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If the browser downscales a large image, will it display the same amount of detail on different screen resolutions?

This question relates to web development.

I'm going to start by apologizing for the horrible title but I wasn't sure how to title this. CSS pixels and how they interact with screen resolutions confuse me a bit already, when you add "intermediate pixel layers" and image drawing it's even more nebulous. So here's my actual question:

Let's say that we have an image set to display as 2x1 (css pixels) on a web page. And we instead feed it a 4x1 image (rasterized). The browser will fit this image to the 2x1 css pixels we specified earlier. But, would we technically be able to see all 4x1 pixels on a setup where 1 CSS pixels = 2 device pixels? Or would it instead resize the 4x1 image to 2x1 then display each pixel twice? And does this change on a per browser/device basis?

Bonus points: How does this play into accessibility tools that zoom into web content for the visually impaired? (if at all).

Solution

  • Answering my own question. The short answer is YES browsers will show different amounts of detail based on screen resolution.

    CSS pixels ARE NOT screen pixels. If an image is 600x600 pixels and you decide to display it in a CSS-defined area of 300x300 (img:{width:300px;height:300px}), it could display as 300x300 screen pixels or it could display as 600x600 screen pixels. Which of the two it is will depend on the end user's OS screen resolution.

    Example:

    I created a 4x1 pixel png image:

    4 by 1 pixel image

    I added this image to an html page and sized it with a img:{width:2px; height:1px;}. For good measure, I also added a blue div with div:{width:2px; height:1px;} bellow it.

    I then set the "css resolution" of the page to half that of my OS screen resolution (If the OS was set to 2000x2000 I made sure that the full html page had a css widthxheight of 1000x1000px). So each css pixel would contain 4 screen pixels.

    This is how it displayed:

    enter image description here

    Both the image and the blue div are 2px wide (that's css pixels) but they display as 4 screen pixels. In the case of the blue div, it duplicates the pixels to fit the screen resolution. But in the case of the image, it displays all 4 individual pixels. In both cases, it duplicates the pixels vertically to fit the screen resolution (technically it's probably more of a stretch than a duplication, but you get the idea).

    I'm not entirely sure about this but I'm assuming at this stage that all browsers act in a similar fashion.