大部分计算方式可以用这四条规则来概括。这些规则基本上涵盖了大部分情况除了个别边缘问题。
总体来说，上列尺寸计算规则关心的是一个图像有无定义的尺寸和比例，与图片格式没有关系。具有固定尺寸的 SVG 图像依然被视为大小相同的光栅图像。
在深入研究使用{{ cssxref("background-size") }}并不同类型图片的影响并且得到结果之前，我们先来看看不同尺寸和大小的图像示例。
在每个例子中，图像被渲染在 150x150 的容器内，并且在下方提供了 SVG 文件资源
下面这个图片既没有尺寸也没有比例。这种情况不会关心它的尺寸也不关心它的长宽比例。无论你的屏幕尺寸和长宽比如何，这都是一个很好的渐变桌面背景。
这个图片指定了 100px 的宽但是没有高度也没有固定的比例。我们可以说这是一个可以在一条街道上无线延伸的壁纸。
这个图片指定了 100px 的高但没有宽。 同时指定了一个 3:4 的比例，除非是故意放大到不成比例的尺寸（也就是说，通过显式指定宽度和高度到不是这个比例）
这个图片既没有指定高度也没有指定宽度。它指定的是 1:1 的固定比例。就像软件的图标一样。它总是保持正方形，而且可以用于任何尺寸，例如 32x32，128x128，和 512x512。
现在让我们来看看这些图片在各种缩放情景下怎么展现。以下例子都是宽 300 高 200 像素的矩形。此外， {{ cssxref("background-repeat") }} 都设为了 no-repeat ，以便看得出来缩放的情况。
如果你使用 {{ cssxref("background-size") }} 指定了两条边的长度， those lengths are always used, per rule 1 above. In other words, the image will always get stretched to the dimensions you specify, regardless of whether or not the source image has specified its dimensions and/or aspect ratio.
Given this CSS:
background: url(no-dimensions-or-ratio.svg); background-size: 125px 175px;
The rendered output would look like this:
Given this CSS:
background: url(100px-wide-no-height-or-ratio.svg); background-size: 250px 150px;
The rendered output would look like this:
Given this CSS:
background: url(100px-height-3x4-ratio.svg); background-size: 275px 125px;
The rendered output would look like this:
Given this CSS:
background: url(no-dimensions-1x1-ratio.svg); background-size: 250px 100px;
The rendered output would look like this:
当 {{ cssxref("background-size") }} 指定为 cover 时，图片能多小就多小，只要依然能覆盖整个背景区域。而指定为 contain 则会使得图片能多大就多大，只要不被背景裁切就好。
For an image with an intrinsic ratio, exactly one size matches the cover/fit criteria alone. But if there is no intrinsic ratio specified, cover/fit isn't sufficient, so the large/small constraints choose the resulting size.
If an image doesn't specify either dimensions or an intrinsic ratio, neither rule 2 nor rule 3 apply, so rule 4 takes over: the background image is rendered covering the entire background area. This satisfies the largest-or-smallest constraint.
background: url(no-dimensions-or-ratio.svg); background-size: contain;
The rendered output looks like this:
Similarly, if the image has one dimension specified but no intrinsic ratio, rule 4 applies, and the image is scaled to cover the entire background area.
background: url(100px-wide-no-height-or-ratio.svg); background-size: contain;
The rendered output looks like this:
Things change when you specify an intrinsic ratio. In this case, rule 1 isn't relevant, so rule 2 is applied: we try to preserve any intrinsic ratio (while respecting contain or cover). For example, preserving a 3:4 intrinsic aspect ratio for a 300x200 box with contain means drawing a 150x200 background.
background: url(100px-height-3x4-ratio.svg); background-size: contain;
The rendered output looks like this:
Notice how the entire image is rendered, fitting as best as possible into the box without clipping any of it away.
background: url(100px-height-3x4-ratio.svg); background-size: cover;
The rendered output looks like this:
Here, the 3:4 ratio is preserved while still stretching the image to fill the entire box. That causes the bottom of the image to be clipped away.
When using an image with no intrinsic dimensions but an intrinsic ratio, things work similarly.
background: url(no-dimensions-1x1-ratio.svg); background-size: contain;
The rendered output looks like this:
Notice that the image is sized to fit the smallest dimension while preserving the 1:1 aspect ratio.
background: url(no-dimensions-1x1-ratio.svg); background-size: cover;
The rendered output looks like this:
Here, the image is sized so that it fills the largest dimension. The 1:1 aspect ratio has been preserved, although with this source image, that can be difficult to see.
If {{ cssxref("background-size") }} is set to auto or auto auto, rule 2 says that rendering must preserve any intrinsic ratio that's provided.
When no intrinsic ratio or dimensions are specified by the source image, rule 4 takes effect, and the image is rendered to fill the background area.
background: url(no-dimensions-or-ratio.svg); background-size: auto auto;
The rendered output looks like this:
If no intrinsic ratio is specified, but at least one dimension is specified, rule 3 takes effect, and we render the image obeying those dimensions.
background: url(100px-wide-no-height-or-ratio.svg); background-size: auto auto;
The rendered output looks like this:
Note here that the width, which is specified in the source SVG at 100 pixels, is obeyed, while the height fills the background area since it's not specified (either explicitly or by an intrinsic ratio).
If we have an intrinsic ratio with a fixed dimension, that fixes both dimensions in place. Knowing one dimension and a ratio is, as has been mentioned already, the same as specifying both dimensions explicitly.
background: url(100px-height-3x4-ratio.svg); background-size: auto auto;
The rendered output looks like this:
Since this image has an explicit 100 pixel height, the 3:4 ratio explicitly sets its width at 75 pixels, so that's how it's rendered in the auto case.
When an intrinsic ratio is specified, but no dimensions, rule 4 is applied -- except that rule 2 also applies. The image is therefore rendered just like for the contain case.
background: url(no-dimensions-1x1-ratio.svg); background-size: auto auto;
The rendered output looks like this:
Given rule 1, specified dimensions are always used, so we need to use our rules only to determine the second dimension.
If the image has no dimensions or intrinsic ratio, rule 4 applies, and we use the background area's dimension to determine the value for the auto dimension.
background: url(no-dimensions-or-ratio.svg); background-size: auto 150px;
Here, the width is determined using the background area's width per rule 4, while the height is the 140px specified in the CSS.
If the image has one specified dimension but no intrinsic ratio, that specified dimension is used per rule 3 if that dimension is set to auto in the CSS.
background: url(100px-wide-no-height-or-ratio.svg); background-size: 200px auto;
Here, the 200px specified in the CSS overrides the 100px width specified in the SVG, per rule 1. Since there's no intrinsic ratio or height provided, auto selects the height of the background area as the height for the rendered image.
background: url(100px-wide-no-height-or-ratio.svg); background-size: auto 125px;
In this case, the width is specified as auto in the CSS, so the 100px width specified in the SVG is selected, per rule 3. The height is set at 125px in the CSS, so that is selected per rule 1.
When a dimension is specified, rule 1 applies that dimension from the SVG to the rendered background unless specifically overridden by the CSS. When an intrinsic ratio is also specified, that's used to determine the other dimension.
background: url(100px-height-3x4-ratio.svg); background-size: 150px auto;
In this case, we've overridden the height of the image in the CSS to be 150px, so rule 1 is applied. The intrinsic 3:4 aspect ratio then determines the width for the auto case.
If no dimensions are specified in the SVG, the specified dimension in the CSS is applied, then the intrinsic ratio is used to select the other dimension, per rule 2.
background: url(no-dimensions-1x1-ratio.svg); background-size: 150px auto;
The width is set by the CSS to 150px. The auto value for the height is computed using that width and the 1:1 aspect ratio to be 150px as well, resulting in the image above.
