[PNG icon]

PNG Transparency

Screenshots from WebTV

These are half-scale JPEG screen shots of the indicated pages. They were taken with a Bt878-based video-capture card connected to a Philips WebTV Internet Terminal--i.e., the box that comes with WebTV Classic service. Because the box's output is analog (NTSC video) and therefore noisy at the bit level, the full-scale linked images are also high-quality JPEGs rather than larger (much larger) PNGs. The first three linked images are 343087, 152212 and 133776 bytes, respectively.

[pngs-img.html half-scale screen shot (78k)]
[inline-alpha.html half-scale screen shot (33k)]
[inline-alpha-table.html half-scale screen shot (35k)]

WebTV's handling of PNG transparency in the 32-bit RGBA case is acceptable; it uses nine discrete levels of transparency (including fully transparent and fully opaque) rather than 256, as can be seen in the two screen shots on the right. Its handling of palette transparency is much poorer. It appears to support only binary transparency, and that only some of the time (for example, in the owl and redbrush images but not in the magnolia or icicles ones). The problem is not related to the subject matter itself; in the larger versions of the images, the redbrush flower retains its transparency, but the owl loses its, and the icicles gain a tiny amount:

[png-MagnoliaAlpha.html half-scale screen shot (17k)]
[png-IceAlpha.html half-scale screen shot (17k)]
[png-OwlAlpha.html half-scale screen shot (12k)]
[png-RedbrushAlpha.html half-scale screen shot (15k)]

The magnolia tree (or, more specifically, its background) remains opaque, which is incorrect. Stephan Litke's simple-transparency test page indicates the problem: WebTV apparently supports palette transparency if and only if it is associated with the first palette entry, as demonstrated by the image on the left:

[idx-png-trnsp-tst.html half-scale screen shot (17k)]
[gamma_test half-scale screen shot (13k)]

On the other hand, WebTV does implement gamma correction (with a value slightly less than 1.0), as seen in the image on the right.

Here are some related PNG pages at this site:

[primary site hosted by SourceForge] Last modified 14 March 2009.

Copyright © 2000-2009 Greg Roelofs.