How to grayscale a gif using bit-swapping

[Lars Erik Bryld]

Hello group

I am trying to convert a (supposedly) working python script into php.
The script reads a colour gif-picture and produces a greyscale copy of
it.

The method is to first copy the gif header, then establish the size of
the colour palette, then do a simple averaging of each colour byte
triplet into a grayscale equivalent, and finally to copy the remaining
part of the gif file unaltered over into the new file. I believe, it's
the last procedure that goes wrong, partly because of my inability to
understand the file manipulation issues.


The original Python script:

header = f.read(13)
o.write(header)
for i in range (1 << (( ord(header)[10]) & 7) + 1)):
gray = (ord(f.read(1) + ord(f.read(1) + ord(f.read(1)))/3
o.write(3*chr(g ray))
o.write(f.read( ))


My feeble and as yet non-functional PHP translation:

$fi = fopen("in.gif", 'r');
$fo = fopen("out.gif" , 'w');

$header = fread($fi, 13);
fwrite($fo, $header);

$l = (1 << (ord($header[10]) & 7) + 1);

for ($i = 0; $i <= $l; $i++) {
$gray = (ord(fread($fi, 1)) + ord(fread($fi, 1)) + ord(fread($fi,
1)))/3;
fwrite($fo, 3*chr($gray));
}

$rest = fread($fi, 999999999999999 99999);
fwrite($fo, $rest);

fclose($fi);
fclose($fo);




I hope some kind spirit will point out any blatant errors in my use of
the functions above. I only get small 2-300 byte long output files
regardless of input file size.


--
Regards
Lars Erik Bryld

[Kimmo Laine]

Re: How to grayscale a gif using bit-swapping

"Lars Erik Bryld" <larserik@dadln et.invalidwrote in message
news:31ddofla5n 3x.dlg@lebryld. fqdn...

Hello group
>
I am trying to convert a (supposedly) working python script into php.
The script reads a colour gif-picture and produces a greyscale copy of
it.
>
The method is to first copy the gif header, then establish the size of
the colour palette, then do a simple averaging of each colour byte
triplet into a grayscale equivalent, and finally to copy the remaining
part of the gif file unaltered over into the new file. I believe, it's
the last procedure that goes wrong, partly because of my inability to
understand the file manipulation issues.
>
>
The original Python script:
>
header = f.read(13)
o.write(header)
for i in range (1 << (( ord(header)[10]) & 7) + 1)):
gray = (ord(f.read(1) + ord(f.read(1) + ord(f.read(1)))/3
o.write(3*chr(g ray))
o.write(f.read( ))
>
>
My feeble and as yet non-functional PHP translation:
>
$fi = fopen("in.gif", 'r');
$fo = fopen("out.gif" , 'w');
>
$header = fread($fi, 13);
fwrite($fo, $header);
>
$l = (1 << (ord($header[10]) & 7) + 1);

Have you tested that this actually gives the correct value? Just that it
looks a bit.. hmm.. error-bound? :) Although it does seem to be the same as
the original...

>
for ($i = 0; $i <= $l; $i++) {
$gray = (ord(fread($fi, 1)) + ord(fread($fi, 1)) + ord(fread($fi,
1)))/3;
fwrite($fo, 3*chr($gray));

If the goal here is to write the same value three times (for r,g,b) you want
str_repeat(chr( $gray),3). 3*chr($gray) will give you unexpected results.

}
>
$rest = fread($fi, 999999999999999 99999);
fwrite($fo, $rest);
>
fclose($fi);
fclose($fo);
>

--
"Ohjelmoija on organismi joka muuttaa kofeiinia koodiksi" - lpk
http://outolempi.net/ahdistus/ - Satunnaisesti päivittyvä nettisarjis
spam@outolempi. net | rot13(xvzzb@bhg byrzcv.arg)

[C.]

Re: How to grayscale a gif using bit-swapping

On 21 Mar, 18:27, Lars Erik Bryld <larse...@dadln et.invalidwrote :

>
I am trying to convert a (supposedly) working python script into php.
The script reads a colour gif-picture and produces a greyscale copy of
it.
>

<snip>

It would be safer to use GD. There's even an example of how to do this
at http://www.php.net/manual/en/function.imagecolorset.php

(FWIW, both the example above and your own code use an averaging
method to determine the grey scale; while color models vary, it might
be more approriate to measure the size of the orthognal vectors -
(integer)(sqrt( $red*$red + $green*$green + $blue*$blue)*0. 5769)
)

C.

[Lars Erik Bryld]

Re: How to grayscale a gif using bit-swapping

Scripsit Kimmo Laine:

>$l = (1 << (ord($header[10]) & 7) + 1);

>
Have you tested that this actually gives the correct value? Just
that it looks a bit.. hmm.. error-bound? :) Although it does seem
to be the same as the original...

Tested it, at it seems to work. Most GIFS produce the value 256, and a
B/W GIF produces 2 as it should.

If the goal here is to write the same value three times (for r,g,b)
you want str_repeat(chr( $gray),3). 3*chr($gray) will give you
unexpected results.

Tried yoru suggestion, but same result, alas.

>$rest = fread($fi, 999999999999999 99999);
>fwrite($fo, $rest);
>>
>fclose($fi);
>fclose($fo);

This is what I suspect is not doing what I thought it would.


--
Med venlig hilsen
Lars Erik Bryld

[Lars Erik Bryld]

Re: How to grayscale a gif using bit-swapping

Scripsit C.:

It would be safer to use GD. There's even an example of how to do
this at http://www.php.net/manual/en/function.imagecolorset.php

For what it's worth, I'd rather find out why my own little script
doesn't do as intended and thus possibly get a little smarter in the
process of tweaking it into doing what I want.

I'm not really trying to greyscale images, I'm trying to learn gif
palette manipulation.

--
Regards
Lars Erik Bryldi

[Kimmo Laine]

Re: How to grayscale a gif using bit-swapping

spam@outolempi. net | rot13(xvzzb@bhg byrzcv.arg)
"Lars Erik Bryld" <larserik@dadln et.invalidwrote in message
news:ifoxx8z4ay e5.dlg@lebryld. fqdn...

Scripsit Kimmo Laine:
>

>>$l = (1 << (ord($header[10]) & 7) + 1);

>>
>Have you tested that this actually gives the correct value? Just
>that it looks a bit.. hmm.. error-bound? :) Although it does seem
>to be the same as the original...

>
Tested it, at it seems to work. Most GIFS produce the value 256, and a
B/W GIF produces 2 as it should.
>

>If the goal here is to write the same value three times (for r,g,b)
>you want str_repeat(chr( $gray),3). 3*chr($gray) will give you
>unexpected results.

>
>
Tried yoru suggestion, but same result, alas.
>

>>$rest = fread($fi, 999999999999999 99999);
>>fwrite($fo, $rest);
>>>
>>fclose($fi) ;
>>fclose($fo) ;

>
This is what I suspect is not doing what I thought it would.
>

Try this instead:
while(!feof($fi ) ){
fwrite($fo, fread($fi, 256));
}


--
"Ohjelmoija on organismi joka muuttaa kofeiinia koodiksi" - lpk
http://outolempi.net/ahdistus/ - Satunnaisesti päivittyvä nettisarjis

[Lars Erik Bryld]

Re: How to grayscale a gif using bit-swapping

Scripsit Kimmo Laine:

>>>$rest = fread($fi, 999999999999999 99999);
>>>fwrite($fo , $rest);
>>>>
>>>fclose($fi );
>>>fclose($fo );

>>
>This is what I suspect is not doing what I thought it would.
>>

>
Try this instead:
while(!feof($fi ) ){
fwrite($fo, fread($fi, 256));
}

Worked like a charm, thanks. Turned out that PHP balked at the
9999999999999 part.

--
Med venlig hilsen
Lars Erik Bryld