Crear un efecto de fondo enmascarado con CSS

() translation by (you can also view the original English article)

Final product image — What You'll Be Creating

Hoy vamos a crear paso a paso una técnica realmente asombrosa que puedes usar como un efecto muy parecido al desplazamiento parallax, sin usar nada de JavaScript, se puede lograr simplemente a través de solo CSS.

Comienza por revisar la versión online para ver lo que vas a aprender. (Para ver el efecto necesitarás una computadora de escritorio o laptop).

Esta técnica podría ser usada para crear páginas con geniales descripciones de productos, o incluso algo parecido a una presentación PowerPoint/Keynote, y sería genial para las ilustraciones de alguna historia en linea.

Aquí esta cómo se hace.

Todo esta en el CSS

La clave para esta técnica con CSS es background-attachment: fixed;, disponible desde la versión 2.1. Cualquier imagen de fondo con este estilo se aplicará a la misma y se mantendrá en una posición fija con respecto a la ventana (no al elemento que se aplica). Lo usaremos para mantener nuestras ilustraciones en un lugar mientras nuestro contenido se desplaza independientemente de él.

Un par de cosas a tener en cuenta con esta propiedad son que como imágenes de fondo se fijarán en relación a la ventana, su posición no se verá afectada por cosas como márgenes en el camino como otra imagen de fondo lo haría.

También deberías saber que, si bien la propiedad funciona de maravilla en todos los navegadores de escritorio, no funciona correctamente con la versión móvil de Chrome y puede ser un poco distinto en los otros navegadores móviles. Así, mientras los visitantes ven las imágenes bien, el efecto de este desplazamiento puede verse mucho mejor en los navegadores de escritorio.

Cómo se hace

Los pasos básicos para lograr tal efecto como se ve en la versión online son:

Crear un elemento contenedor y agregar el contenido a él.
Ajusta el contenedor (un div en nuestro caso) para tener padding en un lado alrededor del 50% de anchura, y así empuje el contenido hacia el otro lado.
Agrega una imagen de fondo, también con un 50% de anchura, y posiciónala en el lado contrario al contenido.
Ajusta la propiedad background-attachment: fixed; y ve la magia del desplazamiento.

Vamos a ver paso a paso cómo hacer todo esto. Necesitarás los archivos iniciales para este tutorial con las imágenes requeridas.

1. Configuración Básica

Empieza por crear la carpeta del proyecto, y agrega un archivo index.html dentro, también una carpeta css con un archivo de nombre style.css y guárdalo. Copia y pega las cuatro imágenes que descargaste de los archivos iniciales dentro de una carpeta de nombre images.

Agrega este HTML al index.html:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>A Visual Demonstration of background-attachment: fixed;</title>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link href="css/style.css" rel="stylesheet" type="text/css">
    <link href="//fonts.googleapis.com/css?family=Alike|Roboto:900" rel="stylesheet" type="text/css">
  </head>
  <body>
    <div class="content right illustration_01">
        <h2>Scroll Down and Watch What Happens</h2>
        <p>Alice was beginning to get very tired of sitting by her sister on the bank, and of having nothing to do: once or twice she had peeped into the book her sister was reading, but it had no pictures or conversations in it, `and what is the use of a book,' thought Alice `without pictures or conversation?'</p>
        <p>So she was considering in her own mind (as well as she could, for the hot day made her feel very sleepy and stupid), whether the pleasure of making a daisy- chain would be worth the trouble of getting up and picking the daisies, when suddenly a White Rabbit with pink eyes ran close by her.</p>
        <p>There was nothing so very remarkable in that; nor did Alice think it so very much out of the way to hear the Rabbit say to itself, `Oh dear! Oh dear! I shall be late!' (when she thought it over afterwards, it occurred to her that she ought to have wondered at this, but at the time it all seemed quite natural); but when the Rabbit actually took a watch out of its waistcoat- pocket, and looked at it, and then hurried on, Alice started to her feet, for it flashed across her mind that she had never before seen a rabbit with either a waistcoat-pocket, or a watch to take out of it, and burning with curiosity, she ran across the field after it, and fortunately was just in time to see it pop down a large rabbit-hole under the hedge.</p>
        <p>In another moment down went Alice after it, never once considering how in the world she was to get out again.</p>
        <p>The rabbit-hole went straight on like a tunnel for some way, and then dipped suddenly down, so suddenly that Alice had not a moment to think about stopping herself before she found herself falling down a very deep well.</p>
        <p>Either the well was very deep, or she fell very slowly, for she had plenty of time as she went down to look about her and to wonder what was going to happen next. First, she tried to look down and make out what she was coming to, but it was too dark to see anything; then she looked at the sides of the well, and noticed that they were filled with cupboards and book-shelves; here and there she saw maps and pictures hung upon pegs. She took down a jar from one of the shelves as she passed; it was labelled `ORANGE MARMALADE', but to her great disappointment it was empty: she did not like to drop the jar for fear of killing somebody, so managed to put it into one of the cupboards as she fell past it.</p>
        <p>`Well!' thought Alice to herself, `after such a fall as this, I shall think nothing of tumbling down stairs! How brave they'll all think me at home! Why, I wouldn't say anything about it, even if I fell off the top of the house!' (Which was very likely true.) </p>
    </div>  
  </body>
</html>

Lo que estamos haciendo aquí es configurar nuestro archivo HTML, agregando nuestra hoja de estilos y algunos tipos de letra de Google Fonts, después creando nuestro primer div contenedor al que le aplicaremos esta técnica.

El div contenedor tiene tres clases:

.content - usada para establecer algunas propiedades que serán comunes en todos los demás contenedores.
.right - identifica que este contenedor deberá tener el contenido alineado a la derecha (también vamos a trabajar con un contenedor alineado a la izquierda más adelante)
.illustration_01 - usado para establecer la imagen de fondo y el color para este contenedor

Añadiendo estilos

Ahora estamos listos para algo de CSS. Empieza por agregar a tu archivo style.css lo básico para normalizar y formatear el código:

1	* {
2	box-sizing: border-box;
3	}
4	html {
5	font-size: 1em;
6	font-family: 'Alike';
7	background-color: #262626;
8	color: #d9d9d9;
9	}
10	body {
11	margin: 0;
12	}
13	img {
14	max-width: 100%;
15	height: auto;
16	}
17	h1,h2,h3,h4,h5,h6 {
18	font-family: 'Roboto';
19	line-height: 1.313em;
20	}
21	h1 {
22	font-size: 3em;
23	margin: 0.563em 0;
24	}
25	h2 {
26	font-size: 2.25em;
27	margin: 0.625em 0;
28	}
29	h3 {
30	font-size: 1.5em;
31	margin: 1.313em 0;
32	}
33	h4 {
34	font-size: 1.313em;
35	margin: 1.313em 0;
36	}
37	h5 {
38	font-size: 1.125em;
39	margin: 1.313em 0;
40	}
41	h6 {
42	font-size: 1em;
43	margin: 0.75em 0;
44	}

Ahora, para la clase .content. Agrega esto al final de tu hoja de estilos:

.content {
  font-size: 1.875rem;
  color: #262626;
  background-size: 49% auto;
  background-attachment: fixed;
  background-repeat: no-repeat;
}

En esta clase es donde la mayor parte de la magia sucede. Para el texto, establecemos nuestro tamaño de fuente y color. Para el fondo como verás empezamos por fijar la propiedad background-size a un 49% auto.

Esto significa que la imagen de fondo siempre se reducirá o estirará para llenar el 49% del ancho de la página, y la altura se ajustará proporcionalmente. Estamos usando un valor de 49% en lugar de 50% porque de lo contrario Firefox muestra una línea rara en el centro de la pantalla.

A continuación establecemos background-fixed que, como usted ya sabe de la descripción anterior, hace que la imagen de fondo se quede fija cuando nos desplazamos, y hace que se posicione con respecto a la ventana en lugar del contenedor al que se aplica.

Finalmente fijamos background-repeat: no-repeat; para asegurarnos que nuestra imagen aparezca solo una vez en la página.

A continuación agrega la clase .right a tu hoja de estilos:

1	.right {
2	padding: 1.618em 6.472em 3.236em 50%;
3	background-position: 0 50%;
4	}

Esta última clase coloca el contenido del texto en una mitad de la pantalla y la imagen de fondo en otra.

La propiedad background-position nos dice que la posición de la imagen de fondo será de cero píxeles desde el lado izquierdo de la ventana y alineado a la mitad del mismo hacia abajo desde la parte superior de la ventana.

Por último agrega la clase .illustration_01:

1	.illustration_01 {
2	background-color: #00c17b;
3	background-image: url("../images/minipadwhite.png");
4	}

Esto especifica la imagen de fondo y el color que queremos para el contenido de este contenedor.

Echa un vistazo a tu sitio y debería verse como esto:

Cuando te desplazas hacia abajo deberías ver el contenido deslizarse mientras la imagen permanece exactamente donde está.

2. Agrega un Segundo Contenedor

Vamos a agregar un segundo contenedor, éste con el contenido alineado a la izquierda.

Añade este contenido HTML a tu contenedor después del último div:

    <div class="content left illustration_02">
      <h2>Fixed Background Scrolling Effect</h2>
      <p>Down, down, down. Would the fall never come to an end! `I wonder how many miles I've fallen by this time?' she said aloud. `I must be getting somewhere near the centre of the earth. Let me see: that would be four thousand miles down , I think--' (for, you see, Alice had learnt several things of this sort in her lessons in the schoolroom, and though this was not a very good opportunity for showing off her knowledge, as there was no one to listen to her, still it was good practice to say it over) `--yes, that's about the right distance--but then I wonder what Latitude or Longitude I've got to?' (Alice had no idea what Latitude was, or Longitude either, but thought they were nice grand words to say .)</p>
      <p>Presently she began again. `I wonder if I shall fall right through the earth! How funny it'll seem to come out among the people that walk with their heads downward! The Antipathies, I think--' (she was rather glad there was no one listening, this time, as it didn't sound at all the right word) `--but I shall have to ask them what the name of the country is, you know. Please, Ma' am, is this New Zealand or Australia?' (and she tried to curtsey as she spoke-- fancy curtseying as you're falling through the air! Do you think you could manage it?) `And what an ignorant little girl she'll think me for asking! No, it'll never do to ask: perhaps I shall see it written up somewhere.'</p>
      <p>Down, down, down. There was nothing else to do, so Alice soon began talking again. `Dinah'll miss me very much to-night, I should think!' (Dinah was the cat .) `I hope they'll remember her saucer of milk at tea-time. Dinah my dear! I wish you were down here with me! There are no mice in the air, I'm afraid, but you might catch a bat, and that's very like a mouse, you know. But do cats eat bats, I wonder?' And here Alice began to get rather sleepy, and went on saying to herself, in a dreamy sort of way, `Do cats eat bats? Do cats eat bats?' and sometimes, `Do bats eat cats?' for, you see, as she couldn't answer either question, it didn't much matter which way she put it. She felt that she was dozing off, and had just begun to dream that she was walking hand in hand with Dinah, and saying to her very earnestly, `Now, Dinah, tell me the truth: did you ever eat a bat?' when suddenly, thump! thump! down she came upon a heap of sticks and dry leaves, and the fall was over.</p>
      <p>Alice was not a bit hurt, and she jumped up on to her feet in a moment: she looked up, but it was all dark overhead; before her was another long passage, and the White Rabbit was still in sight, hurrying down it. There was not a moment to be lost: away went Alice like the wind, and was just in time to hear it say, as it turned a corner, `Oh my ears and whiskers, how late it's getting!' She was close behind it when she turned the corner, but the Rabbit was no longer to be seen: she found herself in a long, low hall, which was lit up by a row of lamps hanging from the roof.</p>
      <p>There were doors all round the hall, but they were all locked; and when Alice had been all the way down one side and up the other, trying every door, she walked sadly down the middle, wondering how she was ever to get out again. </p>
    </div>

Observa que esta vez usamos la clase .left en lugar de .right y hemos aumentado el número de ilustración en la clase .illustration_01 por .illustration_02

Agrega las siguientes dos clases a tu hoja de estilos:

.left {
  padding: 1.618em 50% 3.236em 6.472em;
  background-position: 100% 50%;
}
.illustration_02 {
  background-color: #e8697b;
  background-image: url("../images/minipadblack.png");
}

Esta vez hemos aplicado el 50% de padding del lado derecho del contenedor de manera que el contenido sea empujado hacia la izquierda, y el fondo está posicionado 100% horizontalmente, es decir, todo a la derecha. También vamos a agregar un color y una imagen de fondo diferente a este contenedor.

Revisa de nuevo tu sitio y empieza por desplazarte hacia abajo. Cuando encuentres el final del primer contenedor deberías empezar a ver el segundo venir, desapareciendo la primer imagen en la parte superior y revelando poco a poco la segunda.

3. Agrega un Separador

Sería mejor el efecto de esta técnica si agregamos un separador entre los dos contenedores, así que lo vamos añadir ahora.

En medio de los dos div contenedores agrega este HTML:

1	<section class="separator">
2	<h3>Another Section Starts Here</h3>
3	</section>

Y agrega la clase .separator a tu hoja de estilos:

.separator {
  font-size: 1.875rem;
  padding: 1.618em 0;
  text-align: center;
}

Cuando actualices tu sitio ahora deberías tener un lindo separador entre tus contenedores:

4. Tercer y Cuarto Contenedor

Ahora puedes introducir el siguiente código para agregar más separadores y contenedores.

Añade este HTML después de los divs creados anteriormente:

    <section class="separator">
      <h3>Another Section Starts Here</h3>
    </section>
    <div class="content right illustration_03">
      <h2>Great For Product Presentations</h2>
      <p>Suddenly she came upon a little three-legged table, all made of solid glass; there was nothing on it except a tiny golden key, and Alice's first thought was that it might belong to one of the doors of the hall; but, alas! either the locks were too large, or the key was too small, but at any rate it would not open any of them. However, on the second time round, she came upon a low curtain she had not noticed before, and behind it was a little door about fifteen inches high: she tried the little golden key in the lock, and to her great delight it fitted!</p>
      <p>Alice opened the door and found that it led into a small passage, not much larger than a rat-hole: she knelt down and looked along the passage into the loveliest garden you ever saw. How she longed to get out of that dark hall, and wander about among those beds of bright flowers and those cool fountains, but she could not even get her head though the doorway; `and even if my head would go through,' thought poor Alice, `it would be of very little use without my shoulders. Oh, how I wish I could shut up like a telescope! I think I could, if I only know how to begin.' For, you see, so many out-of-the-way things had happened lately, that Alice had begun to think that very few things indeed were really impossible.</p>
      <p>There seemed to be no use in waiting by the little door, so she went back to the table, half hoping she might find another key on it, or at any rate a book of rules for shutting people up like telescopes: this time she found a little bottle on it, (`which certainly was not here before,' said Alice,) and round the neck of the bottle was a paper label, with the words `DRINK ME' beautifully printed on it in large letters.</p>
      <p>It was all very well to say `Drink me,' but the wise little Alice was not going to do that in a hurry. `No, I'll look first,' she said, `and see whether it's marked "poison" or not'; for she had read several nice little histories about children who had got burnt, and eaten up by wild beasts and other unpleasant things, all because they would not remember the simple rules their friends had taught them: such as, that a red-hot poker will burn you if you hold it too long; and that if you cut your finger very deeply with a knife, it usually bleeds; and she had never forgotten that, if you drink much from a bottle marked `poison,' it is almost certain to disagree with you, sooner or later.</p>
      <p>However, this bottle was NOT marked `poison,' so Alice ventured to taste it, and finding it very nice, (it had, in fact, a sort of mixed flavour of cherry- tart, custard, pine-apple, roast turkey, toffee, and hot buttered toast,) she very soon finished it off. </p>
    </div>
    <section class="separator">
      <h3>Another Section Starts Here</h3>
    </section>
    <div class="content left illustration_04">
      <h2>Simple Technique Using Pure CSS</h2>
      <p> `What a curious feeling!' said Alice; `I must be shutting up like a telescope .'</p>
      <p>And so it was indeed: she was now only ten inches high, and her face brightened up at the thought that she was now the right size for going though the little door into that lovely garden. First, however, she waited for a few minutes to see if she was going to shrink any further: she felt a little nervous about this; `for it might end, you know,' said Alice to herself, `in my going out altogether, like a candle. I wonder what I should be like then?' And she tried to fancy what the flame of a candle is like after the candle is blown out, for she could not remember ever having seen such a thing.</p>
      <p>After a while, finding that nothing more happened, she decided on going into the garden at once; but, alas for poor Alice! when she got to the door, she found he had forgotten the little golden key, and when she went back to the table for it, she found she could not possibly reach it: she could see it quite plainly through the glass, and she tried her best to climb up one of the legs of the table, but it was too slippery; and when she had tired herself out with trying, the poor little thing sat down and cried.</p>`Come, there's no use in crying like that!' said Alice to herself, rather sharply; `I advise you to leave off this minute!' She generally gave herself very good advice, (though she very seldom followed it), and sometimes she scolded herself so severely as to bring tears into her eyes; and once she remembered trying to box her own ears for having cheated herself in a game of croquet she was playing against herself, for this curious child was very fond of pretending to be two people. `But it's no use now,' thought poor Alice, `to pretend to be two people! Why, there's hardly enough of me left to make ONE respectable person!'
      <p>Soon her eye fell on a little glass box that was lying under the table: she opened it, and found in it a very small cake, on which the words `EAT ME' were beautifully marked in currants. `Well, I'll eat it,' said Alice, `and if it makes me grow larger, I can reach the key; and if it makes me grow smaller, I can creep under the door; so either way I'll get into the garden, and I don't care which happens!'</p>
      <p>She ate a little bit, and said anxiously to herself, `Which way? Which way?', holding her hand on the top of her head to feel which way it was growing, and she was quite surprised to find that she remained the same size: to be sure, this generally happens when one eats cake, but Alice had got so much into the way of expecting nothing but out-of-the-way things to happen, that it seemed quite dull and stupid for life to go on in the common way.</p>
      <p>So she set to work, and very soon finished off the cake. </p>
    </div>
    <section class="separator">
      <h1>THE END</h1>
    </section>

Agrega este CSS a tu hoja de estilos:

.illustration_03 {
  background-color: #14b29a;
  background-image: url("../images/miniwhite.png");
}
.illustration_04 {
  background-color: #80b9f1;
  background-image: url("../images/miniblack.png");
}

Ahora deberías ver en tu pantalla el tercer y cuarto contenedor mostrándose:

Y para rematar, un separador final:

5. Haz que el diseño sea Adaptable

La última cosa que necesitarás hacer será adaptar el sitio a los distintos tamaños de pantalla. Cuando la vista se vuelva demasiado pequeña para mostrar cómodamente nuestras imagenes las acomodaremos para que se muestren en linea una después de la otra.

En la parte superior de tus contenedores, dentro de los div de apertura y por encima del texto, agrega un elemento figure con la clase .smallscreen y dentro de ese elemento agrega una etiqueta img para mostrar cada una de las imágenes que se usan actualmente:

Contenido del Primer Contenedor:

1	<figure class="smallscreen">
2	<image src="images/minipadwhite.png">
3	</figure>

Contenido del Segundo Contenedor:

1	<figure class="smallscreen">
2	<image src="images/minipadblack.png">
3	</figure>

Contenido del Tercer Contenedor:

1	<figure class="smallscreen">
2	<image src="images/miniwhite.png">
3	</figure>

Contenido del Cuarto Contenedor:

1	<figure class="smallscreen">
2	<image src="images/miniblack.png">
3	</figure>

Vamos a usar la clase .smallscreen para ocultar esta imagen por defecto, pero para mostrarla cuando se visualice en un tamaño pequeño de pantalla.

Agrega la siguiente clase a tu hoja de estilos:

1	.smallscreen {
2	display: none;
3	}

Bien, ahora agregaremos media queries que se encargarán de mostrar las imágenes de fondo en su totalidad o no. También escalarán progresivamente el tamaño y espacio del texto en el diseño y así podremos acomodar todos los elementos al ancho de la página.

Agrega éstas media queries a tu hoja de estilos:

@media (max-width: 106.25rem) {
  .wrapper,
  .separator {
    font-size: 1.6875rem;
  }
}
@media (max-width: 93.75rem) {
  .content,
  .separator {
    font-size: 1.5rem;
  }
  .right {
    padding: 1.618em 4.854em 1.618em 50%;
  }
  .left {
    padding: 1.618em 50% 1.618em 4.854em;
  }
}
@media (max-width: 81.25rem) {
  .content,
  .separator {
    font-size: 1.3125rem;
  }
  .right {
    padding: 1.618em 3.236em 1.618em 45%;
    background-size: 44% auto;
    background-position: 0 55%;
  }
  .left {
    padding: 1.618em 45% 1.618em 3.236em;
    background-size: 44% auto;
    background-position: 100% 55%;
  }
}
@media (max-width: 68.75rem) {
  .content,
  .separator {
    font-size: 1.125rem;
  }
  .right {
    padding: 1.618em 3.236em 1.618em 40%;
    background-size: 39% auto;
    background-position: 0 60%;
  }
  .left {
    padding: 1.618em 40% 1.618em 3.236em;
    background-size: 39% auto;
    background-position: 100% 60%;
  }
}
@media (max-width: 50rem) {
  .smallscreen {
    display: block;
  }
  .right {
    padding: 1.618em 3.236em;
    background-image: none;
  }
  .left {
    padding: 1.618em 3.236em;
    background-image: none;
  }
}
@media (max-width: 31.25rem) {
  .right {
    padding: 1.618em 1.618em;
  }
  .left {
    padding: 1.618em 1.618em;
  }
}
@media (max-width: 12rem) {
  html {
    min-width: 12rem;
  }
}

Las primeras cuatro media queries simplemente reducen el tamaño de fuente del texto y el padding dentro del contenido y el contenedor, de manera progresiva para así adaptarse al ancho de pantalla disponible.

En la quinta media query de max-width: 50rem incluimos código que hace visible la clase .smallscreen, remueve nuestro 50% de padding del contenido y el contenedor, y oculta las imágenes de fondo. Cuando esta media query se active no veremos más imágenes de fondo fijas, en su lugar vamos a ver imágenes normales en la parte superior de cada contenedor.

Ahora, cuando actualices tu sitio se debería ver sin problemas al momento de reducir la anchura de la vista, hasta que vea su tamaño más pequeño:

Conclusión

Incluso después de muchos años trabajando con CSS no dejo de sorprenderme por el gran número de cosas increibles que puedes hacer con él. Y cuando más simple es la técnica más impresionante el resultado.

Prueba esta pequeña joya, ¡Es tan rápido y fácil que probablemente quedarás enganchado!

¡Sé el primero en conocer las nuevas traducciones–sigue @tutsplus_es en Twitter!