# Does the SIFT feature be calculated in the original image or the corresponding octave(s)?

1. Given there are 4 octaves(the 1st octave is the original image, the 2nd octave is half size of the original image, and so on.), does SIFT calculate the keypoint's orientation in the original image or the octave the keypoint extracted from?

For example, there is a keypoint found in the 3rd octave. Does SIFT calculate its orientation in the original image or in the 3rd octave (one fourth of the original image size)

2. Similar to 1st question, does SIFT calculate the keypoint descriptor in the original image or the octave the keypoint extracted from?

• The answer relied by Rakshit Kothari in Quora: "The original image. The octave/scale information is used to select the region around the key points in the image to be used for the feature." – willSapgreen Jan 13 '16 at 23:51
• This title is a lot better, thank you! – Raphael Apr 21 '16 at 14:13