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 Posted: Jun 17, 2009 - 08:05 PM |
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Joined: Oct 29, 2007
Posts: 134
Location: Como, Italy
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Hello,
I'm playing with an RGB led. I use a Tiny85 with 3 hw PWM. I created a struct that defines the current state of the led (R, G, B values). Then I can set the next color and the time it needs to reach that. The routine change the color softly with a linear interpolation.
Two questions:
1) I know it's better to use a linear control of brightness in color mixing application. Anyway, I'd like to try an exponential control. I can map the whole exp curve in flash as an array[0..255] but I'm wondering if there is a better way to achieve that.
2) To random the color I random each channel, but it's not very nice. There are a lot of bright colors and few pure ones. Do you know any smart way to get a cute effect?
Thanks
Marco |
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Posted: Jun 17, 2009 - 08:13 PM |
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Joined: Jan 12, 2002
Posts: 7004
Location: Canada
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if by "pure" colour you mean colours that are 100% intensity of any combination of the 3 primaries, including a single primary. Then simply generate 3 random bits, each bit represents one of the primaries. Then generate a random time. Now run the fade from the current colour, to the new "pure" colour over the specified time. And then repeat.
If you want more complex "pure" colours, create a palette of these colours, and then randomly select the entry, and again simply fade to it, and repeat.
As for the exponential mixing, you'll need at least 12 bits of output, for the 8 bit input. Look-up table works well for this. Though if you have plenty of time between lookups, simply calculate it by brute force. |
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Posted: Jun 17, 2009 - 09:57 PM |
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Joined: Oct 30, 2002
Posts: 3922
Location: The Netherlands
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I fairly recently made a 'color wheel' with an RGB LED, and linear intensity control does not work properly. The difference in perceived brightness from 40 to 50% is much bigger then from 80-90%; so you need gamma correction, I believe it's around 0.76 for LEDs.
I calculated the required RGB intensity from hue, saturation and brightness, with the latter two being fixed to maximum.
My goal was to end up with a LED that has fixed brightness but only changing colours; it did not work, but my 1 year old daughter likes it anyway 
Anyway, this is the code I wrote for a T85:
Code:
hi=hue/60; // Hue varies between 0 and 359
f=(hue % 60)*425/100;
q=(255-f);
switch(hi)
{
case 0: r=255; g=f; b=0; break;
case 1: r=q; g=255; b=0; break;
case 2: r=0; g=255; b=f; break;
case 3: r=0; g=q; b=255; break;
case 4: r=f; g=0; b=255; break;
case 5: r=255; g=0; b=q; break;
}
OCR0B=255-r;
OCR1B=255-g;
OCR0A=255-b;
I would use a simple lookup table, you can generate these easily in a spreadsheet. |
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Posted: Jun 18, 2009 - 02:48 PM |
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Joined: Nov 11, 2003
Posts: 934
Location: Chicago Illinois USA
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| For exponential, just multiply your level instead of incrementing. Like a shift left. |
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