Set background image with opacity

Question

So, I can see in CSS references how to set image transparency and how to set a background image. But how can I combine these two in order to set a transparent background image?

I have an image that I'd like to use as a background, but it is too bright - I'd like to turn the opacity down to about 0.2. How can I do this?

#main {
    background-image: url(/wp-content/uploads/2010/11/tandem.jpg); 
}

Answer 1

Simple solution

if you need to set the gradient to background-image only:

background-image: url(IMAGE_URL); /* fallback for older browsers */

background-image: linear-gradient(to bottom, rgba(0,0,0,0.6) 0%,rgba(0,0,0,0.6) 100%), url(IMAGE_URL);

Answer 2

A great way to do it for a simple image is to do it using only CSS to set the background of the HTML element like so.

HTML {
    background: url('http://www.earthtimes.org/newsimage/eat-lead-by-example-obesity-expert-tells-parents_139.jpg');
    width: 100%;
    height: 100%;
}

If you want to get fancy, and set its opacity, then, in IE9+*, you can set a transparent background color of the body. This works by overlaying semitransparent white to make the image whiter, and appear to be brighter. For example, white with 75% opacity (rgba(255,255,255,.75)) would produce the following effect.

HTML {
  background: url('http://www.earthtimes.org/newsimage/eat-lead-by-example-obesity-expert-tells-parents_139.jpg');
  width: 100%;
  height: 100%;
  position: relative;
}

body {
  width: 100%;
  min-height: 100%;
  position: absolute;
  top: 0;
  left: 0;
  margin: 0;
  background: rgba(255, 255, 255, .75);
}

<p>Lorem ipsum dolor sit amet, et id, maecenas a convallis, elit facilisi congue augue id ac. Suscipit luctus egestas rutrum, amet tincidunt porttitor ante convallis magna, vivamus amet at turpis, ante lacinia blandit vel metus mauris rutrum. Ipsum nam adipiscing.
  Est sapien quis sem vestibulum, quis cubilia turpis, suspendisse mattis vehicula enim risus pede, placerat suspendisse dui. Convallis nostrud pede, mollit lobortis, ornare ipsum tempor faucibus tortor, vel pede, porttitor nulla nonummy vestibulum purus.
  Eros placerat tenetur augue ipsum aliquam, pellentesque congue condimentum sed vitae lectus.</p>
<p>Aliquam venenatis curabitur pellentesque sociosqu quam. Tincidunt id erat vestibulum in, est fermentum ipsum et augue, nascetur etiam. Lorem elit, sed donec, leo vivamus ac id enim faucibus vel. Nullam sit feugiat sed massa consectetuer auctor, nulla
  et erat lacinia nec, eget ut ante nullam est non in, elit porttitor in in donec eget porttitor. Interdum ultricies sem morbi facilisis nibh erat. Id suspendisse, sed tincidunt fringilla sit, sapien odio vel, at culpa augue sed taciti neque inceptos,
  massa class non. Vel tristique condimentum at pellentesque, commodo nulla sagittis rhoncus lorem. Viverra maecenas tellus pretium urna mauris proin, vel libero morbi, ante volutpat vestibulum augue. Itaque leo mauris turpis, vivamus nisl congue nisl
  nulla in turpis, eget vitae accumsan dolor ipsum leo venenatis, feugiat vehicula in risus, donec eu pede vivamus itaque nam. Justo amet vitae pellentesque sed, posuere fusce sapien, sed nam placerat libero sed etiam curabitur, suspendisse justo, amet
  porttitor.</p>
<p>Condimentum eget. Sem elementum a et mauris sem a, velit justo consectetuer in turpis mauris, sit sed elit cum, fusce suspendisse pretium dictum, mattis dui tortor tellus. Volutpat ut ante lorem nec laoreet aliquam, lorem est magna amet, integer mauris
  purus tellus. Porta enim repellendus aliquam eros. Turpis posuere elementum suscipit wisi lobortis, nec nunc consequat dictum ut unde at, mi lorem amet nunc. Cubilia pede, integer dolor, eget platea felis elit enim rhoncus, integer proin quam in ipsum
  lorem, diam curabitur netus pretium pellentesque. Donec rutrum ultrices placerat, curabitur maecenas, feugiat pede varius accumsan quam lorem, dui et dictumst asperiores nulla, vivamus urna nam leo libero. Posuere non convallis amet justo parturient,
  imperdiet consectetuer arcu praesent cursus risus, quis pretium dolor.</p>

• Tip: notice how the HTML is position: relative, while the body is position: absolute. This is to prevent the background color of the body from behaving more like a highlighter of the text in the body.

This could even be expanded to something comparable to, but still very distinct from, CSS filters by changing around the body's RGBA color background. For example, rgba(0,255,0,.75) would create a very green tint as you can see in the code snippet.

HTML {
    background: url('http://www.earthtimes.org/newsimage/eat-lead-by-example-obesity-expert-tells-parents_139.jpg');
    width: 100%;
    height: 100%;
    position: relative;
}
body {
    width: 100%;
    min-height: 100%;
    position: absolute;
    top: 0;
    left: 0;
    margin: 0;
    background: rgba(0,255,0,.75);
}

<p>Aliquam venenatis curabitur pellentesque sociosqu quam. Tincidunt id erat vestibulum in, est fermentum ipsum et augue, nascetur etiam. Lorem elit, sed donec, leo vivamus ac id enim faucibus vel. Nullam sit feugiat sed massa consectetuer auctor, nulla et erat lacinia nec, eget ut ante nullam est non in, elit porttitor in in donec eget porttitor. Interdum ultricies sem morbi facilisis nibh erat. Id suspendisse, sed tincidunt fringilla sit, sapien odio vel, at culpa augue sed taciti neque inceptos, massa class non. Vel tristique condimentum at pellentesque, commodo nulla sagittis rhoncus lorem. Viverra maecenas tellus pretium urna mauris proin, vel libero morbi, ante volutpat vestibulum augue. Itaque leo mauris turpis, vivamus nisl congue nisl nulla in turpis, eget vitae accumsan dolor ipsum leo venenatis, feugiat vehicula in risus, donec eu pede vivamus itaque nam. Justo amet vitae pellentesque sed, posuere fusce sapien, sed nam placerat libero sed etiam curabitur, suspendisse justo, amet porttitor.</p>
<p>Condimentum eget. Sem elementum a et mauris sem a, velit justo consectetuer in turpis mauris, sit sed elit cum, fusce suspendisse pretium dictum, mattis dui tortor tellus. Volutpat ut ante lorem nec laoreet aliquam, lorem est magna amet, integer mauris purus tellus. Porta enim repellendus aliquam eros. Turpis posuere elementum suscipit wisi lobortis, nec nunc consequat dictum ut unde at, mi lorem amet nunc. Cubilia pede, integer dolor, eget platea felis elit enim rhoncus, integer proin quam in ipsum lorem, diam curabitur netus pretium pellentesque. Donec rutrum ultrices placerat, curabitur maecenas, feugiat pede varius accumsan quam lorem, dui et dictumst asperiores nulla, vivamus urna nam leo libero. Posuere non convallis amet justo parturient, imperdiet consectetuer arcu praesent cursus risus, quis pretium dolor.</p>
<p>Lorem ipsum dolor sit amet, et id, maecenas a convallis, elit facilisi congue augue id ac. Suscipit luctus egestas rutrum, amet tincidunt porttitor ante convallis magna, vivamus amet at turpis, ante lacinia blandit vel metus mauris rutrum. Ipsum nam adipiscing. Est sapien quis sem vestibulum, quis cubilia turpis, suspendisse mattis vehicula enim risus pede, placerat suspendisse dui. Convallis nostrud pede, mollit lobortis, ornare ipsum tempor faucibus tortor, vel pede, porttitor nulla nonummy vestibulum purus. Eros placerat tenetur augue ipsum aliquam, pellentesque congue condimentum sed vitae lectus.</p>

• Tip: RGBA corresponds to RedGreenBlueAlpha. So, the browser interprets rgba(0,255,0,.75) as something exemplified by {red:0, green:255, blue:0, alpha:'75%'}.

*A full compatibility table can be found at Can I Use. However, please also note that you need to click the "Show All" to see that IE9 supports it.

Addendum

Since I have already answered the question but I have more I want to add, I am titling this section addendum and having it add some potentially helpful information. So, to extrapolate even further on the content above, you could use an SVG as a background image to do wicked awesome things. For example, you could create a loading screen background featuring a cool website icon as you can see in the example of a base64 encoded SVG below.

HTML {
    background: url('data:image/svg+xml;base64,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');
    width: 100%;
    height: 100%;
    position: relative;
    background-size: cover;
}
body {
    width: 100%;
    min-height: 100%;
    position: absolute;
    top: 0;
    left: 0;
    margin: 0;
    background: rgba(255,255,255,.5);
}

<p>Condimentum eget. Sem elementum a et mauris sem a, velit justo consectetuer in turpis mauris, sit sed elit cum, fusce suspendisse pretium dictum, mattis dui tortor tellus. Volutpat ut ante lorem nec laoreet aliquam, lorem est magna amet, integer mauris purus tellus. Porta enim repellendus aliquam eros. Turpis posuere elementum suscipit wisi lobortis, nec nunc consequat dictum ut unde at, mi lorem amet nunc. Cubilia pede, integer dolor, eget platea felis elit enim rhoncus, integer proin quam in ipsum lorem, diam curabitur netus pretium pellentesque. Donec rutrum ultrices placerat, curabitur maecenas, feugiat pede varius accumsan quam lorem, dui et dictumst asperiores nulla, vivamus urna nam leo libero. Posuere non convallis amet justo parturient, imperdiet consectetuer arcu praesent cursus risus, quis pretium dolor.</p>
<p>Lorem ipsum dolor sit amet, et id, maecenas a convallis, elit facilisi congue augue id ac. Suscipit luctus egestas rutrum, amet tincidunt porttitor ante convallis magna, vivamus amet at turpis, ante lacinia blandit vel metus mauris rutrum. Ipsum nam adipiscing. Est sapien quis sem vestibulum, quis cubilia turpis, suspendisse mattis vehicula enim risus pede, placerat suspendisse dui. Convallis nostrud pede, mollit lobortis, ornare ipsum tempor faucibus tortor, vel pede, porttitor nulla nonummy vestibulum purus. Eros placerat tenetur augue ipsum aliquam, pellentesque congue condimentum sed vitae lectus.</p>
<p>Aliquam venenatis curabitur pellentesque sociosqu quam. Tincidunt id erat vestibulum in, est fermentum ipsum et augue, nascetur etiam. Lorem elit, sed donec, leo vivamus ac id enim faucibus vel. Nullam sit feugiat sed massa consectetuer auctor, nulla et erat lacinia nec, eget ut ante nullam est non in, elit porttitor in in donec eget porttitor. Interdum ultricies sem morbi facilisis nibh erat. Id suspendisse, sed tincidunt fringilla sit, sapien odio vel, at culpa augue sed taciti neque inceptos, massa class non. Vel tristique condimentum at pellentesque, commodo nulla sagittis rhoncus lorem. Viverra maecenas tellus pretium urna mauris proin, vel libero morbi, ante volutpat vestibulum augue. Itaque leo mauris turpis, vivamus nisl congue nisl nulla in turpis, eget vitae accumsan dolor ipsum leo venenatis, feugiat vehicula in risus, donec eu pede vivamus itaque nam. Justo amet vitae pellentesque sed, posuere fusce sapien, sed nam placerat libero sed etiam curabitur, suspendisse justo, amet porttitor.</p>

• Tip: The background-size: cover CSS causes the SVG logo in the background to be resized to the size of the HTML element.