如何缩放和居中图像到方形尺寸？

Question

这些问题很相似但没有帮助：this, this, this, and this。

目标是将图像绘制到正方形 canvas 上，同时保留原始纵横比并在原始纵横比不是正方形的情况下将图像居中。

例如，拍摄所附的 1262x2688 图片。下面的代码将其调整为 100x100，但它扭曲了纵横比。

代码应：(1) 缩放图像以适合 100x100 canvas； (2) 保持纵横比； (3) 在 canvas.

范围内垂直和水平居中图像

    // Create canvas element.
    var canvas = $(document.createElement("canvas"));

    // Get canvas context.
    var context = canvas[0].getContext("2d");

    // Set canvas size.
    canvas[0].width = 100;
    canvas[0].height = 100;

    // Write image to canvas.
    context.drawImage(image, 0, 0, newWidth, newHeight);

图片

Answer 1

假设我的计算是正确的，下面的方法可能有用吗？

ratio = image.width/image.height;
if (image.width > image.height) {
    output.height = ( 100 / ratio ) 
    output.width = 100
    output.x = 0
    output.y = (100 - output.height) / 2
} else {
    output.height = 100
    output.width = 100 * ratio
    output.x = (100 - output.width) / 2
    output.y = 0
}

Answer 2

要使图像适合 canvas 同时保留纵横比，请使用以下

const w = image.naturalWidth;
const h = image.naturalHeight;

// Get the min scale to fit the image to the canvas
const scale = Math.min(canvas.width / w, canvas.height / h);

// Set the transform to scale the image, and center to the canvas
ctx.setTransform(scale, 0, 0, scale, canvas.width / 2, canvas.height / 2);

// draw the image offset by half its width and height to center and fit
ctx.drawImage(image, -w / 2, -h / 2, w, h);

// to reset the transform
// ctx.setTransform(1,0,0,1,0,0);

Answer 3

这是我们使用的代码：

    // Create canvas element.
    var canvas = $(document.createElement("canvas"));

    // Get canvas context.
    var context = canvas[0].getContext("2d");

    // Set canvas size.
    canvas[0].width = canvasWidth;
    canvas[0].height = canvasHeight;

    // Set image size, must use image.naturalWidth and image.naturalHeight -- not image.width and image.height.
    const imageWidth = image.naturalWidth;
    const imageHeight = image.naturalHeight;

    // Set scale to fit image to canvas, 
    const scale = Math.min(canvasWidth/imageWidth, canvasHeight/imageHeight);

    // Set new image dimensions.
    const scaledWidth = imageWidth * scale;
    const scaledHeight = imageHeight * scale;

    // Draw image in center of canvas.
    context.drawImage(image, (canvasWidth - scaledWidth)/2, (canvasHeight - scaledHeight)/2, scaledWidth, scaledHeight);

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