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unable to scroll in overlay if Trans Background set to True... is this a feature?
#1
I have an overlay container in the center of my interface that I load my screens into, some of my screens have been over-sized horizontally or vertically so they can be scrolled/swiped.  However when I set the overlay container to use a transparent background, its stopping my ability to scroll on that page.  I need to set this to transparent so the image I am displaying in the center of the overlay container behind the screen being displayed.

So just wondering if this is expected behavior and if there may be a workaround that may allow me still achieve the ability to scroll/swipe those pages while still keeping that background image centered.


thanks!

-Todd


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#2
Yeh, that's the way it works. The reason being that Windows doesn't really support drawing transparently. Images are really the only thing that can be moved transparently across a background. Text can be drawn 'transparently' but it is anti-aliased to make it blend better with the background and those edge pixels are based on a mixture of the color of the text and the background content it is drawn over. If you try to draw some text and then move it across a background it will look really bad because those edge pixels will not just not blend they will make it look really ratty.

That would mean that all the content would have to be redrawn every time the scroll position moves. That's not really practical from a performance standpoint, so it's just disabled for overlays. You can only scroll the content of overlays against a flat background, or a gradient that doesn't change in the direction of travel. I.e. a vertical gradient doesn't change left to right, so you can scroll horizontally over it.

Some of the widgets support scrolling over a transparent background, because in those cases they only have to understand how to scroll their specific content. So list browsers and tool bars can do it. Even then, the text may not look great if there's too much contrast change in the background over which it travels. For an overlay to support that, every single widget type would have to be able to draw its contents transparently, which would be a lot of work and it would still look quite bad.

Some day I'd like to transition the IV to DirectShow, which is designed to do these types of things inherently. You can create things in layers and composite them. I guess in theory, once we drop the old RIVA clients, that could be done because I think that the HTML5 canvas (being based on graphics engines on each platform) could support it. But it would be a lot of work and an ongoing thing. It would open the door for some spectacular interfaces though.
Dean Roddey
Software Geek Extraordinaire
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