Re: how to sort by luminosity?

Alex Drinkwater Mon, 04 Aug 2008 13:48:30 -0700

Intriguingly, if you convert both of your examples to greyscale inPhotoshop (I used CS3) by desaturating, the averaging example gives amore accurate gradation of grey values.


Not sure what that means....


Sorry for the bad screenshot, incidentally.


a|x

<<inline: LuminosityExamples.png>>







On 4 Aug 2008, at 21:40, Alessandro Sabatelli wrote:

Using a simple average (r+g+b)/3 we get:
<avg.png>


And using r*0.2126 + g+0.7152 + b*0.0722 we get:
<lum.png>


I "think" I prefer the latter...
.xX

On Aug 4, 2008, at 1:33 PM, Alessandro Sabatelli wrote:

I simply averaged the rgb in the provided example, but Anton'ssolution for computing the luminosity is much better (at least forhumans).

.xX

On Aug 4, 2008, at 1:20 PM, vade wrote:

he mean externally form the javascript patch. Computer the luma byconverting to black and white, or use rec709 luma values in aCIKernel, and get the area average, and pass that into the JS.


I think external reffered to outside the JS patch, not outside QC :)

CI kernel for rec 709 luma coefficients (as per HDTV spec):

/* luma coeffs Y' = 0.2126 R' + 0.7152 G' + 0.0722 B */

const vec3 lumacoeff = vec3(0.2126,0.7152,0.0722);

kernel vec4 lumaKernel(sampler image)
{
        vec4 inputImage = sample(image, samplerCoord(image));
        float luma = dot(inputImage.rgb, lumacoeff);

        return vec4(luma,luma,luma,inputImage.a);
}


On Aug 4, 2008, at 4:14 PM, Alex Drinkwater wrote:

On 4 Aug 2008, at 21:10, Alessandro Sabatelli wrote:

Create an object:

var myObject = new Object()
Then add the image and the brightness, which you can computeexternally:


That's the point though:
he wants to compute the brightness IN Quartz Composer.


a|x
http://machinesdontcare.wordpress.com



myObject.image = image
myObject.brightness = brightness

Then store the image in an array

myArray[index] = myObject

Once you've downloaded and processed all of the images then sort

myArray.sort(f)

Where f simply compares the brightness

function f(a, b) {
        return a.brightness-b.brightness
}

Here's an example which shows it working:

<Brightness Sort.qtz>

.xX

On Aug 3, 2008, at 1:58 PM, aienn wrote:

On Aug 4, 2008, at 12:12 AM, Alessandro Sabatelli wrote:
You can store the average brightness along with the image andthen return the sorted list of images from JS.
Yep, I already figured out the JS and averaging stuff, but howexactly do I store the average brightness along with the image,as you're suggesting?
a.


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