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Quick Start Guide for Drawing Images by calling the Canvas API of HTML5, html5canvas

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Quick Start Guide for Drawing Images by calling the Canvas API of HTML5, html5canvas

1. Canvas Element
The following html code defines a canvas element.

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  1. <! DOCTYPE html>
  2. <Html>
  3. <Head>
  4. <Title> Canvas Quick Start </title>
  5. <Meta charset = "UTF-8"/>
  6. </Head>
  7. <Body>
  8. <Div>
  9. <Canvas id = "mainCanvas" width = "640" height = "480"> </canvas>
  10. </Div>
  11. </Body>
  12. </Html>

Use the following Javascript statement to access the canvas element:

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  1. // DOM writing
  2. Window. onload = function (){
  3. Var canvas = document. getElementById ("mainCanvas ");
  4. Var context = canvas. getContext ("2d ");
  5. };
  6. // JQuery Writing Method
  7. $ (Document). ready (function (){
  8. Var canvas =$ ("# mainCanvas ");
  9. Var context = canvas. get (0). getContext ("2d ");
  10. });
  11. // You can call the context method to call the drawing API.

2. Basic API
2.1 Coordinate System
The Canvas 2D rendering context uses the Cartesian coordinate system of the plane. The origin is (0, 0) in the upper left corner. one unit of the coordinate system is equivalent to one pixel on the screen. For example:

2.2 draw basic Images
2.2.1 rectangle

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  1. // Draw a filled rectangle
  2. Context. fillRect (x, y, width, height)
  3. // Draw a border rectangle
  4. Context. strokeRect (x, y, width, height)
  5. // Clear a rectangle
  6. Context. clearRect (x, y, width, height)

2.2.2 line
There are some differences between drawing a line and drawing a graph. A line is actually called a path. To draw a simple path, you must first call the beginPath method, then call moveTo to set the start coordinate of the path, and then call lineTo to set the end coordinate of the line segment (which can be set multiple times ), call closePath to complete the path painting. Finally, call stroke to draw the outline (or call the fill path ). The following is an example:

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  1. // Example
  2. Context. beginPath (); // start path
  3. Context. moveTo (40, 40); // move to a point (40, 40)
  4. Context. lineTo (300, 40); // draw a line to a point (, 30)
  5. Context. lineTo (40,300); // draw a line to point (40,300)
  6. Context. closePath (); // end path
  7. Context. stroke (); // draw the outline
  8. // Or fill in with context. fill ();

2.2.3 circle
Canvas does not actually have a method to draw a circle. You can draw an arc to simulate a circle. Because an arc is a path, the API for drawing an arc should be included between beginPath and closePath.
2.3 Style
2.3.1 modify the line color

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  1. Var color;
  2. // Specify the RGB Value
  3. Color = "rgb (255, 0, 0 )";
  4. // Specify the RGBA value (the last parameter is the alpha value; value range: 0.0 ~ 1.0)
  5. Color = "rgba (255, 0, 0, 1 )";
  6. // Specify the hexadecimal code
  7. Color = "# FF0000 ";
  8. // Specify with words
  9. Color = "red ";
  10. // Set the fill color
  11. Context. fillStyle = color;
  12. // Set the border color
  13. Context. strokeStyle = color;

2.3.2 modify the line width

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  1. // Specify the line width.
  2. Var value = 3;
  3. // Set the border color
  4. Context. linewidth = value;

2.4 draw text

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  1. // Specify the Font Style
  2. Context. font = "italic 30px ";
  3. // Draw text at a vertex (40, 40)
  4. Context. fillText ("Hello world! ", 40, 40 );

2.5 draw images
Before creating an image, you must define the image and load it.

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  1. Var img = new Image ();
  2. Img. src = "myImage.png ";
  3. Img. onload = function (){
  4. // Image Loading completed
  5. };

The following is an explanation of the drawImage API:

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  1. // Draw an image in (x, y)
  2. Context. drawImage (image, x, y)
  3. // In (x, y), the image that draws the width * height
  4. Context. drawImage (image, x, y, width, height)
  5. // Capture the sWidth * sHeight image at image (sx, sy), and draw the dWidth * dHeight image at (dx, dy ).
  6. Context. drawImage (image, sx, sy, sWidth, sHeight, dx, dy, dWidth, dHeight)

3. Advanced functions
3.1 fill the Canvas with the browser window
The simplest way is to precisely set the width and height of the canvas element to the width and height of the browser window, and eliminate white gaps with CSS.
CSS code:

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  1. *{
  2. Margin: 0;
  3. Padding: 0;
  4. }
  6. Html, body {
  7. Height: 100%;
  8. Width: 100%;
  9. }
  11. Canvas {
  12. Display: block;
  13. }

Javascript code:

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  1. Function resizeCanvas (){
  2. // Canvas is obtained by jQuery.
  3. Canvas. attr ("width", $ (window). get (0). innerWidth );
  4. Canvas. attr ("height", $ (window). get (0). innerHeight );
  5. Context. fillRect (0, 0, canvas. width (), canvas. height ());
  6. }
  7. $ (Window). resize (resizeCanvas );
  8. ResizeCanvas ();

3.2 drawing status
In a canvas, a drawing refers to the complete set of attributes that describe the 2D rendering context appearance at a certain time point, including: transform matrix, crop area, globalAlpha, struct, strokeStyle, fillStyle, lineWidth, lineCap, lineJoin, miterLimit, shadowOffsetX, shadowOffsetY, shadowBlur, shadowColor, font, textAlign, and textBaseline.
To change the global status of the canvas, save the current status first-call the save method to push the status to the drawing state stack). After the operation, call the restore method to restore the drawing status.

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  1. // Example
  2. Context. save ();
  3. Context. globalAlpha = 0.5;
  4. Context. fillRect (0, 0,200,100 );
  5. Context. restore ();

Deformation 3.3
3.3.1 Translation
Moves the origin of the 2D rendering context from one position to another. Note that the coordinates are moved to the global drawing position. The API is as follows:

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  1. // Move the coordinate origin to (x, y)
  2. Context. translate (x, y)

3.3.2 Scaling

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  1. // Scale the global horizontal and vertical dimensions to x and y (that is, multiply the multiplication factor by the original value)
  2. Context. scale (x, y)

3.3.3 Rotation

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  1. // Rotate the radius radians of the canvas around the origin
  2. Context. rotate (radius)

This article is an English version of an article which is originally in the Chinese language on aliyun.com and is provided for information purposes only. This website makes no representation or warranty of any kind, either expressed or implied, as to the accuracy, completeness ownership or reliability of the article or any translations thereof. If you have any concerns or complaints relating to the article, please send an email, providing a detailed description of the concern or complaint, to info-contact@alibabacloud.com. A staff member will contact you within 5 working days. Once verified, infringing content will be removed immediately.

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