CSS custom properties (variables) in <picture> <source> sizes - html

I'm trying to best describe to the browser which image to pick from a srcset list using the sizes attribute.
Basic is:
<picture>
<source
type="image/jpeg"
media="(min-width: 1024px)"
srcset="a.jpg 1024w, b.jpg 1200w, c.jpg 2048w, d.jpg 2400w"
sizes="100vw"
>
<img src="e.jpg">
</picture>
Now you might have a "max viewport" at which point your webpage "stops growing", so you add a media query in your sizes:
<source
...
sizes="(min-width: 1200px) 1200px, 100vw"
>
Your image might also have some padding, so you might be tempted to use 98vw instead of 100vw but that's not always accurate if the paddings are in pixels for example, so what you can do instead it use calc:
<source
...
sizes="(min-width: 1200px) 1200px, calc(100vw - 20px)"
>
So far we've only been using absolute units, but I've read you can also use context-dependent units like em:
<source
...
sizes="(min-width: 1200px) 1200px, calc(100vw - 2em)"
>
Interestingly, that means the browser must be aware of the styles being applied to the image (since em can change based on CSS). So if this is true, could you use CSS custom properties (or "CSS variables") in your sizes?
<source
...
sizes="(min-width: 1200px) 1200px, calc(var(--six-columns) - var(--image-padding))"
>
Thanks!
As a bonus question: any idea where I could find the browser support for such a thing?

The answer to this is NO!
When using em as a unit inside the sizes attribute, the browser will use a fixed predetermined size (14px in chrome apparently, determined experimentally).
In the end, no information actually comes from CSS, and CSS custom properties aren't actually usable that way.

Related

Understanding srcset and sizes in combination with HiDPI monitors

I have been into CSS for quite a while now, but srcset and sizes for the image element confuse me. Here is an example that I thought would work.
<img alt="Background image flowers"
srcset="img/flowers-480.jpg 480w,
img/flowers-480#2x.jpg 480w 2x,
img/flowers-768.jpg 768w,
img/flowers-768#2x.jpg 768w 2x,
img/flowers-960.jpg 960w,
img/flowers-960#2x.jpg 960w 2x,
img/flowers-1280.jpg 1280w,
img/flowers-1280#2x.jpg 1280w 2x"
sizes="(max-width: 1279px) 100vw,
(min-width: 1280) 1280px"
src="img/flowers-960.jpg">
The idea is to have an image that's 100% of the viewport until the viewport is 1280px wide or wider, then the image will be fixed size. However, to compensate for higher DPI devices I thought it was recommended to add DPI descriptors (1.5x, 2x and so on), as suggested here and here.
What I thought the above code would do is:
check sizes, see what kind of size to expect for the image (and if a relative unit is given such as % or vw, calculate the pixel width)
find the images in srcset width that is closest to that width
from these images, filter out the one with a DPI descriptor closest to the device's DPI
However, when I put this through a validator I get the following error:
Error: Bad value for attribute srcset on element img: Width for image img/flowers-480#2x.jpg is identical to width for image
img/flowers-480.jpg
So clearly I am completely missing the point of how srcset and sizes work. What am I doing wrong?
As defined on MDN for <img srcset="...">:
Each string is composed of:
a URL to an image, optionally, whitespace followed by one of:
a width descriptor, or a positive integer directly
followed by 'w'. The width descriptor is divided by the source size
given in the sizes attribute to calculate the effective pixel density.
a pixel density descriptor, which is a positive floating point number
directly followed by 'x'.
You tried to use both, and that's illegal.
For a combination of HPDI settings and responsive sizes for images you should actually use <picture> with a few <source> and a fallback <img> elements.
Details are in this article.
According to MDN, "It is incorrect to mix width descriptors and pixel density descriptors in the same srcset attribute. Duplicate descriptors (for instance, two sources in the same srcset which are both described with '2x') are invalid, too."
You have 2x listed 4 times. That's invalid.
Here is an example from MDN:
Example 4: Using the srcset and sizes attributes
The src attribute is ignored in user agents that support srcset when using 'w' descriptors. When the (max-width: 600px) media condition matches, the image will be 200px wide, otherwise it will be 50vw wide (50% of the viewport width).
<img src="clock-demo-thumb-200.png"
alt="Clock"
srcset="clock-demo-thumb-200.png 200w,
clock-demo-thumb-400.png 400w"
sizes="(max-width: 600px) 200px, 50vw">
The "art direction use case", linked in #vbarinov's answer, quoted from developers.google.com, shows how to combine both width and pixel density as criteria for which image source to load and display:
<picture>
<source media="(min-width: 800px)" srcset="head.jpg, head-2x.jpg 2x">
<source media="(min-width: 450px)" srcset="head-small.jpg, head-small-2x.jpg 2x">
<img src="head-fb.jpg" srcset="head-fb-2x.jpg 2x" alt="a head carved out of wood">
</picture>
In viewports that are at least 800 pixels wide, head.jpg should be displayed on a single density display, while head-2x.jpg should be displayed on a display with double or greater density.
Viewports below 800 pixels with, but at least 450 pixels wide, single-density displays should show head-small.jpg, while head-small-2x.jpg is for double density or greater.
The fallback image definition inside the <img> tag has another srcset that defines a double-density version (head-fb-2x.jpg) for viewports below 450 pixels width.
Single-density devices and browsers that don't support source sets or picture elements will fall back to display head-fb.jpg.
In a more complex scenario, we can also add alternative image formats, to take advantage of webp compression while providing jpg versions for browsers without webp support.
<picture>
<source media="(min-width: 800px)" srcset="head.webp, head-2x.webp 2x" with="800" height="941">
<source media="(min-width: 800px)" srcset="head.jpg, head-2x.jpg 2x" with="800" height="941">
<source media="(min-width: 450px)" srcset="head-small.webp, head-small-2x.webp 2x" width="800" height="941">
<source media="(min-width: 450px)" srcset="head-small.jpg, head-small-2x.jpg 2x" width="800" height="941">
<img src="head-fb.jpg" srcset="head-fb-2x.jpg 2x" width="400" height="471" alt="a head carved out of wood">
</picture>
Here is a codepen of the complex example.
As of 2022, I'd argue it's just easier to assume high DPI screens and leverage the breakpoint idea of srcset:
<img alt="Background image flowers"
srcset="img/flowers-480.jpg 480w,
img/flowers-768.jpg 768w,
img/flowers-960.jpg 960w,
img/flowers-1280.jpg 1280w,
img/flowers-1920.jpg 1920w,
img/flowers-4096.jpg 4096w"
sizes="(max-width: 1279px) 100vw,
(min-width: 1280) 1280px"
src="img/flowers-960.jpg">

Can anyone see where I'm going wrong with this srcset?

I'm building a responsive Wordpress website. The blog feed has a list of stories which have a title and an image. The image sizes should be as follows:
up to 479px wide - 140x140px (stories are displayed in a list)
over 480px wide - 360x190px (stories are displayed in a row of 3)
I've been trying to use SRCSET for this so that the 140x140px image would be loaded for browsers up to 479px and the 360px image would be loaded for browsers 480px and over.
I've googled and read literally every bit of documentation out there on sizes and srcset but I just can't get my head around it. So far I've come up with the following:
<img
src="http://placekitten.com/140/140"
srcset="
http://placekitten.com/140/140 140w,
http://placekitten.com/360/190 360w"
sizes="
(max-width: 479px) 140px,
(min-width: 480px) 360px,
100vw"
alt=""
class="lazyload"
/>
Unfortunately all this does is display the 360x190px at every width, despite the actual src of the image being set to the 140x140px image.
Can anyone see what I've missed? I think it's the sizes that I'm most confused about. I added in media queries like documented but they don't seem to be applied?
Thank you!
Note that this srcset strategy on the <img> element depends on the fact that the browser has not cached the image yet. This strategy is meant to save the bandwidth for the browser. So, if you start off with a wide viewport, the browser simply fetches the larger of the two images and will no longer fallback to the smaller one even if you resize.
If you want to force the browser to load images at various viewport breakpoints, use <picture> instead:
<picture alt="" class="lazyload">
<source srcset="https://via.placeholder.com/360x190" media="(min-width: 480px)" />
<img src="https://via.placeholder.com/140x140" />
</picture>
Check out the code below.
<picture>
<source class="img-fluid" srcset="//www.fillmurray.com/140/140" media="(max-width: 479px)"><img class="img-fluid" src="//www.fillmurray.com/360/190">
</picture>

Does html picture tag saves bandwidth

currently i am using big images using <img> tag to fit for all screen sizes. If i use <picture> tag will it saves bandwidth or page loading time when small image will load for small screen?
Need opinion please.
If you are talking about the same, single source as in the example below (which wouldn't make sense anyway), then no.
<picture>
<img srcset="default.jpg" alt="Default">
</picture>
If you are talking about using different image sources, then yes. A browser will only load the most appropriate image for the current media. E.g. (source):
<picture>
<source srcset="smaller_landscape.jpg" media="(max-width: 40em) and (orientation: landscape)">
<source srcset="smaller_portrait.jpg" media="(max-width: 40em) and (orientation: portrait)">
<source srcset="default_landscape.jpg" media="(min-width: 40em) and (orientation: landscape)">
<source srcset="default_portrait.jpg" media="(min-width: 40em) and (orientation: portrait)">
<img srcset="default_landscape.jpg" alt="My default image">
</picture>
Note that you'll need a polyfill to use picture in IE.
No. A picture tag requires more characters to type than an img tag so it will take up more (although not significantly more) bandwidth.
You might be conflating the picture element with responsive images (which can be achieved using both picture and img elements).

Mixing srcset and picture for responsive images

At my work we have our own framework and we're trying to think of the best way to implement responsive images. Images are generated from the backend code and we would like to have a single control that solves all use cases.
Since srcset seems to be the best solution for responsive images and Picture seems to solve two other cases (Art direction and modern image formats like WebP and Jpeg XR) we want to see if we can combine them.
Since the picture element is mostly used with a element with a media query inside it is an instruction to browser to use this if it meets certain conditions.
Srcset however let's the browser choose the picture bases on some calculations.
Would it somehow be possible to keep the choice to the browser within the picture element?
Something like this for example.
<picture>
<source sizes="(min-width: 800px) 50vw, 100vw" srcset="small.webp 100w, medium.webp 200w, large.webp 300w">
<source sizes="(min-width: 800px) 50vw, 100vw srcset="small.JPG 100w, medium.JPG 200w, large.JPG 300w">
<img src="small.jpg" alt="Some image" />
</picture>
Would appreciate any help :)
You are correct. The first snippet is right:
<source sizes="(min-width: 800px) 50vw, 100vw" srcset="small.webp 100w, medium.webp 200w, large.webp 300w">
The second one is not:
<source sizes="(min-width: 800px) 50vw, 100vw srcset="small.JPG 100w, medium.JPG 200w, large.JPG 300w">
You can also use the type attribute within the source element to find out support of that image type on the browser. Something like this:
<picture>
<source media = "(max-width: 30em)" type="image/vnd.ms-photo" srcset="images/small/space-needle.jxr 1x, images/small/space-needle-2x.jxr 2x, images/small/space-needle-hd.jxr 3x >
<source media = "(max-width: 30em)" type="image/jpg" srcset="images/small/space-needle.jpg 1x, images/small/space-needle-2x.jpg 2x, images/small/space-needle-hd.jpg 3x >
<img src="space-needle.jpg" alt="Space Needle">
</picture>
More details available here.

How do I tell srcset attribute to load NO images when viewport smaller than certain size

I'm having trouble understanding how to keep srcset from loading any images on screens < 768px.
I've tried the code below but the sizes attribute doesn't seem to do what you may think.
Below loads 1024.jpg on all screen sizes:
<img
src="default.jpg"
srcset="img/1024.jpg 1024w"
sizes="(min-width: 768px) 768px, 100vw"
/>
Or the picture element, if it would honor an empty srcset but it only "hints" to which image a browser should load.
The other answer isn't really satisfying. In general with srcset you give the browser the choice to select an image candidate. While you can assume which image is taken on certain devices. You don't have any control. Each image in srcset can be taken.
So if you want to control, what is used or not used, you need to use the picture element.
Here are 3 examples. If the viewport is 768px or wider the 'img/1024.jpg' image is downloaded, otherwise a data uri or a broken img is selected.
<picture>
<!--[if IE 9]><video style="display: none;"><![endif]-->
<source srcset="img/1024.jpg" media="(min-width: 768px)">
<!--[if IE 9]></video><![endif]-->
<img src="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" alt="Image">
</picture>
<picture>
<!--[if IE 9]><video style="display: none;"><![endif]-->
<source srcset="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" media="(max-width: 768px)">
<!--[if IE 9]></video><![endif]-->
<img src="img/1024.jpg" alt="Image">
</picture>
<!-- you can also write (but this makes it invalid) -->
<picture>
<!--[if IE 9]><video style="display: none;"><![endif]-->
<source srcset="img/1024.jpg" media="(min-width: 768px)">
<!--[if IE 9]></video><![endif]-->
<img alt="Image">
</picture>
Although the first and the second code example are absolutely valid. There is currently some discussion to allow this by simply omitting the srcset (see code example 2). See this discussion here: https://github.com/ResponsiveImagesCG/picture-element/issues/243
There's a really simple solution to this which works without <picture>.
It is likely not intended to be used this way, but it works very well.
Attach a 1x1px image and it will be used whenever sizes == 0vw. In this case this would be true for everything under 768px:
<img
src="default.jpg"
srcset="img/null.jpg 1w
img/1024.jpg 1024w"
sizes="(max-width: 768px) 0vw, 100vw"
/>
I'm aware that this is not answering the OP's question of "loading NO images", but it's relatively close and you can reference the same 1x1 px image in all other situations, which means it's not gonna be downloaded there.
One caveat:
In the following situation I believe the browser might decide to use the 1x1px image already from 512px and downwards even though we specify 100px
<img
src="default.jpg"
srcset="img/null.jpg 1w
img/1024.jpg 1024w"
sizes="(max-width: 100px) 0vw, 100vw"
/>
This is due to the fact that the browser takes whatever is closest to the desired size in this case 1 is closer to 511 than 1024 is... not sure though
Edit: This situation could quite easily be fixed by attaching a 200w image as well, which you would likely do in any case
EDIT:
To say it simply, you can't.
Removing/hiding an image element must be done in CSS with media queries, or with Javascript.
The srcset and sizes tags are useful for choosing the content source of an image element, but it cannot control the existence or visibility of the element.
The srcset and sizes tags are intended to augment responsive CSS. Their values should follow whatever breakpoints are defined in CSS.
srcset
srcset is a list of available images the browser can choose from with their respective widths.
According to the latest spec, it will choose only from that list when populated:
For backwards compatibility, one of the URLs is specified in the img element's src attribute. In new user agents, the src attribute is ignored when the srcset attribute uses w descriptors.
Therefore, it sees 1024.jpg as the only choice and ignores default.jpg.
Add the default image to srcset (with the correct w descriptor - here I assume default.jpg is 768px wide):
<img
src="default.jpg"
srcset="default.jpg 768w, img/1024.jpg 1024w"
sizes="(min-width: 768px) 768px, 100vw"
/>
sizes
sizes tells the browser how wide the image will be when a given media query is true. This helps the browser calculate which image to pick from srcset.
Currently, sizes="(min-width: 768px) 768px, 100vw" is telling the browser:
"The image will be 768px wide if the viewport is larger than 768px, otherwise the image will be full width when the viewport is less than 768px."
I assume you don't want to use a 1024px image when the viewport is less than 768px.
To hint at a small image when the viewport is less than 768px, use max-width: 768px instead:
<img
src="default.jpg"
srcset="default.jpg 768w, img/1024.jpg 1024w"
sizes="(max-width: 768px) 768px, 100vw"
/>