如果您需要获取矩形区域的平均颜色,而不是单个像素的颜色,请参考以下问题:
[JavaScript - 从图像的特定区域获取平均颜色](link1)
无论如何,两者的实现方式非常相似:
从图像或画布中获取单个像素的颜色/值
要获取单个像素的颜色,您首先需要将该图像绘制到画布上,这一步您已经完成了:
const image = document.getElementById('image');
const canvas = document.createElement('canvas');
const context = canvas.getContext('2d');
const width = image.width;
const height = image.height;
canvas.width = width;
canvas.height = height;
context.drawImage(image, 0, 0, width, height);
然后像这样获取单个像素的值:
const data = context.getImageData(X, Y, 1, 1).data;
通过一次性获取所有ImageData来加快速度
您需要使用相同的CanvasRenderingContext2D.getImageData()来获取整个图像的值,方法是更改其第三个和第四个参数。该函数的签名如下:
ImageData ctx.getImageData(sx, sy, sw, sh);
sx
: 矩形左上角的x坐标,从该矩形中提取ImageData。
sy
: 矩形左上角的y坐标,从该矩形中提取ImageData。
sw
: 从中提取ImageData的矩形的宽度。
sh
: 从中提取ImageData的矩形的高度。
您可以看到它返回一个ImageData对象,不管那是什么。这里重要的部分是该对象具有一个.data属性,其中包含所有像素值。
然而,请注意.data属性是一个一维的Uint8ClampedArray
,这意味着所有像素的组件都被展平,所以你得到的是这样的东西:
假设您有一个2x2的图像,像这样:
RED PIXEL | GREEN PIXEL
BLUE PIXEL | TRANSPARENT PIXEL
然后,你会像这样得到它们:
[ 255, 0, 0, 255, 0, 255, 0, 255, 0, 0, 255, 255, 0, 0, 0, 0 ]
| RED PIXEL | GREEN PIXEL | BLUE PIXEL | TRANSPAERENT PIXEL |
| 1ST PIXEL | 2ND PIXEL | 3RD PIXEL | 4TH PIXEL |
由于调用getImageData
是一个较慢的操作,您只能调用一次来获取所有图像的数据(sw
= 图像宽度,sh
= 图像高度)。
然后,在上面的示例中,如果您想访问TRANSPARENT PIXEL
的组件,即这个虚构图像在位置x = 1,y = 1
处的像素,您可以在其ImageData
的data
属性中找到其第一个索引i
:
const i = (y * imageData.width + x) * 4;
✨ 让我们看看它的实际效果
const solidColor = document.getElementById('solidColor');
const alphaColor = document.getElementById('alphaColor');
const solidWeighted = document.getElementById('solidWeighted');
const solidColorCode = document.getElementById('solidColorCode');
const alphaColorCode = document.getElementById('alphaColorCode');
const solidWeightedCOde = document.getElementById('solidWeightedCode');
const brush = document.getElementById('brush');
const image = document.getElementById('image');
const canvas = document.createElement('canvas');
const context = canvas.getContext('2d');
const width = image.width;
const height = image.height;
const BRUSH_SIZE = brush.offsetWidth;
const BRUSH_CENTER = BRUSH_SIZE / 2;
const MIN_X = image.offsetLeft + 4;
const MAX_X = MIN_X + width - 1;
const MIN_Y = image.offsetTop + 4;
const MAX_Y = MIN_Y + height - 1;
canvas.width = width;
canvas.height = height;
context.drawImage(image, 0, 0, width, height);
const imageDataData = context.getImageData(0, 0, width, height).data;
function sampleColor(clientX, clientY) {
if (clientX < MIN_X || clientX > MAX_X || clientY < MIN_Y || clientY > MAX_Y) {
requestAnimationFrame(() => {
brush.style.transform = `translate(${ clientX }px, ${ clientY }px)`;
solidColorCode.innerText = solidColor.style.background = 'rgb(0, 0, 0)';
alphaColorCode.innerText = alphaColor.style.background = 'rgba(0, 0, 0, 0.00)';
solidWeightedCode.innerText = solidWeighted.style.background = 'rgb(0, 0, 0)';
});
return;
}
const imageX = clientX - MIN_X;
const imageY = clientY - MIN_Y;
const i = (imageY * width + imageX) * 4;
const R = imageDataData[i];
const G = imageDataData[i + 1];
const B = imageDataData[i + 2];
const A = imageDataData[i + 3] / 255;
const iA = 1 - A;
const wR = (R * A + 255 * iA) | 0;
const wG = (G * A + 255 * iA) | 0;
const wB = (B * A + 255 * iA) | 0;
requestAnimationFrame(() => {
brush.style.transform = `translate(${ clientX }px, ${ clientY }px)`;
solidColorCode.innerText = solidColor.style.background
= `rgb(${ R }, ${ G }, ${ B })`;
alphaColorCode.innerText = alphaColor.style.background
= `rgba(${ R }, ${ G }, ${ B }, ${ A.toFixed(2) })`;
solidWeightedCode.innerText = solidWeighted.style.background
= `rgb(${ wR }, ${ wG }, ${ wB })`;
});
}
document.onmousemove = (e) => sampleColor(e.clientX, e.clientY);
sampleColor(MIN_X, MIN_Y);
body {
margin: 0;
height: 100vh;
display: flex;
flex-direction: row;
align-items: center;
justify-content: center;
cursor: none;
font-family: monospace;
overflow: hidden;
}
#image {
border: 4px solid white;
border-radius: 2px;
box-shadow: 0 0 32px 0 rgba(0, 0, 0, .25);
width: 150px;
box-sizing: border-box;
}
#brush {
position: absolute;
top: 0;
left: 0;
pointer-events: none;
width: 1px;
height: 1px;
mix-blend-mode: exclusion;
border-radius: 100%;
}
#brush::before,
#brush::after {
content: '';
position: absolute;
background: magenta;
}
#brush::before {
top: -16px;
left: 0;
height: 33px;
width: 100%;
}
#brush::after {
left: -16px;
top: 0;
width: 33px;
height: 100%;
}
#samples {
position: relative;
list-style: none;
padding: 0;
width: 250px;
}
#samples::before {
content: '';
position: absolute;
top: 0;
left: 27px;
width: 2px;
height: 100%;
background: black;
border-radius: 1px;
}
#samples > li {
position: relative;
display: flex;
flex-direction: column;
justify-content: center;
padding-left: 56px;
}
#samples > li + li {
margin-top: 8px;
}
.sample {
position: absolute;
top: 50%;
left: 16px;
transform: translate(0, -50%);
display: block;
width: 24px;
height: 24px;
border-radius: 100%;
box-shadow: 0 0 16px 4px rgba(0, 0, 0, .25);
margin-right: 8px;
}
.sampleLabel {
font-weight: bold;
margin-bottom: 8px;
}
.sampleCode {
}
<img id="image" src="data:image/gif;base64,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" >
<div id="brush"></div>
<ul id="samples">
<li>
<span class="sample" id="solidColor"></span>
<div class="sampleLabel">solidColor</div>
<div class="sampleCode" id="solidColorCode">rgb(0, 0, 0)</div>
</li>
<li>
<span class="sample" id="alphaColor"></span>
<div class="sampleLabel">alphaColor</div>
<div class="sampleCode" id="alphaColorCode">rgba(0, 0, 0, 0.00)</div>
</li>
<li>
<span class="sample" id="solidWeighted"></span>
<div class="sampleLabel">solidWeighted (with white)</div>
<div class="sampleCode" id="solidWeightedCode">rgb(0, 0, 0)</div>
</li>
</ul>
⚠️ 注意:我使用了一个小的数据URI来避免跨域问题,如果我包含一个外部图像或者一个超过允许长度的答案,那么这个数据URI就会变长。
️这些颜色看起来很奇怪,不是吗?
如果你将光标移动到星号形状的边界周围,有时你会看到
avgSolidColor
是红色的,但你采样的像素看起来是白色的。这是因为即使该像素的
R
分量可能很高,但是alpha通道很低,所以颜色实际上是一种几乎透明的红色,但
avgSolidColor
忽略了这一点。
另一方面,
avgAlphaColor
看起来是粉色的。嗯,实际上并不是这样,只是看起来是粉色的,因为我们现在使用了alpha通道,这使得它半透明,并且可以看到页面的背景,而在这种情况下是白色。
Alpha加权颜色
那么,我们该怎么解决这个问题呢?事实证明,我们只需要使用 alpha 通道及其反向作为权重来计算我们新样本的分量,这种情况下我们将其与白色合并,因为白色是我们用作背景的颜色。
这意味着,如果一个像素是 R、G、B、A,其中 A 在区间 [0, 1] 内,我们将计算 alpha 通道的反向 iA,以及加权样本的分量,如下所示:
const iA = 1 - A;
const wR = (R * A + 255 * iA) | 0;
const wG = (G * A + 255 * iA) | 0;
const wB = (B * A + 255 * iA) | 0;
请注意,像素越透明(
A
接近 0),颜色越浅。