[PD-dev] pd file format: color settings

[Robert Schwarz]

Thanks for the quick answer.

The concept of embedding three 8 bit components in one integer was clear
to me, but I think that pd doesn't really use all 8 bits for the colors.
Or maybe there is some issue with 2-complements or something.

For example, if I want to create three bang objects, in red (#ff0000),
green (#00ff00) and blue (#0000ff), your formula gives values of:
16711680, 65280, 255 for the three colors.

But I insert them in a patch, like:

#N canvas 825 10 450 300 10;
#X obj 0 0 bng 15 200 50 0 target empty empty 0 0 0 8 16711680 0 0 ;
#X obj 0 15 bng 15 200 50 0 target empty empty 0 0 0 8 65280 0 0 ;
#X obj 0 30 bng 15 200 50 0 target empty empty 0 0 0 8 255 0 0 ;

I see the colors white, white, yellow.

Now, when I change the colors by hand, to really get red, blue and green
on the bang objects and save the file, it reads:

#N canvas 825 10 450 300 10;
#X obj 0 0 bng 15 200 50 0 target empty empty 0 0 0 8 -258049 -1 -1 ;
#X obj 0 15 bng 15 200 50 0 target empty empty 0 0 0 8 -4033 -1 -1 ;
#X obj 0 30 bng 15 200 50 0 target empty empty 0 0 0 8 -64 -1 -1 ;

So it uses negative numbers, and -64 means "full blue".
Now, when I re-open the same file and look at the properties of the blue
bang object, the color now reads: #0000fc instead of the #0000ff I
entered just before saving.

That's why I suspect some lower resolution going on. I tried to browse
this part in the sources, but all the GUI code confuses me.

For your interest, this patch results for colors of #040000, #000400 and
#000004 set by hand in the properties window:

#X obj 0 0 bng 15 200 50 0 target empty empty 0 0 0 8 -4097 -1 -1;
#X obj 0 15 bng 15 200 50 0 target empty empty 0 0 0 8 -65 -1 -1;
#X obj 0 30 bng 15 200 50 0 target empty empty 0 0 0 8 -2 -1 -1;

Setting colors to lower values, like #010000 results in getting them
rounded down to #000000.

So, the resolution is apparently 256/4 = 64 values, or 6 bits.

Indeed, if I replace the formula with:

color = (-([red]+1)/4*64*64) - (([green]+1)/4*64) - ([blue]+1)/4

I get the same values that Pure Data produces.
Hm, I might just have solved my problem.

It's still weird and some developer could check this our or change the
documentation.

Cheers, Robert

On 02/13/2010 11:08 PM, Martin Peach wrote:
> That formula should read:
>  color = ([red] * 65536) + ([green] * 256) + ([blue])
> In binary the idea is to shift the 8 'red' bits 16 to the left, then add
> 8 'green' bits shifted 8 bits, and finally 8 'blue' bits, so in all 24
> bits are occupied.
> Multiplying the blue value by -1 in the original formula has the effect
> of setting the 16 bits to the left of it to 1, so you get different
> shades of pure blue.
> 
> Martin
> 
> Robert Schwarz wrote:
>> Hi all,
>>
>> I recently tried writing patches in a text editor (or from scripts) and
>> had problems getting the color settings right, for bang elements.
>>
>> There is some documentation at
>> http://puredata.info/docs/developer/fileformat
>> with the explanation:
>>
>>> Color: Some graphical elements have color attributes. Per color only
>>> one signed integer value is stored that contains the three 8-bit
>>> color components (RGB). Formula to calculate color attribute values:
>>>
>>> color = ( [red] * -65536) + ( [green] * -256) + ( [blue] * -1)
>>>
>>> Where [red], [green], [blue] obviously represent the three color
>>> components, their values range from 0 to 255. They apply to the
>>> attributes [background color], [front color], [label color] of
>>> various elements.
>>
>> I tried that, but it didn't work. Instead of showing the whole spectrum
>> I just got different shades of blue. Also, when I opened one of my
>> handwritten patches in PureData, looked at the color settings and saved,
>> the resulting numbers changed. I assume that some kind of rounding is
>> happening, and colors are actually saved in lower resolution.
>>
>> Do you have any ideas?
>>
>> Also, my application is a 13x13 button matrix, each triggering different
>> chords via MIDI. The buttons should be color coded. Obviously, it's too
>> much work setting all colors individually and I might want to create
>> several of these patches with different colors.
>> Maybe there is another obvious solution I didn't see.
>>
>> Any help is appreciated!
>>
>> (I'm using standard pd 0.42_5 on Arch Linux, but this shouldn't make a
>> difference.)
>>
> 

-- 
Robert Schwarz <mail at rschwarz.net>

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