%sift算法的描述符
%計算主方向
%在特征點的一個區域內計算梯度直方圖（分36個角度）
%特征描述是32維
function?[desc??locs?]=features_description（fim_features）
f?=?mat2gray（rgb2gray（im2double（f）））;
g?=?gaussian_filter（?1.5?*?2?）;?%?9?
zero_pad?=?ceil（?length（g）?/?2?）;?%?5
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%?1）計算梯度方向和幅值
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fprintf（?‘Computing?gradient?magnitude?and?orientation...\n‘?）;
tic;
??diff_x?=?0.5*（f（2:（end-1）3:（end））-f（2:（end-1）1:（end-2）））;????%L（x+1y）-L（x-1y）
??diff_y?=?0.5*（f（3:（end）2:（end-1））-f（1:（end-2）2:（end-1）））;??????%L（xy+1）-L（xy-1）
??mag?=?zeros（size（f））;??????
??mag（2:（end-1）2:（end-1））?=?sqrt（?diff_x?.^?2?+?diff_y?.^?2?）;%梯度幅值
??mag_pad?=?zeros（size（mag）+2*zero_pad）;
??mag_pad（（zero_pad+1）:（end-zero_pad）（zero_pad+1）:（end-zero_pad））?=?mag;??????
??
??grad?=?zeros（size（f））;?
??grad（2:（end-1）2:（end-1））?=?atan2（?diff_y?diff_x?）;???%梯度方向
??grad（grad?==?pi）?=?-pi;
??grad_pad?=?zeros（size（grad）+2*zero_pad）;
??grad_pad（（zero_pad+1）:（end-zero_pad）（zero_pad+1）:（end-zero_pad））?=?grad;
??clear?mag?grad
grad_time?=?toc;
fprintf（?‘Gradient?calculation?time?%.2f?seconds.\n‘?grad_time?）;
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%?2）確定特征點的主方向：將特征點周圍鄰域的梯度方向列直方圖（36個方向），尋找極大值，同時超過極大值80%的方向為輔方向
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fprintf（?‘Assigining?keypoint?orientations...\n‘?）;
num_bins?=?36;
hist_step?=?2*pi/num_bins;?%?10“c
hist_orient?=?[-pi:hist_step:（pi-hist_step）];%?-180:10:170
pos?=?[];????????%?初始化關鍵點的位置、方向及尺度信息
orient?=?[];
scale?=?[];
tic;
????keypoint_count?=?0;
????%?構造高斯加權掩膜
????g?=?gaussian_filter（?1.5?*?2?）;?%?sigma=3?n=5
????hf_sz?=?floor（length（g）/2）;%?取比length（g）/2小的最大整數
????g?=?g‘*g;??????
????loc_pad?=?zeros（size（f）+2*zero_pad）;?
????loc_pad（（zero_pad+1）:（end-zero_pad）（zero_pad+1）:（end-zero_pad））?=?im_features;
????[iy?ix]=find（loc_pad==1）;??%ix是圖像的行，iy是圖像的列
????for?k?=?1:length（iy）
?????????x?=?ix（k）;
?????????y?=?iy（k）;
?????????wght?=?g.*mag_pad（（y-hf_sz）:（y+hf_sz）（x-hf_sz）:（x+hf_sz））;%加權極值點的鄰域，length（g）Xlength（g）
?????????grad_window?=?grad_pad（（y-hf_sz）:（y+hf_sz）（x-hf_sz）:（x+hf_sz））;%?選取鄰域的角度
?????????orient_hist=zeros（length（hist_orient）1）;%?36X1
?????????for?bin=1:length（hist_orient）%?1?:?36
????????????diff?=?mod（?grad_window?-?hist_orient（bin）?+?pi?2*pi?）?-?pi;?
?????????????%絕對值小于10度；或大于10度
????????????orient_hist（bin）=orient_hist（bin）+sum（sum（wght.*max（1?-?abs（diff）/hist_step0）））;?
?????????end
?????????%?用非極大值抑制法找到直方圖的極值
?????????peaks?=?orient_hist;?????
?????????rot_right?=?[?peaks（end）;?peaks（1:end-1）?];?%?1-D?array?with?36?elements
?????????rot_left?=?[?peaks（2:end）;?peaks（1）?];??%?1-D?array?with?36?elements
?????????peaks（peaks??????????peaks（peaks????????