
attribute, the alignment specified by the
textAlign
attribute, and the baseline specified by the
textBaseline
attribute. For example:
context.fillText("Text to draw",x,y,maximumWidth);
where
x
and
y
are the coordinates at which the drawing begins (the anchor point), and
maximumWidth
is the maximum width of the text string (optional). If the
width
returned in step 2 is larger than the
maximumWidth
, the font is scaled down until the width of the text string is less than the
maximumWidth
指定。
If you don't specify the
font
attribute, the text inherits the font size and style of the
<canvas>
element itself.
strokeText
is the same as the
fillText
method, except that a stroke style is applied to the text instead of a fill style, creating outlines of glyphs. For example:
context.fillText("Text to stroke",x,y,maximumWidth);
You can insert existing images onto your canvas, you can scale or crop them, and you can combine them to create composite images. You can also draw new images by creating an
Image()
object with JavaScript.
To insert an existing image onto a canvas, call the
drawImage
方法：
context.drawImage(image, dx, dy, dw, dh)
where:
image
is a reference to an HTML
<image>
or
<canvas>
element. The image must be fully loaded before you can draw it on the canvas. The reference cannot be a URL. Instead, it should be referenced using standard DOM methods such as
document.images()
or
document.getElementById()
。例如：
<canvas id="demo1" width="100" height="150"></canvas> var canvas = document . getElementById( "demo1" ); var context = canvas . getContext( "2d" );
dx
is the x coordinate of the upper left corner of the image to be drawn on the canvas (the destination image)
dy
is the y coordinate of the upper left corner of the destination image
dw
is the width of the destination image (optional)
dh
is the height of the destination image (optional)
若
dw
and
dh
are not specified, the image retains its source dimensions when drawn on the canvas. When
dw
and
dh
are specified, the image is scaled to width
dw
和高度
dh
when drawn on the canvas.
If you want to crop the source image, the
drawImage
method can be overloaded with the following arguments:
context.drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)
where:
sx
is the x coordinate of the upper left corner of the cropped source image
sy
is the y coordinate of the upper left corner of the cropped source image
sw
is the width of the cropped source image
sh
is the height of the cropped source image
Use this method if you want to crop the source image to the rectangle (sx, sy, sw, sh) before drawing it on the canvas. The destination image will have width
dw
, height
dh
, and upper left corner at coordinates
(dx,dy)
on the canvas.
To create a new image using JavaScript, create an
图像
object and define its source. Use an
onload
event handler to ensure that the
drawImage
method is not called until the image has finished loading. For example:
var graphic = new Image(); graphic.src = "clipart/graphic.png";
The image begins to load.
graphic.onload = function(){ context.drawImage(graphic,x,y); };
You can create patterns with an image by repeating it horizontally, vertically, or both. The top left corner of the first image must be anchored at the origin of the coordinate space. To repeat an image, call the
createPattern
方法：
context.createPattern(image, repetition);
where:
image
is a reference to an HTML
<image>
or
<canvas>
element that is repeated to form a pattern. The image must be fully loaded before you can draw it on the canvas. The reference cannot be a URL. Instead, it should be referenced via standard DOM methods such as
document.images
and
document.getElementById
。例如：
<canvas id="demo1" width="100" height="150"></canvas>
var canvas
=
document
.
getElementById(
"demo1"
);
var context
=
canvas
.
getContext(
"2d"
);
repetition
is the direction in which the image repeats to form the pattern. Possible values are:
repeat
(default) the image repeats both horizontally and vertically
repeat-x
the image repeats horizontally
repeat-y
the image repeats vertically
The repeated images are the same size as the source image. The
createPattern
method does not scale the images.
For example, to create a horizontal pattern of roses, create an
图像
object to use as a pattern and define its source. Use an
onload
event handler to ensure that the
createPattern
method is not called until the image has finished loading. For example:
var roses = new Image();
roses.src = "clipart/roses.jpg";
The image begins to load.
roses.onload = function(){
var pattern = context.createPattern(roses,repeat-x);
};
To draw the outline of a shape in color, set the
strokeStyle
attribute to any valid
CSS color value
. The color value can be in hexadecimal notation or in RGB/HSL notation, as described in
Specifying Color and Opacity
. For example, either of the following sets a shape's outline to red:
context.strokeStyle = "#FF0000" context.strokeStyle = "rgb(255,0,0)"
To fill a shape with color, set the
fillStyle
attribute to a l{CSS Color Value}{CSS color value}. The color value can be in hexadecimal notation or in RGB/HSL notation. For example, either of the following colors a shape's interior as blue:
context.fillStyle = "#0000FF" context.fillStyle = "rgb(0,0,255)"
The CSS3 Color Module specification extends both RGB and HSL color models to include a color's opacity, referred to as its alpha . These extended models are known as RGBA and HSLA. There are no hexadecimal notations for RGBA and HSLA values. The following specifies varying levels of opacity for a blue shape:
context.fillStyle = rgba(0, 0, 255, 0) // transparent context.fillStyle = rgba(0, 0, 255, 0.5) // semi-transparent context.fillStyle = rgba(0, 0, 255, 1) // opaque
When you set the
context.strokeStyle
or
context.fillStyle
attributes, whatever value you set becomes the default value for all subsequently drawn shapes, until you set a new value.
A gradient is a smooth transition between colors. There are two types of gradients: linear and radial.
A linear gradient transitions the color along a line between two points. To create a linear gradient, call the
createLinearGradient
方法：
createLinearGradient(x0, y0, x1, y1)
where
(x0, y0)
is the starting point and
(x1, y1)
is the ending point for the linear gradient.
A radial gradient transitions the color along a cone between two circles. To create a radial gradient, call the
createRadialGradient
方法：
createRadialGradient(x0, y0, r0, x1, y1, r1)
where:
(x0, y0, r0)
represents the starting circle, whose origin is
(x0, y0)
and whose radius is
r0
.
(x1, y1, r1)
represents the ending circle, whose origin is
(x1, y1)
and whose radius is
r1
.
Gradients must have two or more
color stops
, representing color shifts positioned from
0
to
1
between to the gradient's starting and end points or circles:
addColorStop(position,color)
where:
位置
specifies the position of the color within the already defined starting and end points or circles, expressed as a number from
0
to
1
.
color
specifies the CSS color at that position.
For example, to define a gradient that varies from red to blue horizontally along a rectangular area:
var redbluegradient = context.createLinearGradient(0,0,100,0);
redbluegradient.addColorStop(0, "rgb(255,0,0)"); // red at the left side of the rectangle
redbluegradient.addColorStop(1, "rgb(0,0,255)"); // blue at the right side of the rectangle
fillStyle
or
strokeStyle
:
context.fillStyle = redbluegradient;
context
.
fillRect(
0
,
0
,
100
,
150
);
To define a gradient that varies from red to blue vertically along a rectangle:
var redbluegradient = context.createLinearGradient(0,0,0,150);
redbluegradient.addColorStop(0, "rgb(255,0,0)"); // red at the top of the rectangle
redbluegradient.addColorStop(1, "rgb(0,0,255)"); // blue at the bottom of the rectangle
fillStyle
or
strokeStyle
:
context.fillStyle = redbluegradient;
context
.
fillRect(
0
,
0
,
100
,
150
);
注意：
A canvas gradient's color stops behave slightly differently than those used within non-canvas
gradients
. Webkit gradients specify mandatory
from
and
to
colors, with optional
color-stop
values for additional color shifts within the overall range of the gradient. For canvas gradients, even the initial and final colors are defined as color stops.
To add a shadow effect to a drawing on a canvas, set the following attributes:
shadowColor
sets the color of the shadow. The value can be any CSS color value. The default value is transparent black (
"rgba(0,0,0,0)"
).
shadowBlur
sets the amount of blur in the shadow, in pixels. The value can be any positive number or 0. A value of 0 produces a sharp shadow with no blur.
shadowOffsetX
sets the number of pixels the shadow extends horizontally from the object drawn. If this value is a positive number, the shadow extends to the right of the object. If negative, the shadow extends to the left of the object. The default value is 0 pixels.
shadowOffsetY
sets the number of pixels the shadow extends vertically from the object drawn. If this value is a positive number, the shadow extends below the object. If negative, the shadow extends above the object. The default value is 0 pixels.
The following example code adds a semi-transparent black shadow to the bottom right of a blue rectangle:
var context = canvas.getContext("2d");
context.shadowOffsetX = 5;
context.shadowOffsetY = 5;
context.shadowBlur = 10;
context.shadowColor = "rgba(0,0,0,0.5)";
context.fillStyle = "#0000FF";
context.fillRect = (0,0,100,50)
When drawing shapes and paths, you can translate the canvas's origin, rotate the canvas around the origin, scale the units in the canvas grid, and modify the transformation matrix directly.
Translating the origin enables you to draw patterns of different objects on the canvas without having to measure the coordinates manually for each shape. To translate the origin of the canvas, use the
translate
方法：
context.translate(x,y);
where:
x
is the horizontal distance that the origin is translated, in coordinate space units
y
is the vertical distance that the origin is translated, in coordinate space units
To rotate the canvas around the current origin, call the
rotate()
方法：
context.rotate(angle);
where
angle
is the clockwise rotation angle in radians. The number of radians is the number of degrees multiplied by Π/180, expressed in JavaScript as:
var radians = (Math.PI/180)*degrees;
To increase or decrease the size of each unit in the canvas grid, call the
scale
方法：
context.scale(x,y);
where:
x
is the scale factor in the horizontal direction
y
is the scale factor in the vertical direction
The scale factors are in multiples. For example,
scale(2.0, 0.5)
would double the horizontal size of an object drawn on the canvas and half its vertical size, as shown below:
Modifying the transformation matrix directly enables you to perform scaling, rotating, and translating transformations in a single step.
The transformation matrix is an affine transformation matrix from linear algebra. Affine transformations preserve colinearity and relative distance in the transformed coordinate space. This means that points in a line remain in a line, parallel lines remain parallel, and the distance between lines and objects maintains the same ratio, even if a scale factor is applied. Repositioning by translation, rotation, or skewing is also possible.
Each point on the canvas is multiplied by the matrix before anything is drawn. The HTML5 Canvas API defines the transformation matrix as:
where:
a
is the scale factor in the horizontal (x) direction
c
is the skew factor in the x direction
e
is the translation in the x direction
b
is the skew factor in the y (vertical) direction
d
is the scale factor in the y direction
f
is the translation in the y direction
The scale factors and skew factors are multiples;
e
and
f
are coordinate space units, just like the units in the
translate(x,y)
方法。
The rotation transformation matrix is as follows:
在哪里
angle
of rotation is in radians.
See Also: senocular.com for a good explanation of how transformation matrices are used identically within Adobe Flash.
You can animate a canvas drawing by repeatedly redrawing the canvas for each frame and translating, rotating, skewing, and scaling the drawn objects.
To draw each frame by employing the HTML5 canvas API, you should define the original canvas state and save it for future reference. The drawing context maintains a stack of drawing states. Each state consists of the current transformation matrix, current clipping region, and current values of the following attributes:
strokeStyle
fillStyle
globalAlpha
lineWidth
lineCap
lineJoin
miterLimit
shadowOffsetX
shadowOffsetY
shadowBlur
shadowColor
globalCompositeOperation
font
textAlign
textBaseline
The current path and the current bitmap are NOT part of the drawing state. The path can be reset only by invoking the
beginPath()
method. The current bitmap is a property of the canvas, not of the context.
To save the original canvas state, call the
save()
方法：
context.save();
Before drawing each new frame, you must clear the canvas:
canvas.clearRect(x,y,width,height);
where:
x
is the position of the top left corner of the canvas on the horizontal axis
y
is the position of the top left corner of the canvas on the vertical axis
width
is the width of the canvas
height
is the height of the canvas
Draw the new frame using any of the methods provided by the canvas API. Then save it by calling the
save()
方法。
If you wish to return to the state of the original frame as the basis for each new frame that you draw, call the
context.restore()
方法。
To execute the drawing methods repeatedly, use the standard JavaScript-based animation technique, calling the
setInterval()
and
clearInterval()
methods. The following shows how to execute an animation function every
50
milliseconds (corresponding to 20 times per second, a typical animation frame rate), then subsequently halt the animation:
var id = setInterval(functionName, 50); clearInterval(id);
See Also: