Computing desk
< December 2	<< Nov \| December \| Jan >>	December 4 >

Welcome to the Wikipedia Computing Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

December 3[edit]

RGBA hex interpretation[edit]

Why is the RGB colour #00000080 interpreted as black at exactly 0.5 opacity (at least by Firefox)? #…7f is treated as 0.498 ≈ 127⁄255 and #…81 as 0.506 ≈ 129⁄255, for what it's worth. Jc86035 (talk) (notify me) 14:01, 3 December 2017 (UTC)[reply]

The answer is "it shouldn't be" - 0x80 should be 0.502 (128⁄255). This is the official W3C specification, linked from this page on the Mozilla site, and neither say anything about a special case for 0x80. See also RGBA color space. Perhaps a bug report to the Firefox developers is in order? Tevildo (talk) 20:18, 3 December 2017 (UTC)[reply]

It's very likely that the implementation of Alpha compositing uses hardware-accelerated, low-bit-depth (e.g. 8 or 16 bits) math for intermediate results, which could yield all types of varieties of rounding or loss of precision. Here's some more on layer compositing in Gecko, the engine used by Mozilla Firefox. Casually glancing at that system schematic diagram, I see about six zillion places where a minor math or rounding error might cause a small but non-zero error in the final pixel value.

If I had a dollar for every time I've seen a professional image-software programmer make a single-bit- numerical error in a compositing- or color space conversion- operation that resulted in a user-visible effect, ...I'd have a lot of dollars. Attention, all programmers: today's a good day to review the mathematics in error analysis and error propagation!

As Tevildo suggests, a bug-report is in order: it won't be the first - accelerating the graphical operations in Gecko creates havoc ranging from minor glitches to catastrophic layout problems to total machine-failures - and those are just my cursory samplings of the bugs reported this week!

Nimur (talk) 02:50, 4 December 2017 (UTC)[reply]

Another level of 3D experience[edit]

(I posted this question at the Entertainment RD, but I'll continue it here.)

Looking for a digital 3D CGI animation video format where I can hover the animating object over (similar to ZygoteBody, but I can freely move to any angle including top and bottom). It will be great if it does not rely on Adobe Flash. JSH-alive/^{talk/cont/mail} 16:31, 3 December 2017 (UTC)[reply]

Preview / Mac OS provides this function with some file formats (eg .dae / Collada). The WP article lists a few more. --Cookatoo.ergo.ZooM (talk) 16:47, 4 December 2017 (UTC)[reply]

Can anyone recommend me a Windows equivalent or web browser-based ones? And it should have a time bar, so I'm able to skip forward or go back. JSH-alive/^{talk/cont/mail} 17:10, 6 December 2017 (UTC)[reply]