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If you want to detect numbers using opencv you can simply get the best answer's by little search :

1st link

Investigate SIFT: Scale Invariant Feature Transform. OpenCV implements this feature, check: Implementing SIFT in OpenCV.

Also these pages offer valuable content:

• Finding shapes in an image using opencv
• Are there any fast alternatives to SURF and SIFT for scale-invariant feature extraction?
• Sift implementation with OpenCV 2.2
• training SIFT features in OpenCV

Basic OCR in OpenCV

Scene Text Detection