Facebook API 覆盖背景位置属性中的图像偏移

Facebook API cover images offsets in background-position attributes

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我环顾四周,但找不到这个问题的答案:

我正在从 fb api 获取活动封面图片,并且还存储 offset_x 和 offset_y 值。然后我将图像放置为 css 背景图像,如下所示:

CSS

  .event-image {
    margin-bottom: 30px;
    width: auto;
    width: 500px;
    height: 178px;
    max-width: 100%;
    min-width: 300px;
    background-size: cover;
    background-position: center center;
    background-repeat: no-repeat;
    outline: none;
  }

height/width 是基于 facebook 使用的确切比例:2,8:1

HTML

<div class="event-image" style="background-image: url([url of img]);  background-position: [offset_x]% [offset_y]%;" >

(我确实有一些内部逻辑,如果 fb api 中有一组,则只添加背景位置 属性。)

问题是这只在 90% 的时间有效。大约 10% 的图像略微未对齐。通常只有几个百分点:-(

这是一个例子。

.event-image {
  margin-bottom: 30px;
  width: auto;
  width: 500px;
  height: 178px;
  max-width: 100%;
  min-width: 300px;
  background-size: cover;
  background-position: center center;
  background-repeat: no-repeat;
  outline: none;
}
<div class="event-image" style="background-image: url(https://scontent.xx.fbcdn.net/t31.0-0/p180x540/14566476_1295215523823549_1334771895810946224_o.jpg);  background-position: 0% 50%; "> </div>
<!-- Offsets are taken directly from API -->

现在是 actual event

您可以看到实际上偏移量在 46% 时是完美的 - 那么为什么 fb 给出 50%?

我能找到的最多信息是 pixel calculations,但考虑到我使用的是百分比,这没有用。

编辑

实施 elfan 解决方案的新问题:

Here is an event 在 fb 上,图像在 -7px 的实际 fb 页面中有 offset_x - 但根据 api,offset_x = 50%

{
  "cover": {
    "offset_x": 50,
    "offset_y": 0,
    "source": "https://scontent.xx.fbcdn.net/t31.0-8/s720x720/14681104_1307094859330152_7540791723420117074_o.jpg",
    "id": "1307094859330152"
  },
  "id": "544220282434185"
}

所以,使用计算500px (width of my image) * offset_x % = 250px

我做错了什么:-)

我还注意到有些事件的偏移量非常大,例如 1800。根据 fb 文档,它应该在 0-100 之间。

我已经看到这种方法被用于 FB 的偏移量。

FB.api(artist, function (data) {
    $('.ed-cover img').attr('src', data.cover.source)
    .css("top", (-1 * data.cover.offset_y) + '%');
});

解释有问题

fb api 中的值 50 显然是指在 top 中使用它时以百分比表示的偏移量,这意味着包含块高度的百分比(spec here). And that is different to when using it in background-position (spec here). There is also an article here 直观地显示了差异。

如果要使用background-position,解决方法是使用px。 或者,您可以将 top 用作 %px.

我制作了以下代码来比较您的代码、fb 代码以及修复应该是什么(对于所有替代方案):

/* Your original code */
.event-image {
 width: 500px;
 height: 178px;
 background-size: cover;
 background-image: url("https://scontent-sit4-1.xx.fbcdn.net/t31.0-0/p600x600/14566476_1295215523823549_1334771895810946224_o.jpg");
 background-position: 0 50%;  /* "50" is from fb api, but not used correctly */
}

/* FB actual code */
.cover {
 width: 826px;
 height: 294px;
 position: relative;
 overflow: hidden;
}
.cover img {
 position: absolute;
 width: 100%;
 left: 0;
 top: -147px;  /* 294px * 50% = 147px, this is the correct usage of "50" from fb api */
}

/* Your code if showing as big as FB image */
.bigger-image {
 width: 826px;
 height: 294px;
 background-size: cover;
 background-image: url("https://scontent-sit4-1.xx.fbcdn.net/t31.0-0/p600x600/14566476_1295215523823549_1334771895810946224_o.jpg");
 background-position: 0 50%; /* "50" is from fb api, but not used correctly */
}

/* The correct code using "background-position" */
.correct-image {
 width: 500px;
 height: 178px;
 background-size: cover;
 background-image: url("https://scontent-sit4-1.xx.fbcdn.net/t31.0-0/p600x600/14566476_1295215523823549_1334771895810946224_o.jpg");
 background-position: 0 -89px;   /* 178px * 50% = 89px, this is the correct usage of "50" from fb api */
}

/* The correct code using "top" in pct */
.correct-cover {
 width: 500px;
 height: 178px;
 position: relative;
 overflow: hidden;
}
.correct-cover img.pct {
 position: absolute;
 width: 100%;
 left: 0;
 top: -50%;    /* the correct usage of "50" from fb api */
}

/* The correct code using "top" in px */
.correct-cover img.px {
 position: absolute;
 width: 100%;
 left: 0;
 top: -89px;  /* 178px * 50% = 89px, this is the correct usage of "50" from fb api */
}
<h3>Your original code</h3>
<div class="event-image"></div>
<br />

<h3>FB actual code</h3>
<div class="cover">
 <img src="https://scontent-sit4-1.xx.fbcdn.net/t31.0-0/p600x600/14566476_1295215523823549_1334771895810946224_o.jpg" />
</div>
<br />

<h3>Your code if showing as big as FB image</h3>
<div class="bigger-image"></div>
<br />

<h3>The correct code using "background-position"</h3>
<div class="correct-image"></div>
<br />

<h3>The correct code using "top" in pct</h3>
<div class="correct-cover">
 <img class="pct" src="https://scontent-sit4-1.xx.fbcdn.net/t31.0-0/p600x600/14566476_1295215523823549_1334771895810946224_o.jpg" />
</div>
<br />

<h3>The correct code using "top" in px</h3>
<div class="correct-cover">
 <img class="px" src="https://scontent-sit4-1.xx.fbcdn.net/t31.0-0/p600x600/14566476_1295215523823549_1334771895810946224_o.jpg" />
</div>
<br />

关于为什么 46% 在您的原始代码中看起来正确的补充说明:

background-position: 0% 等同于 top: 0px

background-position: 100% 等同于 top: -197px

background-position: 50%top: -98.5px 相同 (197 x 50%)

background-position: 46%top: -90.62px (197 x 46%) 相同,而正确的是 top: -89px,所以看起来很接近。

197从何而来? 框大小为 500 x 178 像素。实际图像大小为 800 x 600 像素。由于 background-size: cover,rendered/scaled 图片大小为 500 x 375 像素。 图片高度比框高大 375-178 = 197px。请记住,当使用 background-position: 100% 时,图像的底部边缘将与框的底部边缘相交,这意味着 top: -197px.