CSS Shapes: Shapes From Images

In this guide we will take a look at how we can create a shape from an image file with an alpha channel or even from a CSS Gradient. This is a very flexible way to create shapes. Rather than drawing a path with a complex polygon in CSS, you can create the shape in a graphics program and then use the path created by the pixels less opaque than a threshold value.

A simple shape from an image

To use an image for the shape the image needs to have an Alpha Channel, an area that is not fully opaque. The shape-image-threshold property is used to set a threshold for this opacity. Pixels that are more opaque than this value will be used to calculate the area of the shape.

As a simple example, I have an image of a star with a solid red area and an area which is fully transparent. I use the path to the image file as the value of the shape-outside property. The content now wraps around the star shape.

I can use shape-margin to move the text away from the shape, giving a margin around the created shape and the text.

CORS compatibility

Something that you will run into when creating shapes from an image is that the image you use must be CORS compatible. An image hosted on the same domain as your site should work, however if your images are hosted on a different domain such as on a CDN you should ensure that they are sending the correct headers to enable them to be used for Shapes. Due to this requirement for CORS compatible images, if you are previewing your file locally without using a local web server, your shape will not work.

Is it a CORS issue?

DevTools can help you to identify CORS errors. In Chrome the Console will alert you to CORS problems. In Firefox if you inspect the property you will be alerted to the fact that the image could not be loaded. This should alert you to the fact that your image cannot be used as the source of a shape due to CORS.

Setting a threshold

The shape-image-threshold property enables the creation of shapes from areas which are not fully transparent. If the value of shape-image-threshold is 0.0 (which is the initial value) then the area must be fully transparent. If the value is 1.0 then it is fully opaque. Values in between mean that you can set a semi-transparent area as the defining area.

In the example below I am using a similar image to the initial example, however in this image the background of the star is not fully transparent, it has a 20% opacity as created in my graphics programme. If I set shape-image-threshold to 0.3 then I see the shape, if I set it to something smaller than 0.2 I do not get the shape.

Using images with generated content

In the above example I have both used the image as the value of shape-outside and also added it to the page. Many demos do this as it helps to show the shape we are following, however the shape-outside property is not related to the image that is displayed on the page and so you do not need to display an image to use an image to create a shape.

You do need something to float, but that could be some generated content as in the below example. I am floating generated content and using a larger star image to shape my content without displaying any image on the page.

Creating shapes using a gradient

As a CSS gradient is treated as an image, you can use a gradient to generate your shape, by having transparent or semi-transparent areas as part of the gradient.

In this next example I have used generated content and floated this content, giving it a background image of a linear gradient. I am using that same value as the value of shape-outside. The linear gradient goes from purple to transparent, so by changing the value of shape-image-threshold I can decide how transparent the pixels need to be that create my shape. You can play with that value in the example below to see how the diagonal line will move across the shape depending on that value.

You could also try removing the background image completely, thus using the gradient purely to create the shape and not displaying it on the page at all.

In this next example I am using a radial gradient with an ellipse, once again using a transparent part of the gradient to create the shape.

You can experiment directly in these live examples, to see how changing the gradient will change the path of your shape.