Home > Tutorials > Html Tutorials

Detailed description of the non-zero surround principle when HTML5 Canvas is used to draw irregular images. html5canvas

Source: Internet
Author: User
Developer on Alibaba Coud: Build your first app with APIs, SDKs, and tutorials on the Alibaba Cloud. Read more ＞

Detailed description of the non-zero surround principle when HTML5 Canvas is used to draw irregular images. html5canvas

Path direction and non-zero surround Principle
We usually draw well-organized images. If we use a line bar to draw an abstract work, just like the figure below, how do we use fill () to dye our shoes?

Here we need to use a mathematical method-the non-zero surround principle to determine which region is inside and which region is outside. Next, let's take a look at what is a non-zero surround principle.

First, we have to determine a path for the graph. We only need to "draw a stroke" and "do not take the recurring route ., Indicates a path direction. Let's first assume that the positive direction of the path is 1 (in fact, it can be-1, and the positive and negative directions are opposite to each other, not 0 ), the opposite direction is the opposite number-1.
Then, we take a ray in any direction for any point in the several areas cut by the sub-path. Here I only take the rays in three areas as an example, to determine whether the three regions are "Inside" or "outside ".
Next, let's judge. The ray L1 derived from S1 and the positive direction of the Child path of S1 are intersecting. Then we give the counter + 1 and the result is + 1, which is outside.
The X-ray L2 derived from S2 interacts with the positive direction of the two sub-paths. The counter is + 2 and the result is + 2.
The ray L3 in S3 is intersecting with the two sub-paths, but there is a reverse direction. The counter is + 1-1 and the result is 0. Yes, as long as the result is not 0, the ray is located outside.


Draw a ring
Do you remember the arc method? Its last parameter is to determine the direction of the path. If the two concentric circles with the opposite path are together, what is the magical effect of graph coloring?

The following code is used to implement it.

Copy the content to the clipboard using JavaScript Code
  1. <! DOCTYPE html>
  2. <Html lang = "zh">
  3. <Head>
  4. <Meta charset = "UTF-8">
  5. <Title> ring </title>
  6. <Style>
  7. Body {background: url ("./images/bg3.jpg") repeat ;}
  8. # Canvas {border: 1px solid # aaaaaa; display: block; margin: 50px auto ;}
  9. </Style>
  10. </Head>
  11. <Body>
  12. <Div id = "canvas-warp">
  13. <Canvas id = "canvas">
  14. Does your browser support Canvas ?! Just change one !!
  15. </Canvas>
  16. </Div>
  18. <Script>
  19. Window. onload = function (){
  20. Var canvas = document. getElementById ("canvas ");
  21. Canvas. width = 800;
  22. Canvas. height = 600;
  23. Var context = canvas. getContext ("2d ");
  24. Context. fillStyle = "# FFF ";
  25. Context. fillRect (0, 0, 800,600 );
  27. Context. shadowColor = "#545454 ";
  28. Context. shadowOffsetX = 5;
  29. Context. shadowOffsetY = 5;
  30. Context. shadowBlur = 2;
  32. Context. arc (400,300,200, 0, Math. PI * 2, false );
  33. Context. arc (400,300,230, 0, Math. PI * 2, true );
  34. Context. fillStyle = "#00 AAAA ";
  35. Context. fill ();
  36. };
  37. </Script>
  38. </Body>
  39. </Html>

Running result:

Hollow paper-cut effect
Next, we use the non-zero surround principle and shadow to draw a hollow paper-cut effect.

Copy the content to the clipboard using JavaScript Code
  1. <! DOCTYPE html>
  2. <Html lang = "zh">
  3. <Head>
  4. <Meta charset = "UTF-8">
  5. <Title> hollow-out paper-cutting effect </title>
  6. <Style>
  7. Body {background: url ("./images/bg3.jpg") repeat ;}
  8. # Canvas {border: 1px solid # aaaaaa; display: block; margin: 50px auto ;}
  9. </Style>
  10. </Head>
  11. <Body>
  12. <Div id = "canvas-warp">
  13. <Canvas id = "canvas">
  14. Does your browser support Canvas ?! Just change one !!
  15. </Canvas>
  16. </Div>
  18. <Script>
  19. Window. onload = function (){
  20. Var canvas = document. getElementById ("canvas ");
  21. Canvas. width = 800;
  22. Canvas. height = 600;
  23. Var context = canvas. getContext ("2d ");
  24. Context. fillStyle = "# FFF ";
  25. Context. fillRect (0, 0, 800,600 );
  27. Context. beginPath ();
  28. Context. rect (200,100,400,400 );
  29. Fig (context, 250,150,300,150 );
  30. Fig (context, 345,350,420,450,270,450 );
  31. Context. arc (500,400, 50, 0, Math. PI * 2, true );
  32. Context. closePath ();
  34. Context. fillStyle = "#058 ";
  35. Context. shadowColor = "gray ";
  36. Context. shadowOffsetX = 10;
  37. Context. shadowOffsetY = 10;
  38. Context. shadowBlur = 10;
  39. Context. fill ();
  41. };
  43. // Draw a rectangle counterclockwise
  44. Function drawPathRect (cxt, x, y, w, h ){
  45. /**
  46. * BeginPath and closePath cannot be used here,
  47. * Otherwise, it will not belong to the sub-path but to another new path,
  48. * The non-zero surround principle cannot be used.
  49. */
  50. Cxt. moveTo (x, y );
  51. Cxt. lineTo (x, y + h );
  52. Cxt. lineTo (x + w, y + h );
  53. Cxt. lineTo (x + w, y );
  54. Cxt. lineTo (x, y );
  56. }
  58. // Draw a triangle counterclockwise
  59. Function drawPathTriangle (cxt, x1, y1, x2, y2, x3, y3 ){
  60. Cxt. moveTo (x1, y1 );
  61. Cxt. lineTo (x3, y3 );
  62. Cxt. lineTo (x2, y2 );
  63. Cxt. lineTo (x1, y1 );
  64. }
  66. </Script>
  67. </Body>
  68. </Html>

Running result:

The reason for drawing the rectangle manually here is that we want to obtain the rectangle of the counterclockwise path, and the rect () method provided by the API is drawn as a clockwise rectangle. In addition, you must note that this paper-cut is a graph and a path. You cannot use beginPath () and closePath () in the method of drawing hollow triangles and hollow rectangles. Otherwise, they will be new paths and images, instead of the sub-paths and sub-images of paper-cut, the non-zero surround principle cannot be used.


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.

Related Keywords:
android canvas draw html5 canvas clock html5 canvas map example html5 canvas examples html5 canvas 3d tutorial draw character from description canvas draw circle
Related Article
What is the META tag in HTML? Introduction to META Tag proper...
An article that allows you to read basic HTML rules and Web p...
How does HTML set the background color? HTML background color...
HTML5 new label--nav label Introduction
What are the canvases and SVG in HTML5? What is the differenc...
How do I use the HTML5 feature of the Rel property in the?

Contact Us

The content source of this page is from Internet, which doesn't represent Alibaba Cloud's opinion; products and services mentioned on that page don't have any relationship with Alibaba Cloud. If the content of the page makes you feel confusing, please write us an email, we will handle the problem within 5 days after receiving your email.

If you find any instances of plagiarism from the community, please send an email to: info-contact@alibabacloud.com and provide relevant evidence. A staff member will contact you within 5 working days.

What's Trending
Top 10 Tags
datastax versions naming convention zookeeper client class definition md5 microsoft sql server 2005 data structures exception handling error handling
Top 10 Keywords
microsoft download center down wordpress address url site address url wordpress address url windows installer 4 0 download 302 not found web address url definition site address url wordpress db2 integer mac os installation step by step pdf abbreviation for return

A Free Trial That Lets You Build Big!

Start building with 50+ products and up to 12 months usage for Elastic Compute Service

Get Started for Free

  • Sales Support

    1 on 1 presale consultation

    Chat Contact Sales

  • After-Sales Support

    24/7 Technical Support 6 Free Tickets per Quarter Faster Response

    Open a Ticket

  • Alibaba Cloud offers highly flexible support services tailored to meet your exact needs.

    Learn More