Generating a rainbow?

[Tobiah]

I'm having a difficult time with this. I want
to display a continuous range of hues using HTML
hex representation (#RRGGBB). How would I go
about scanning through the hues in order to
make a rainbow?

Thanks,

Toby

[Chris Colbert]

> --
> http://mail.python.org/mailman/listinfo/python-list
>

In [43]: possible = []

In [44]: for i in range(2**8):
....: h = hex(i).lstrip('0x')
....: while len(h) < 2:
....: h = '0' + h
....: possible.append(h)
....:
....:

In [45]: full = [r + g + b for r in possible for g in possible for b
in possible]

In [46]: len(full)
Out[46]: 16777216

In [47]: 2**24
Out[47]: 16777216

[Richard Thomas]

Look at the colorsys module.

http://docs.python.org/library/colorsys.html?highlight=colorsys#module-colorsys

[Tobiah]

> Look at the colorsys module.
>
> http://docs.python.org/library/colorsys.html?highlight=colorsys#module-
colorsys

That so rocks. Thanks!

[Chris Colbert]

> --
> http://mail.python.org/mailman/listinfo/python-list
>

How does that answer your original question?

[Gary Herron]

Use the hue-saturation-value color space, and call hsv_to_rgb from the
standard Python library to convert to RGB. Enjoy!

Gary Herron

from colorsys import hsv_to_rgb

for hue ....:
rgb = hsv_to_rgb(hue, saturation, value)

Let 'hue' run from 0 (red) through 2/3 (blue) Hues from 2/3 to 1 get
into purples and magentas, which are not spectral (i.e., rainbow) colors.

Set 'saturation' to perhaps 0.5 (for a washed out effect) through 1.0
(for pure color). Even an intensely colorful rainbow has lots of white
light mixed in with it; a saturation of 0.5 is probably good.

Set 'value' to something in the range of 0 to 1 to control brightness.

[Tobiah]

> How does that answer your original question?

I was able to do this:

import colorsys

sat = 1
value = 1
length = 1000
for x in range(0, length + 1):
hue = x / float(length)
color = list(colorsys.hsv_to_rgb(hue, sat, value))
for x in range(3):
color[x] = int(color[x] * 255)
hexval = ("#%02x%02x%02x" % tuple(color)).upper()
print "<div style='height: 1; width: 500; background-color: %s'>"
% hexval

http://tobiah.org/rainbow.html

[Peter Parker]

Roy G. Biv would like you to order the colors according to their
wavelength. He would also like to see Orange and Yellow appear in
your rainbow.

[Neil Hodgson]

Tobiah:

> for x in range(0, length + 1):

> ...
> for x in range(3):

You should use different variables for the two loops.

Neil

[Neil Hodgson]

Me:

> You should use different variables for the two loops.

Actually it is closing the divs that makes it work in FireFox:

import colorsys

sat = 1
value = 1
length = 1000

for h in range(0, length + 1):
hue = h / float(length)

color = list(colorsys.hsv_to_rgb(hue, sat, value))
for x in range(3):
color[x] = int(color[x] * 255)
hexval = ("#%02x%02x%02x" % tuple(color)).upper()

print(

"<div style='height: 1; width: 500; background-color: %s'>"

"</div>" % hexval)

[Chris Colbert]

> --
> http://mail.python.org/mailman/listinfo/python-list
>

I see. It appears I misunderstood the question.

The link doesn't work in Chrome btw.

[Tobiah]

>> You should use different variables for the two loops.
>
> Actually it is closing the divs that makes it work in FireFox:
>

Hah. I new that the rainbow wasn't complete and that it
didn't work in Opera. I just fizzled on the closing of the
divs.

I also don't get why it worked at all when I was stomping
on my x variable in the inside loop.

It's better now.

http://tobiah.org/rainbow.html

[Peter Parker]

Better than before ... Oy G. Bivr

[Robert Kern]

Uh, the first color is #FF0000, pure red, not orange.

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

[Peter Parker]

Robert Kern wrote:
> On 2010-04-11 12:54 , Peter Parker wrote:
>> Tobiah wrote:
>>>>> You should use different variables for the two loops.
>>>> Actually it is closing the divs that makes it work in FireFox:
>>>>
>>>
>>> Hah. I new that the rainbow wasn't complete and that it
>>> didn't work in Opera. I just fizzled on the closing of the
>>> divs.
>>> I also don't get why it worked at all when I was stomping
>>> on my x variable in the inside loop.
>>>
>>> It's better now.
>>>
>>> http://tobiah.org/rainbow.html
>>
>> Better than before ... Oy G. Bivr
>
> Uh, the first color is #FF0000, pure red, not orange.
>

Ah, you're right, but it's only 1 pixel high so it's hard to
see. The last one is also #FF0000, pure red ... Roy G. Bivr

[Giacomo Boffi]

Tobiah <to...@rcsreg.com> writes:

if you mean real rainbows when you say "rainbow", as rainbows go from
low to high wavelengths and you have to specify colors to your display
in RGB, i think that you have to read, e.g.,

http://www.physics.sfasu.edu/astro/color.html

otoh if you mean simply a continuos palette with varying hues,
excellent answers were already posted

hth
g
--
compro mobili vecchi - vendo mobili antichi