Monday, May 19, 2008

Introducing CSS Gradients

There are two types of gradients: linear gradients and radial gradients.

The syntax is as follows:

-gradient(, [, ]?, [, ]? [, ]*)

The type of a gradient is either linear or radial.

A point is a pair of space-separated values. The syntax supports numbers, percentages or the keywords top, bottom, left and right for point values.

A radius is a number and may only be specified when the gradient type is radial.

A stop is a function, color-stop, that takes two arguments, the stop value (either a percentage or a number between 0 and 1.0), and a color (any valid CSS color). In addition the shorthand functions from and to are supported. These functions only require a color argument and are equivalent to color-stop(0, …) and color-stop(1.0, …) respectively.

Paraphrasing the HTML5 spec and adjusting the language slightly to not be canvas-specific:

“The color of the gradient at each stop is the color specified for that stop. Between each such stop, the colors and the alpha component must be linearly interpolated over the RGBA space without premultiplying the alpha value to find the color to use at that offset. Before the first stop, the color must be the color of the first stop. After the last stop, the color must be the color of the last stop. When there are no stops, the gradient is transparent black.

If multiple stops are added at the same offset on a gradient, they must be placed in the order added, with the first one closest to the start of the gradient, and each subsequent one infinitesimally further along towards the end point (in effect causing all but the first and last stop added at each point to be ignored).”

The points of a linear gradient specify a line. Linear gradients must be rendered such that at and before the starting point the color at offset 0 is used, that at and after the ending point the color at offset 1 is used, and that all points on a line perpendicular to the line that crosses the start and end points have the color at the point where those two lines cross (with the colors coming from the interpolation described above).

If x0 = x1 and y0 = y1, then the linear gradient must paint nothing.

For a radial gradient, the first two arguments represent a start circle with origin (x0, y0) and radius r0, and the next two arguments represent an end circle with origin (x1, y1) and radius r1.

Radial gradients must be rendered by following these steps:

If x0 = x1 and y0 = y1 and r0 = r1, then the radial gradient must paint nothing. Abort these steps.

Let x(ω) = (x1-x0)ω + x0

Let y(ω) = (y1-y0)ω + y0

Let r(ω) = (r1-r0)ω + r0

Let the color at ω be the color of the gradient at offset 0.0 for all values of ω less than 0.0, the color at offset 1.0 for all values of ω greater than 1.0, and the color at the given offset for values of ω in the range 0.0 ≤ ω ≤ 1.0

For all values of ω where r(ω) > 0, starting with the value of ω nearest to positive infinity and ending with the value of ω nearest to negative infinity, draw the circumference of the circle with radius r(ω) at position (x(ω), y(ω)), with the color at ω, but only painting on the parts of the surface that have not yet been painted on by earlier circles in this step for this rendering of the gradient.

This effectively creates a cone, touched by the two circles defined in the creation of the gradient, with the part of the cone before the start circle (0.0) using the color of the first offset, the part of the cone after the end circle (1.0) using the color of the last offset, and areas outside the cone untouched by the gradient (transparent black).”

So what exactly is a gradient in CSS? It is an image, usable anywhere that image URLs were used before. That’s right… anywhere. :)

You can use gradients in the following places:

content property
Gradients as Backgrounds

When specifying a gradient as a background, the gradient becomes a background tile. If no size is specified, then the gradient will size to the box specified by the background-origin CSS3 property. This value defaults to padding, so you the gradient will be as large as the padding-box. This is equivalent to a specified background-size of 100% in both directions.

If you want to achieve effects like tiling of a vertical gradient using a narrow strip, you should specify background-size to give the gradient tile an explicit size.

Gradients used as backgrounds will respect full page zoom, acquiring sharper resolution as the page is zoomed in.

Background Gradients Example
Border Image Gradients

Gradients can be used as border images. The most sensible use for them is specifying only horizontal or only vertical borders (and splitting the gradients up between the top and bottom or left and right borders).

The size of the gradient image is always the size of the border box.

Border Image Gradient Example
List Bullet Gradients

Gradients can be specified as list bullets. One problem with list bullet gradients is that there is currently no way in WebKit to specify the size of the marker box. Therefore the size of the image cannot be specified. WebKit has therefore chosen a default size based off the current font size of the list item.

List Item Gradients Example
Generated Content Gradients

Gradients can be used inside the content property. The image will fill the available width and height of its containing block. Therefore when using gradients inside ::before and ::after content with a specified size, it is important to set the display type to block or inline-block.

Gradients can also be used to do image replacement, so can be used with the img element in HTML or to replace the contents of other elements like spans and divs.

Generated Content Gradients Example
Final Notes

WebKit now supports a generic architecture for generated images, making it easy to add new generator effects to CSS in the future (lenticular halos, checkerboards, starbursts, etc.). The rules for sizing of these generated images will match whatever is decided for SVG content with no intrinsic size (the two are sharing the same rules right now).

We encourage you to try gradients out and file bugs if you see any unexpected or weird behavior. They will degrade safely in other browsers as long as you use multiple declarations (e.g., specify the image in one declaration and the gradient in a following declaration).



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