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<div class="contents">
<p>この親要素の横幅はPC画面で600px、スマホ画面で80%です。テキスト部分は親要素と同じ横幅なのですが、写真は全画面表示になっております。</p>
<div class="image"><img src="https://pulpxstyle.com/wp-main/common/codepen/post21-codepen01.jpg" alt="Calc Margin Demo image01"></div>
<p>写真も本来はPC画面で600px、スマホ画面で80%表示されるはずなのですが、calc()を使うことで全画面表示にすることが出来ます。写真を印象的に表現したいページに使えますね。</p>
<p>これは、テキスト部分の外側にある余白の値を計算して、写真をその分左右に広げることにより全画面表示をしてます。</p>
<p>文章の横半分の値を『50%』で、全画面の横半分の値を『50vw』で出すことができるので、それらを使って文章横にある余白の値をcalc()で計算します。</p>
<div class="image"><img src="https://pulpxstyle.com/wp-main/common/codepen/post21-codepen02.jpg" alt="Calc Margin Demo image02"></div>
<p>計算式は、calc((50vw - 50%)* -1)</p>
<p>『-1』をかけているのはネガティブマージンを値を出して、その値で写真を左右に広げるから。仮にテキスト部分の外側にある余白が左右それぞれ200pxだとしたら、marginの左右の値を『-200px』にすることで画像部分の要素が左右に200px分ずつ広がります。</p>
<p>そのため『-1』をかけてネガティブマージンにしてます。</p>
<p>それを簡略化させるため、実際の記述は『calc(50% - 50vw)』にして-1を掛けなくてもネガティブマージンの値を求められるようにします。</p>
<div class="image"><img src="https://pulpxstyle.com/wp-main/common/codepen/post21-codepen03.jpg" alt="Calc Margin Demo image03"></div>
<p>この部分的な全画面表示は写真や画像だけではなく、さまざまな要素に適用することができます。</p>
</div>
.contents {
margin-right: auto;
margin-left: auto;
width: 600px;
}
@media not all and (min-width: 600px){
.contents {
width: 80%;
}
}
.contents p {
margin-bottom: 50px;
}
.image {
margin-right: calc(50% - 50vw);
margin-left: calc(50% - 50vw);
margin-bottom: 50px;
}
@media not all and (min-width: 600px){
.contents p {
margin-bottom: 30px;
}
.image {
margin-bottom: 30px;
}
}
.image img {
display: block;
width: 100%;
height: auto;
}
@media not all and (min-width: 600px){
.image img {
height: 200px;
object-fit: cover;
}
}
Also see: Tab Triggers