How to use image qualifiers in WebView in Windows 8 without specifying height and width

Question

I have a webview in my Windows 8 (Metro) app that I will use to display much of my content. The webview automatically scales any CSS dimensions to 100%, 140%, and 180%. This means that when I specify:

#square {
    width:100px;
    height:100px;
    background-color:white;
}

...we get a nice square that is 100, 140, or 180 device pixels, depending on the display. So far, so good.

Further, if I supply an image that is 100px square, the OS correctly scales it to 140 and 180 as appropriate on higher density screens.

Further still, if I supply versions of the image that are 100px, 140px, and 180px, and I indicate the size as 100px in the CSS, like this:

#my_image {
    width:100px;
    height:100px;
}

The OS uses an area that is 100 dp square (that is to say, 100, 140, or 180 device pixels square as appropriate) and automatically selects the right image. So far, still good.

The problem occurs when I try to use images with density qualifiers without naming a literal size in CSS. Why would I want to do this? I have lots of images with variable sizes. I'd prefer to just allow the webview to infer the appropriate size based on the dimensions of the images.

So I expect that if I supply 100, 140, and 180 versions of an image, the OS will be smart enough to say, "Ah, this is a 100-dp image that happens to have additional versions available."

What actually happens, however, is this.

I supply images:

square.scale-100.png
square.scale-140.png
square.scale-180.png

The OS picks the appropriate one. On a 180% screen, it picks the version that is 180 device pixels square. Recall, however, that we made it 180 device pixels because it was the 180% version of the 100 dp image. We want it to actually take up only 100x100 dp of space.

However, the OS takes 180 as the size in DP. So it scales it by 180% again.

How can I avoid this double-scaling? Any pointers would be awesome!

Brian Rak Brian Rak · Accepted Answer · 2014-04-17T01:59:38

We've figured out a solution to this that I'm sharing in case it helps anyone else.

The solution is to include dynamically written CSS that zooms images using a factor that is the inverse of the current scale factor.

The current scale factor is available to app code, but not to Javascript running in a WebView. From the point of view of such Javascript, all dimensions are already scaled. So, for example, window.devicePixelRatio is no good -- it always returns 1.

You can obtain the current scale factor in app code like so:

ResolutionScale scale = DisplayInformation.GetForCurrentView().ResolutionScale;

From this, you can derive a zoom factor like this:

float imageZoomFactor = 1.0F;
switch (DisplayInformation.GetForCurrentView().ResolutionScale)
{
    case ResolutionScale.Scale140Percent:
        imageZoomFactor = (1.0F / 1.4);
        break;

    // etc
}

You can pass this into your Javascript in one of several ways. Since we were reading HTML from the app bundle and using WebView.NavigateToString, we created a placeholder in the Javascript that we replace in the string. The Javascript looks like this:

<!-- Scale images so that their sizes map 1:1 to device pixels -->
<script type="text/javascript" language="javascript">
    document.write('<style type="text/css">img {zoom:' + HOSTING_CODE_SUPPLIED_ZOOM_FACTOR + ';}</style>');
</script>

Now when you use an image tag without specifying explicit dimensions, it works properly. For example, an image that is 180px x 180px, created for the purpose of being displayed at 100dp x 100dp in 180% mode, is displayed correctly using 180 device pixels.

One caveat is that if you have CSS that explicitly sizes an image, you need to cancel the zoom. So, for example:

#my_explicit_image { 
    width:200px;
    height:200px;
    zoom:normal;    /* cancel the zoom on img since we're being explicit */
}

If you don't include the last line, the zoom specified on img will apply to the dimensions you specify here.

Hope this helps someone out there!

How to use image qualifiers in WebView in Windows 8 without specifying height and width

1 Answers