i want to write plate detection

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First of all you need to learn about it from chp 5 of book Packtpub.Mastering.OpenCV.with.Practical.Computer.Vision.Projects in which they detect the vehicle licence plate (Book is free to download , so search it out) . You can view the same question and helpfull answer from @Mathieu here .

Some algorithms which i remember you have use are :

Sobel filter
Threshold operation (otsu)
Close Morphological operation
Mask of one filled area
Svm classifier