HTML5 Canvas progressive filling and transparency

Describes how to set and use parameters for progressive padding in HTML5 Canvas, and how to set and enable transparency in Canvas.

With incremental filling and transparency support to achieve the image's Mask effect.

 

1. Progressive filling (Gradient Fill)

Canvas supports two progressive filling modes: Line Gradient Fill and

Fill the radial gradient (RadialGradient Fill ). The APIs are:

CreateLinearGradient (x1, y1, x2, y2 );

X1 and y1 are the first coordinate, x2 and y2 are the Second coordinate.

CreateRadialGradient (x1, y1, r1, x2, y2, r2 );

X1 and y1 are the coordinates of the first center, r1 is the radius, x2, y2 is the coordinates of the second center, and r2 is the radius.

Set color for each vertex

AddColorStop (position, color );

Position indicates the position. The size range is [0 ~ 1] 0 indicates the first vertex, and 1 indicates the Second coordinate.

Color indicates the Color value, which is the Color value of any CSS.

After the incremental filling object is created and configured, it can be used to set the strokeStyle and fillStyle of context to implement text,

Fill in the progressive color of the geometric shape.

Code demonstration of linear progressive mode:

1. Gradual color in the vertical (Y) Direction

[Javascript]
// Vertical/Y direction
Var lineGradient = ctx. createLinearGradient (50, 0, 50,200 );
LineGradient. addColorStop (0, 'rgba (255, 0, 0, 1 )');
LineGradient. addColorStop (1, 'rgba (255,255, 0, 1 )');
Ctx. fillStyle = lineGradient;
Ctx. fillRect (0, 0,300,300 );

// Vertical/Y direction
Var lineGradient = ctx. createLinearGradient (50, 0, 50,200 );
LineGradient. addColorStop (0, 'rgba (255, 0, 0, 1 )');
LineGradient. addColorStop (1, 'rgba (255,255, 0, 1 )');
Ctx. fillStyle = lineGradient;
Ctx. fillRect (0, 0,300,300 );

2. Horizontal (X) Direction gradual color

[Javascript]
// Horizontal/X direction
Var lineGradient = ctx. createLinearGradient (0, 50,200, 50 );
LineGradient. addColorStop (0, 'rgba (255, 0, 0, 1 )');
LineGradient. addColorStop (1, 'rgba (255,255, 0, 1 )');
Ctx. fillStyle = lineGradient;
Ctx. fillRect (0, 0,300,300 );

// Horizontal/X direction
Var lineGradient = ctx. createLinearGradient (0, 50,200, 50 );
LineGradient. addColorStop (0, 'rgba (255, 0, 0, 1 )');
LineGradient. addColorStop (1, 'rgba (255,255, 0, 1 )');
Ctx. fillStyle = lineGradient;
Ctx. fillRect (0, 0,300,300 );

3. vertical and horizontal (XY direction) colors gradually

[Javascript]
// Vertical and horizontal ction
Var lineGradient = ctx. createLinearGradient (0, 0,200,200 );
LineGradient. addColorStop (0, 'rgba (255, 0, 0, 1 )');
LineGradient. addColorStop (1, 'rgba (255,255, 0, 1 )');
Ctx. fillStyle = lineGradient;
Ctx. fillRect (0, 0,300,300 );

// Vertical and horizontal ction
Var lineGradient = ctx. createLinearGradient (0, 0,200,200 );
LineGradient. addColorStop (0, 'rgba (255, 0, 0, 1 )');
LineGradient. addColorStop (1, 'rgba (255,255, 0, 1 )');
Ctx. fillStyle = lineGradient;
Ctx. fillRect (0, 0,300,300 );

2. Transparency)

The transparency settings in Canvas can be divided into global and local transparency settings.

Context. globalAlpha. You can use fillStyle to set the alpha channel in the color value.

. The Code is as follows:

// Change global alpha value

Ctx. globalAlpha = 0.5;

Ctx. fillRect (50,50, 75,50 );

// Change fill style color's alphachannel

Ctx. fillStyle = 'rgba (225,225,225, 0.5 )';

Ctx. fillRect (50,50, 75,50 );

The two effects are the same.

3. Transparent and progressive photo Mask effect

The translucent mask effect on the image is achieved through radial color gradient and transparency change. The script running effect is as follows:

 

[Javascript]
Var myImage = document. createElement ('img ');
MyImage. src = "../test.png ";
MyImage. onload = function (){
Ctx. drawImage (myImage, 80, 30, myImage. naturalWidth, myImage. naturalHeight );
Var radialGradient = ctx. createRadialGradient (canvas. width/2, canvas. height/2, 10, canvas. width/2, canvas. height/2,200 );
RadialGradient. addColorStop (0, 'rgba (247,247,247, 0 )');
RadialGradient. addColorStop (0.7, 'rgba (120,120,120, 0.5 )');
RadialGradient. addColorStop (0.9, 'rgba (0, 0, 0, 0.8 )');
RadialGradient. addColorStop (1, 'rgba (238,238,238, 1 )');
Ctx. beginPath ();
Ctx. arc (canvas. width/2, canvas. height/2,300, 0, Math. PI * 2, true );
Ctx. closePath ();
Ctx. fillStyle = radialGradient;
Ctx. fill ();
}

Var myImage = document. createElement ('img ');
MyImage. src = "../test.png ";
MyImage. onload = function (){
Ctx. drawImage (myImage, 80, 30, myImage. naturalWidth, myImage. naturalHeight );
Var radialGradient = ctx. createRadialGradient (canvas. width/2, canvas. height/2, 10, canvas. width/2, canvas. height/2,200 );
RadialGradient. addColorStop (0, 'rgba (247,247,247, 0 )');
RadialGradient. addColorStop (0.7, 'rgba (120,120,120, 0.5 )');
RadialGradient. addColorStop (0.9, 'rgba (0, 0, 0, 0.8 )');
RadialGradient. addColorStop (1, 'rgba (238,238,238, 1 )');
Ctx. beginPath ();
Ctx. arc (canvas. width/2, canvas. height/2,300, 0, Math. PI * 2, true );
Ctx. closePath ();
Ctx. fillStyle = radialGradient;
Ctx. fill ();
}