%?下一步是計算特征點的主方向.
%?在特征點的一個區域內計算其梯度直方圖
g?=?gaussian_filter（?1.5?*?absolute_sigma（1intervals+3）?/?subsample（1）?）;
zero_pad?=?ceil（?length（g）?/?2?）;


%?計算高斯金字塔圖像的梯度方向和幅值
if?interactive?>=?1
????fprintf（?2?‘Computing?gradient?magnitude?and?orientation...\n‘?）;
end
tic;
mag_thresh?=?zeros（size（gauss_pyr））;
mag_pyr?=?cell（size（gauss_pyr））;
grad_pyr?=?cell（size（gauss_pyr））;
for?octave?=?1:octaves
????for?interval?=?2:（intervals+1）
????????
????????%?計算xy的差分
????????diff_x?=?0.5*（gauss_pyr{octaveinterval}（2:（end-1）3:（end））-gauss_pyr{octaveinterval}（2:（end-1）1:（end-2）））;
????????diff_y?=?0.5*（gauss_pyr{octaveinterval}（3:（end）2:（end-1））-gauss_pyr{octaveinterval}（1:（end-2）2:（end-1）））;
????????
????????
????????%?計算梯度幅值
????????mag?=?zeros（size（gauss_pyr{octaveinterval}））;
????????mag（2:（end-1）2:（end-1））?=?sqrt（?diff_x?.^?2?+?diff_y?.^?2?）;
????????
????????
????????%?存儲高斯金字塔梯度幅值
????????mag_pyr{octaveinterval}?=?zeros（size（mag）+2*zero_pad）;
????????mag_pyr{octaveinterval}（（zero_pad+1）:（end-zero_pad）（zero_pad+1）:（end-zero_pad））?=?mag;
????????
????????
????????%?計算梯度方向
????????grad?=?zeros（size（gauss_pyr{octaveinterval}））;
????????grad（2:（end-1）2:（end-1））?=?atan2（?diff_y?diff_x?）;???%?[-pipi]
????????grad（find（grad?==?pi））?=?-pi;
????????
????????
????????%?存儲高斯金字塔梯度方向
????????grad_pyr{octaveinterval}?=?zeros（size（grad）+2*zero_pad）;
????????grad_pyr{octaveinterval}（（zero_pad+1）:（end-zero_pad）（zero_pad+1）:（end-zero_pad））?=?grad;
????end
end
clear?mag?grad
grad_time?=?toc;
if?interactive?>=?1
????fprintf（?2?‘Gradient?calculation?time?%.2f?seconds.\n‘?grad_time?）;
end


%?下一步是確定特征點的主方向
%?方法：通過尋找每個關鍵點的子區域內梯度直方圖的峰值（注：每個關鍵點的主方向可以有不止一個）

%?g?=?gaussian_filter（?1.5?*?absolute_sigma（1intervals+3）?/?subsample（1）?）;
%?zero_pad?=?ceil（?length（g）?/?2?）;
%?將灰度直方圖分為36等分，每隔10度一份
num_bins?=?36;
hist_step?=?2*pi/num_bins;
hist_orient?=?[-pi:hist_step:（pi-hist_step）];


%?初始化關鍵點的位置、方向和尺度信息
pos?=?[];
orient?=?[];
scale?=?[];


%?給關鍵點確定主方向
if?interactive?>=?1
????fprintf（?2?‘Assigining?keypoint?orientations...\n‘?）;
end
tic;
for?octave?=?1:octaves
????if?interactive?>=?1
????????fprintf（?2?‘\tProcessing?octave?%d\n‘?octave?）;
????end
????for?interval?=?2:（intervals?+?1）
????????if?interactive?>=?1
????????????fprintf（?2?‘\t\tProcessing?interval?%d?‘?interval?）;
????????end
????????keypoint_count?=?0;??

????????%?構造高斯加權掩模
????????g?=?gaussian_filter（?1.5?*?absolute_sigma（octaveinterval）/subsample（octave）?）;
????????hf_sz?=?floor（length（g）/2）;
????????g?=?g‘*g;
????????
????????
????????loc_pad?=?zeros（size（loc{octaveinterval}）+2*zero_pad）;
????????loc_pad（（zero_pad+1）:（end-zero_pad）（zero_pad+1）:（end-zero_pad））?=?loc{octaveinterval};
????????
????????
????????[iy?ix]=find（loc_pad==1）;
????????for?k?=?1:length（iy）
????????????
????????????x?=?ix（k）;
????????????y?=?iy（k）;
????????????%?%?對其值進行高斯平滑
????????????wght?=?g.*mag_pyr{octaveinterval}（（y-hf

?屬性????????????大小?????日期????時間???名稱
-----------?---------??----------?-----??----

?????文件???????6283??2017-05-08?12:21??sift\assign_consistent_orientation.m

?????文件???????2646??2017-05-08?12:24??sift\display_keypoints.m

?????文件???????5749??2017-05-18?16:06??sift\DOG_pyramid.m

?????文件???????8509??2017-05-06?21:48??sift\Extreme_point_detection.m

?????文件????????257??2017-05-07?20:41??sift\gaussian_filter.m

?????文件???????4259??2017-05-10?22:22??sift\represent_descriptor.m

?????目錄??????????0??2017-12-04?15:13??sift

-----------?---------??----------?-----??----

????????????????27703????????????????????7