图像本身有点模糊。树木的整个边缘不可见。树存在的唯一线索似乎是阴影。所以我建议使用阴影信息和分水岭变换和局部阈值。我在这里做过类似的练习。结果不是那么完美，但它会给出一些想法。以下是步骤。1. 寻找阴影， 2. 向太阳方向扩展阴影 3. 创建阴影蒙版 4. 模糊图像 5. 分水岭变换 6. 分别对每个分水岭段设置阈值 7. 用霍夫变换拟合圆

im=imread('P0Yvl.png');
imOrig=im;

% Grayscale
im=rgb2gray(im);

% Shadows
shadow=im<60; 
[xs,ys]=find(shadow);

% Grow shadow towards sun direction

% Create structural element for dilation
str_element_initial=flip(eye(25)); 
str_element_initial(:,1:12)=0;

shadow2=zeros(size(str_element_initial));
for i=1:7 
    shadow2 = shadow2 | imresize(imrotate(str_element_initial,40-i*10),size(str_element_initial))>.1;
end

mask_shadow=imdilate(shadow,shadow2);
mask_shadow=mask_shadow-shadow;

% Plot shadow mask and initial shadow locations
figure,imagesc(mask_shadow);axis image;
hold on, plot(ys,xs,'r.')

% Blur image
im2=imfilter(im,fspecial('gaussian',[21 21],3));

% Watershed transform
L = watershed(max(im2(:))-im2);
[xw,yw]=find(L==0);

figure,imagesc(imOrig),axis image
hold on, plot(yw,xw,'r.','MarkerSize',1)

% Threshold each segment individually
tmp=zeros(size(im));

for i=1:max(L(:))
    mask=L==i;
    ind=find(mask);    
    thr =multithresh(im(ind),1);
    tmp(ind)=im(ind)>thr*.8;
end

% Remove detections that don't have shadow
trees=tmp & mask_shadow;
trees=bwareaopen(trees,10);

% Find circles with Hough transform
[centers, radii, metric] = imfindcircles(trees,[5 25]);

% Result
figure,imagesc(imOrig),axis image
viscircles(centers, radii,'EnhanceVisibility',0,'LineStyle','-','LineWidth',1);