Eyes Above The Waves

Daniel Glazman is concerned about the behaviour of our zoom implementation. In particular he's bothered by the fact that zooming can change the layout of a page.

This is intentional. There are actually two ways you can implement zooming:

1. Just scale everything by the zoom factor when drawing, and leave the page layout unchanged. This is what Daniel wants.
   
2. Try make the page look as good as possible when zooming, by changing the layout as necessary. This is what we have implemented.
   

One key difference between the approaches is the way text hinting is handled. When you draw text at a certain size, a good font engine will "hint" the glyphs, e.g. make sure that the vertical lines of an "n" are aligned on pixel boundaries, and this can change glyph metrics. For example, the width of a hinted "n" in 20pt is not necessarily twice the width of "n" in 10pt. So, if we want text to look its best, we must allow the advance width of text to change non-linearly when we change the zoom factor, and that can change layout in arbitrary ways. If we insist on preserving layout, we have to have to use unnatural glyph advance widths, which can't look as good.

Another difference is the handling of native widgets. They may not look good or work well when scaled to odd sizes, so we might not scale them or scale them differently, if we're free to modify the layout.

Another difference is how the available width is handled. If you insist on preserving layout when zooming in, you will almost always force the user to scroll horizontally to see all the content, or when zooming out in a desktop browser, the full width of the window will not be used.

Furthermore as a practical matter we currently have no foolproof way to scale rendering of plugins without actually changing their size, so we can't implement approach 1 for plugins.

For all these reasons I think exposing the second zooming approach in the Firefox 3 UI is the right way to go.

However, Daniel is absolutely right to point out that this is not what you want for a "zoom out and pan" UI in a mobile browser --- or a "magnifying glass" UI, for that matter! For those, it is important to preserve layout. Fortunately, implementing them is easy in principle, now that we're based on cairo; just scale the transform when drawing. There are a few problems, such as handling plugin rendering as I mentioned, but those can be hacked around or fixed. Some work would need to be done to expose this zoom mode as an additional prescontext/presshell API. I'm sure that in the future we'll offer both modes (to embedders and XUL apps, if not to Firefox users). (You can actually mock up this approach today by embedding an <iframe> in an SVG <foreignobject> and scaling that, although there are some problems.)