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#Do I need to use sizes when I use densities?
hollow crestBOT
No, you don't need to use the sizes property when you use densities in the Image component. The densities property is used to generate a list of pixel densities for the image and it will be used to generate a srcset attribute on the <img> tag. However, you should not provide a value for widths when using densities as only one of these two values can be used to generate a srcset.
Here is an example of using densities:
---import { Image } from 'astro:assets';import myImage from '../assets/my_image.png';---
<Image src={myImage} width={myImage.width / 2} densities={[1.5, 2]} alt="A description of my image."/>
This will output:
<img src="/_astro/my_image.hash.webp" srcset=" /_astro/my_image.hash.webp 1.5x /_astro/my_image.hash.webp 2x " alt="A description of my image." width="800" height="450" loading="lazy" decoding="async"/>
For more information, you can refer to the Astro documentation on densities (https://docs.astro.build/en/guides/images/#densities).
Note: you can ask me a follow up question by @ mentioning me again :speech_balloon:
opal narwhal
@hollow crest okay, why when I set densities to {[0.5, 1, 2]} my browser always generates the highest one even on small resolution like 300px
hollow crestBOT
Let me check the knowledge base for you. One moment! :books:
The densities attribute in Astro's Image component generates different versions of an image for different pixel densities, not for different screen resolutions. The browser chooses the image source based on the pixel density of the device's screen, not its resolution.
For example, a device with a pixel density of 2x (like many modern smartphones) will choose the 2x image even if the screen resolution is small. This is because the device can display more pixels in a given space, making images look sharper.
If you want to serve different images based on the screen resolution, you should use the widths and sizes attributes instead. The widths attribute generates different versions of the image for different widths, and the sizes attribute tells the browser how wide the image will be displayed at different screen resolutions.
Here is an example:
---import { Image } from 'astro:assets';import myImage from '../assets/my_image.png'; // Image is 1600x900---
<Image src={myImage} widths={[240, 540, 720, myImage.width]} sizes={`(max-width: 360px) 240px, (max-width: 720px) 540px, (max-width: 1600px) 720px, ${myImage.width}px`} alt="A description of my image."/>
This will output:
<img src="/_astro/my_image.hash.webp" srcset=" /_astro/my_image.hash.webp 240w, /_astro/my_image.hash.webp 540w, /_astro/my_image.hash.webp 720w, /_astro/my_image.hash.webp 1600w " sizes=" (max-width: 360px) 240px, (max-width: 720px) 540px, (max-width: 1600px) 720px, 1600px " alt="A description of my image." width="1600" height="900" loading="lazy" decoding="async"/>
In this example, the browser will choose the appropriate image source based on the screen resolution and the sizes specified.
For more information, you can refer to the Astro documentation on widths (https://docs.astro.build/en/guides/images/#widths).
Note: you can ask me a follow up question by @ mentioning me again :speech_balloon:
opal narwhal
okay, got it. Can I use just sizes? or i need to also privde widths
@hollow crest okay, got it. Can I use just sizes? or i need to also privde widths
hollow crestBOT
Let me check the knowledge base for you. One moment! :books:
Yes, you can use the sizes attribute in the Image component, but it should be used in conjunction with the widths attribute. The widths attribute is used to generate different versions of the image for different widths, and the sizes attribute tells the browser how wide the image will be displayed at different screen resolutions.
Here is an example:
---import { Image } from 'astro:assets';import myImage from '../assets/my_image.png'; // Image is 1600x900---
<Image src={myImage} widths={[240, 540, 720, myImage.width]} sizes={`(max-width: 360px) 240px, (max-width: 720px) 540px, (max-width: 1600px) 720px, ${myImage.width}px`} alt="A description of my image."/>
This will output:
<img src="/_astro/my_image.hash.webp" srcset=" /_astro/my_image.hash.webp 240w, /_astro/my_image.hash.webp 540w, /_astro/my_image.hash.webp 720w, /_astro/my_image.hash.webp 1600w " sizes=" (max-width: 360px) 240px, (max-width: 720px) 540px, (max-width: 1600px) 720px, 1600px " alt="A description of my image." width="1600" height="900" loading="lazy" decoding="async"/>
In this example, the browser will choose the appropriate image source based on the screen resolution and the sizes specified.
For more information, you can refer to the Astro documentation on widths (https://docs.astro.build/en/guides/images/#widths).
Note: you can ask me a follow up question by @ mentioning me again :speech_balloon:
opal narwhal
@hollow crest is there a way to generate these widths and sizes automatically? I would like to just add a "width"
hollow crestBOT
Let me check the knowledge base for you. One moment! :books:
Astro's <Image /> component can automatically infer the width and height of local images based on the original aspect ratio to avoid Cumulative Layout Shift (CLS). However, this automatic inference is not available for images stored in your public/ folder or for remote images unless you set inferSize to true.
Here is an example of using inferSize with a remote image:
---import { Image } from 'astro:assets';---
<Image src="<https://example.com/cat.png"> inferSize alt="A cat sleeping in the sun." />
In this example, Astro will fetch the original dimensions of the remote image and use them for the width and height.
However, for generating multiple versions of an image with different widths (srcset), you currently need to manually specify the widths and sizes attributes. Astro does not provide a way to automatically generate these values.