Asked by Putra A
on 6 Apr 2013

Dear Forum Member.

I want to fit a curve to gamma like curve with current data that i have.

i need to get shape and scale parameter from the data that i assume have a gamma intensity curve.

Here is the grey level as (Y)

Columns 1 through 17 150 149 151 157 163 169 174 176 177 177 177 181 185 186 185 184 181 Columns 18 through 24 176 173 171 169 167 168 168

and the index is

Columns 1 through 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Columns 15 through 24 15 16 17 18 19 20 21 22 23 24

if i create the plot it will be like this

i want to assume the graph as gamma curve, how to get the shape and scale parameter for this curve if i only have x and y ?

new image :

Thank you :)

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Answer by Tom Lane
on 9 Apr 2013

Accepted answer

I'm not sure what you mean by a gamma curve. If you mean a scalar multiple of a curve with the same form as a gamma probability density function, here's an example using nlinfit from the Statistics Toolbox:

function main y = [150 149 151 157 163 169 174 176 177 177 177 181 ... 185 186 185 184 181 176 173 171 169 167 168 168]; x = 1:24; p = nlinfit(x,y,@f,[1 10 150]) plot(x,y,'bo',x,f(p,x),'r-') end

function y = f(abc,x) a = abc(1); b = abc(2); c = abc(3); y = c * x.^(a-1) .* exp(-x/b) / (b^a * gamma(a)); end

Putra A
on 9 Apr 2013

Dear Tom Lane, Thank you for the answer :D.. yes what i want to do is to find the parameter for the curve which have same form with gamma pdf.

I am updated the image there i want to get gamma parameter for the red plot. How do you think about that Tom ? can i use your code above to have it done ?

Tom Lane
on 9 Apr 2013

You can use the X and Y value for the red curve, in place of the X and Y values in your original posting.

Putra A
on 10 Apr 2013

Ok nice, thank you for the answer tom :)

Answer by bym
on 6 Apr 2013

If you have the statistics toolbox, you can use:

gamfit()

Putra A
on 6 Apr 2013

thanks for your answer proecsm, but when i use gamfit(), isnt it used to determine scale and shape parameter from gamma distributed data ? while i only have a curve that i assume follow gamma curve not distribution.

bym
on 7 Apr 2013

Yes, I thought your question pertained to statistics not image processing. You can use

lsqcurvefit()

to fit a nonlinear curve, but it appears from your plot that your data does not follow a power law

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