html：canvas画布绘图简单入门-2

分辨率和元素尺寸

<canvas 
  width="600" 
  height="700" 
  style="width:800px; height:900px">
</canvas>

内容分辨率： 600 * 700

元素尺寸： 800 * 900

示例6：将元素截取后返回画布

像素操作

getImageData()

createImageData()

putImageData()

<canvas id="canvas"
            width="600"
            height="600"></canvas>

<script>
    let canvas = document.querySelector('#canvas');
    let ctx = canvas.getContext('2d');

    ctx.fillStyle = "red";
    ctx.fillRect(0, 0, 50, 50);

    let imageData = ctx.getImageData(0, 0, 50, 50);
    console.log(imageData);

    // 转换元素的颜色数组 RGBA  [红色, 绿色, 蓝色, alpha通道]
    // [255, 0, 0, 255]  => [0, 255, 255, 255]
    for (let i = 0; i< imageData.data.length; i += 4) {
        imageData.data[i] = 255 - imageData.data[i];
        imageData.data[i + 1] = 255 - imageData.data[i + 1];
        imageData.data[i + 2] = 255 - imageData.data[i + 2];
        imageData.data[i + 3] = 255;
    }
    console.log(imageData);

    ctx.putImageData(imageData, 100, 0);
</script>

示例7：移动坐标原点

画布坐标原点左上角(0,0)

<canvas id="canvas"
            width="600"
            height="600"></canvas>

    <script>
        let canvas = document.querySelector('#canvas');
        let ctx = canvas.getContext('2d');

        ctx.fillStyle = "red";
        ctx.fillRect(0, 0, 50, 50);

        // 移动原点坐标(0, 0) => (100, 0)
        ctx.translate(100, 0);
        ctx.fillStyle = "green";
        ctx.fillRect(0, 0, 50, 50);
    </script>

示例8：旋转坐标轴

<canvas id="canvas"
            width="600"
            height="600"></canvas>

    <script>
        let canvas = document.querySelector('#canvas');
        let ctx = canvas.getContext('2d');

        // 将坐标轴顺时针旋转45角
        ctx.rotate(Math.PI / 180 * 45);

        ctx.fillStyle = "red";
        ctx.fillRect(25, -25, 50, 50);
    </script>

示例9：缩放坐标轴

<canvas id="canvas"
            width="600"
            height="600"></canvas>

    <script>
        let canvas = document.querySelector('#canvas');
        let ctx = canvas.getContext('2d');

        ctx.fillStyle = "red";
        ctx.fillRect(0, 0, 50, 50);

        // 保存画笔状态
        ctx.save()
        // 坐标轴x, y都放大两倍
        ctx.scale(2, 2);
        ctx.fillStyle = "green";
        ctx.fillRect(50, 0, 50, 50);

        // 恢复画笔状态
        ctx.restore()
        ctx.fillRect(250, 0, 50, 50);
    </script>