How to specify the color and transparency when drawing HTML5Canvas _ html5 tutorial tips-

This article describes how to specify the color and transparency when creating HTML5Canvas, including the introduction of the Global Transparency globalAlpha attribute. For more information, see Color

Black is the default color drawn by Canvas. To change the color, you must specify the color before the actual painting.

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1. Ctx. strokeStyle = color

Specify the line color:

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1. Ctx. fillStyle = color

Specify the fill color:
Let's take a look at the actual example:

JavaScript

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1. Onload = function (){
2. Draw ();
3. };
4. Function draw (){
5. Var canvas = document. getElementById ('c1 ');
6. If (! Canvas |! Canvas. getContext) {return false ;}
7. Var ctx = canvas. getContext ('2d ');
8. Ctx. beginPath ();
9. Ctx. fillStyle = 'rgb (192, 80, 77) '; // red
10. Ctx. arc (70, 45, 35, 0, Math. PI * 2, false );
11. Ctx. fill ();
12. Ctx. beginPath ();
13. Ctx. fillStyle = 'rgb (155,187, 89) '; // green
14. Ctx. arc (45, 95, 35, 0, Math. PI * 2, false );
15. Ctx. fill ();
16. Ctx. beginPath ();
17. Ctx. fillStyle = 'rgb (128,100,162) '; // purple
18. Ctx. arc (95, 95, 35, 0, Math. PI * 2, false );
19. Ctx. fill ();
20. }

The effect is as follows:

Specify transparency

Like in common CSS, we can also include an alpha value when specifying the color (but not much, which is not supported before IE9 ). Check the Code:

JavaScript

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1. Onload = function (){
2. Draw ();
3. };
4. Function draw (){
5. Var canvas = document. getElementById ('c1 ');
6. If (! Canvas |! Canvas. getContext) {return false ;}
7. Var ctx = canvas. getContext ('2d ');
8. Ctx. beginPath ();
9. Ctx. fillStyle = 'rgba (192, 80, 77, 0.7 )';//
10. Ctx. arc (70, 45, 35, 0, Math. PI * 2, false );
11. Ctx. fill ();
12. Ctx. beginPath ();
13. Ctx. fillStyle = 'rgba (155,187, 89, 0.7 )';//
14. Ctx. arc (45, 95, 35, 0, Math. PI * 2, false );
15. Ctx. fill ();
16. Ctx. beginPath ();
17. Ctx. fillStyle = 'rgba (128,100,162, 0.7 )';//
18. Ctx. arc (95, 95, 35, 0, Math. PI * 2, false );
19. Ctx. fill ();
20. }

The result is as follows:

The above code is basically unchanged, that is, the rgb (r, g, B) is changed to rgba (r, g, B, a), and the value of a is also 0 ~ 1. 0 indicates completely transparent, and 1 indicates completely opaque (so the alpha value is actually "opacity ").

Global Transparency globalAlpha
This is also a very simple property. The default value is 1.0, which indicates that it is completely opaque. The value range is 0.0 (completely transparent )~ 1.0. This attribute is the same as the shadow setting. If you do not want to Set opacity for the global image, you must reset globalAlpha before the next painting.

To sum up, what are the status-based attributes?

-- GlobalAlpha

-- GlobalCompositeOpeartion

-- StrokeStyle

-- TextAlign, textBaseline

-- LineCap, lineJoin, lineWidth, miterLimit

-- FillStyle

-- Font

-- ShadowBlur, shadowColor, shadowOffsetX, and shadowOffsetY
Let's use a code to experience the magic of globalAlpha ~

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5. Global Transparency
11. Does your browser support Canvas ?! Just change one !!
15. Script
16. Window. onload = function (){
17. Var canvas = document. getElementById ("canvas ");
18. Canvas. width = 800;
19. Canvas. height = 600;
20. Var context = canvas. getContext ("2d ");
21. Context. fillStyle = "# FFF ";
22. Context. fillRect (0, 0, 800,600 );
24. Context. globalAlpha = 0.5;
26. For (var I = 0; I <= 50; I ++ ){
27. Var R = Math. floor (Math. random () * 255 );
28. Var G = Math. floor (Math. random () * 255 );
29. Var B = Math. floor (Math. random () * 255 );
31. Context. fillStyle = "rgb (" + R + "," + G + "," + B + ")";
33. Context. beginPath ();
34. Context. arc (Math. random () * canvas. width, Math. random () * canvas. height, Math. random () * 100, 0, Math. PI * 2 );
35. Context. fill ();
36. }
37. };
38. Script

Running result:

Is it really cool? Finally, let's look at the artist's fan.