what does := means simply?

[bartc]

On 20/05/2018 01:39, Dennis Lee Bieber wrote:
> On Sat, 19 May 2018 23:14:08 +0100, bartc <bc at freeuk.com> declaimed the
> following:
> 
> 
>> The comments and examples here:
>> https://en.wikipedia.org/wiki/Netpbm_format, and all actual ppm files
>> I've come across, suggest the 3 parts of the header (2 parts for P1/P4)
>> are on separate lines. That is, separated by newlines. The comments are
>> a small detail that is not hard to deal with.
>>
> 
> 	Wikipedia is not a definitive document...
> 
> http://netpbm.sourceforge.net/doc/ppm.html has
> """
> Each PPM image consists of the following:
> 
>      A "magic number" for identifying the file type. A ppm image's magic
> number is the two characters "P6".
>      Whitespace (blanks, TABs, CRs, LFs).
>      A width, formatted as ASCII characters in decimal.
>      Whitespace.
>      A height, again in ASCII decimal.
>      Whitespace.
>      The maximum color value (Maxval), again in ASCII decimal. Must be less
> than 65536 and more than zero.
>      A single whitespace character (usually a newline).
> """

I think if you are going to be generating ppm, then the best choice of 
format, for the widest acceptance, is to separate the header groups with 
a newline. (As I mentioned my downloaded viewer needs a new line after 
the first group. My own viewer, which I only threw together the other 
day to test that benchmark, also expects the newlines. Otherwise I might 
need to do 5 minutes' coding to fix it.)

(Regarding those benchmarks 
(https://benchmarksgame-team.pages.debian.net/benchmarksgame/performance/mandelbrot.html), 
as far as I can tell every language generates the ppm file inline (no 
special ppm library), and they all generate the P4 signature on one line 
and width/height on the next line.

(Click on any source file and look for "P4". Most do it with less fuss 
than Python too.))

> 	That all the ones you've seen have a certain layout may only mean that
> the generating software used a common library implementation:
> http://netpbm.sourceforge.net/doc/libnetpbm.html

Blimey, it makes a meal of it. I got the impression this was supposed to 
be a simple image format, and with the line-oriented all-text formats it 
was.

But it could be worse: they might have used XML.

-- 
bartc