A bug & note on using .png graphics....

[Fred Ridder]

Commenting only on specific issues...


>Art Campbell <art.campbell at gmail.com> wrote (in part):

>>Color depth is generally specified as "bits per pixel (bpp)" -- see this
>>Wikipedia topic:
>>
>>http://en.wikipedia.org/wiki/Color_depth
><snip>
>
>Yes, the Wikipedia article seems to verify what I was saying; that the same 
>file
>can be referred to a 8-bit or 24-bit and both terms can be correct,
>depending on how you
>apportion the bits.

I disagree. Whenever the Wikipedia article refers to images or colors,
it refers to the total number of bits per pixel.  The only times it uses
the terminology you favor it *always* refers to "bits per channel".
And one of the reason why PhotoShop uses the bits per channel
in its menus is that it can deal with either RGB (3 channels per pixel)
or CMYK (4 channels per pixel) color spaces.

>http://www.luminous-landscape.com/tutorials/bit-depth.shtml, among
>other industry sources ...

If this website is one of the places you have been learning from, then
perhaps we have identified the root source of some of the issues. Even
on a quick reading, this page contains some things that are laughably
inaccurate. For one example:

   A Short Digression

   This is why you don't want (for example) to print a B&W image on
   an inkjet printer using just black ink. The printer would only be able
   to lay down 256 shades of gray, from black to white — not nearly
   enough for a decent image. Instead you should print using colour
   inks as well, which means that all three primary colours (Red, Blue
   and Green) will be mixed together to create 16 million shades of
   gray (256X256X256). More than enough.

Even if printers *did* print gray using red, blue, and green inks (which
is not the case), 8-bit per channel color depth is still only capable of
producing 256 shades of gray, *NOT* 16 million. The reason for this
is simple: only colors where R=G=B are (nominally) gray, and there
are only 256 such triplets. If any of the color intensities is not equal
to the others, you get some color other than gray. "16 million shades
of gray" is simply an absurd concept.

My opinions only; I don't speak for Intel.
Fred Ridder
Intel
Parsippany, NJ

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