face alignment by 3000 fps系列学习总结（二）

准备初始数据

mean_shape

mean_shape就是训练图片所有ground_truth points的平均值.那么具体怎么做呢？是不是直接将特征点相加求平均值呢？
显然这样做是仓促和不准确的。因为图片之间人脸是各式各样的，收到光照、姿势等各方面的影响。因此我们求取平均值，应该在一个相对统一的框架下求取。如下先给出matlab代码:

function mean_shape = calc_meanshape(shapepathlistfile)fid = fopen(shapepathlistfile);shapepathlist = textscan(fid, '%s', 'delimiter', '\n');if isempty(shapepathlist)    error('no shape file found');    mean_shape = [];    return;endshape_header = loadshape(shapepathlist{1}{1});if isempty(shape_header)    error('invalid shape file');    mean_shape = [];    return;endmean_shape = zeros(size(shape_header));num_shapes = 0;for i = 1:length(shapepathlist{1})    shape_i = double(loadshape(shapepathlist{1}{i}));    if isempty(shape_i)        continue;    end    shape_min = min(shape_i, [], 1);    shape_max = max(shape_i, [], 1);    % translate to origin point    shape_i = bsxfun(@minus, shape_i, shape_min);    % resize shape    shape_i = bsxfun(@rdivide, shape_i, shape_max - shape_min);    mean_shape = mean_shape + shape_i;    num_shapes = num_shapes + 1;endmean_shape = mean_shape ./ num_shapes;img = 255 * ones(500, 500, 3);drawshapes(img, 50 + 400 * mean_shape);endfunction shape = loadshape(path)% function: load shape from pts filefile = fopen(path);if file == -1    shape = [];    fclose(file);    return;endshape = textscan(file, '%d16 %d16', 'HeaderLines', 3, 'CollectOutput', 2);fclose(file);shape = shape{1};end

解析:

公式表示:

{shapegt−[Region(1),Region(2)]}/[Region(3),Region(4)))]]⇒[0,1]×[0,1]

准备ΔSt

我们知道3000FPS的核心思想是:

ΔSt=WtΦt(I,St−1)

其中ΔSt=Sgt−St为第t个阶段的残差；而Φt(I,St−1)则为特征提取函数；W为线性回归矩阵。由《人脸配准坐标变换解析》我们可以看到所谓的ΔSt需进行相似性变换，而Φt(I,St−1)则不需要.
相似性变换的主要过程是:
先将St，S0中心化变换，再求解如下变换矩阵:

S0=cRSt

,求解完cR后，对ΔSt施加同样的变换，即

St˜=cRΔSt

.我们将使用变化后的St˜去求解线性回归矩阵W.
先贴代码: train_model.m 第103行起

Param.meanshape        = S0(Param.ind_usedpts, :); %选取特定的landmarkdbsize = length(Data);% load('Ts_bbox.mat');augnumber = Param.augnumber; %为每张人脸选取的init_shape的个数for i = 1:dbsize            % initializ the shape of current face image by randomly selecting multiple shapes from other face images           % indice = ceil(dbsize*rand(1, augnumber));      indice_rotate = ceil(dbsize*rand(1, augnumber));      indice_shift  = ceil(dbsize*rand(1, augnumber));      scales        = 1 + 0.2*(rand([1 augnumber]) - 0.5);    Data{i}.intermediate_shapes = cell(1, Param.max_numstage); %中间shape    Data{i}.intermediate_bboxes = cell(1, Param.max_numstage);    Data{i}.intermediate_shapes{1} = zeros([size(Param.meanshape), augnumber]); %68*2*augnumber(augnumber为第i图片设置的初始shape的个数)    Data{i}.intermediate_bboxes{1} = zeros([augnumber, size(Data{i}.bbox_gt, 2)]); %augnumber*4    Data{i}.shapes_residual = zeros([size(Param.meanshape), augnumber]); %shapes_residual为shape 残差 维数:68*2*augnumber    Data{i}.tf2meanshape = cell(augnumber, 1);    Data{i}.meanshape2tf = cell(augnumber, 1);    % if Data{i}.isdet == 1    %    Data{i}.bbox_facedet = Data{i}.bbox_facedet*ts_bbox;    % end         % 如下一段的意思是如果augnumber=1，表明每个图片的Init_shape只有一个，因此这要设置成mean_shape即可,这时你会发现Data{i}.tf2meanshape{1}其实就是    % 单位矩阵，因为他是从mean_shape转化到mean_shape。后面就不一样了.    %；对于augnumber>1的其他init_shape将采用平移、旋转、   % 缩放等方式产生更多的shape，也可以从其他图片的shape中挑选shape    for sr = 1:params.augnumber        if sr == 1            % estimate the similarity transformation from initial shape to mean shape            % Data{i}.intermediate_shapes{1}(:,:, sr) = resetshape(Data{i}.bbox_gt, Param.meanshape);            % Data{i}.intermediate_bboxes{1}(sr, :) = Data{i}.bbox_gt;            Data{i}.intermediate_shapes{1}(:,:, sr) = resetshape(Data{i}.bbox_facedet, Param.meanshape);            Data{i}.intermediate_bboxes{1}(sr, :) = Data{i}.bbox_facedet;            %将mean shape reproject face detection bbox上            meanshape_resize = resetshape(Data{i}.intermediate_bboxes{1}(sr, :), Param.meanshape); %meanshape_resize与 Data{i}.intermediate_shapes{1}(:,:, sr) 是相同的            %计算当前的shape与mean shape之间的相似性变换                     Data{i}.tf2meanshape{1} = fitgeotrans(bsxfun(@minus, Data{i}.intermediate_shapes{1}(1:end,:, 1), mean(Data{i}.intermediate_shapes{1}(1:end,:, 1))), ...                (bsxfun(@minus, meanshape_resize(1:end, :), mean(meanshape_resize(1:end, :)))), 'NonreflectiveSimilarity');            Data{i}.meanshape2tf{1} = fitgeotrans((bsxfun(@minus, meanshape_resize(1:end, :), mean(meanshape_resize(1:end, :)))), ...                bsxfun(@minus, Data{i}.intermediate_shapes{1}(1:end,:, 1), mean(Data{i}.intermediate_shapes{1}(1:end,:, 1))), 'NonreflectiveSimilarity');            % calculate the residual shape from initial shape to groundtruth shape under normalization scale            shape_residual = bsxfun(@rdivide, Data{i}.shape_gt - Data{i}.intermediate_shapes{1}(:,:, 1), [Data{i}.intermediate_bboxes{1}(1, 3) Data{i}.intermediate_bboxes{1}(1, 4)]);            % transform the shape residual in the image coordinate to the mean shape coordinate            [u, v] = transformPointsForward(Data{i}.tf2meanshape{1}, shape_residual(:, 1)', shape_residual(:, 2)');             Data{i}.shapes_residual(:, 1, 1) = u';            Data{i}.shapes_residual(:, 2, 1) = v';         else            % randomly rotate the shape                        % shape = resetshape(Data{i}.bbox_gt, Param.meanshape);       % Data{indice_rotate(sr)}.shape_gt            shape = resetshape(Data{i}.bbox_facedet, Param.meanshape);       % Data{indice_rotate(sr)}.shape_gt             %根据随机选取的scale，rotation，translate计算新的初始shape然后投影到bbox上            if params.augnumber_scale ~= 0                shape = scaleshape(shape, scales(sr));            end            if params.augnumber_rotate ~= 0                shape = rotateshape(shape);            end            if params.augnumber_shift ~= 0                shape = translateshape(shape, Data{indice_shift(sr)}.shape_gt);            end            Data{i}.intermediate_shapes{1}(:, :, sr) = shape;            Data{i}.intermediate_bboxes{1}(sr, :) = getbbox(shape);            meanshape_resize = resetshape(Data{i}.intermediate_bboxes{1}(sr, :), Param.meanshape); %将            Data{i}.tf2meanshape{sr} = fitgeotrans(bsxfun(@minus, Data{i}.intermediate_shapes{1}(1:end,:, sr), mean(Data{i}.intermediate_shapes{1}(1:end,:, sr))), ...                bsxfun(@minus, meanshape_resize(1:end, :), mean(meanshape_resize(1:end, :))), 'NonreflectiveSimilarity');            Data{i}.meanshape2tf{sr} = fitgeotrans(bsxfun(@minus, meanshape_resize(1:end, :), mean(meanshape_resize(1:end, :))), ...                bsxfun(@minus, Data{i}.intermediate_shapes{1}(1:end,:, sr), mean(Data{i}.intermediate_shapes{1}(1:end,:, sr))), 'NonreflectiveSimilarity');            shape_residual = bsxfun(@rdivide, Data{i}.shape_gt - Data{i}.intermediate_shapes{1}(:,:, sr), [Data{i}.intermediate_bboxes{1}(sr, 3) Data{i}.intermediate_bboxes{1}(sr, 4)]);            [u, v] = transformPointsForward(Data{i}.tf2meanshape{1}, shape_residual(:, 1)', shape_residual(:, 2)');            Data{i}.shapes_residual(:, 1, sr) = u';            Data{i}.shapes_residual(:, 2, sr) = v';            % Data{i}.shapes_residual(:, :, sr) = tformfwd(Data{i}.tf2meanshape{sr}, shape_residual(:, 1), shape_residual(:, 2));        end    endend

这段代码的理解需要结合上面给出的那篇文章《人脸配准坐标变换解析》。

按照《人脸配准坐标变换解析》文章所述，

S0¯¯¯¯S1¯¯¯¯=S0−mean(S0)=S1−mean(S1)}⇒S0¯¯¯¯=c1R1S1¯¯¯¯

因此根据

ΔS=Sg−S1

可推出

ΔS˜=c1R1ΔS

但是现在问题比较特殊，需要多操作一下:
由：

 %将mean shape reproject face detection bbox上 meanshape_resize = resetshape(Data{i}.intermediate_bboxes{1}(sr, :), Param.meanshape);

查看resetshape的定义知meanshape被映射到intermediate_bboxes中，使得S0和S1处于同样的尺度下和大致相似的位置上。用数学语言表达为:

S0_resize=S0∗Ratio+[Region(1),Region(2)]

这里Ratio实际上是intermediate_bboxes的大小。
于是同样按照上面的方法计算：

S0˜=S0_Resize−mean(S0_Resize)=S0∗Ratio−mean(S0)∗Ratio=(S0−mean(S0))∗Ratio=S0¯¯¯¯∗Ratio

经过计算得S0˜=Ratio∗S0¯¯¯¯=c1˜R1˜S1¯¯¯¯.（★）
这也就是上面的代码：

 Data{i}.tf2meanshape{1} = fitgeotrans(bsxfun(@minus, Data{i}.intermediate_shapes{1}(1:end,:, 1), mean(Data{i}.intermediate_shapes{1}(1:end,:, 1))), ...                (bsxfun(@minus, meanshape_resize(1:end, :), mean(meanshape_resize(1:end, :)))), 'NonreflectiveSimilarity');

Data{i}.tf2meanshape{1}即为这里算出的c1˜R1˜.
但我们想要的是S0¯¯¯¯=c1R1S1¯¯¯¯,不用着急，(★)为我们指明了方向。
c1R1=c1˜R1˜/Ratio=c1˜R1˜/intermediate_bboxes.因此:

ΔS˜=c1˜R1˜/intermediate_bboxes∗ΔS

也就是代码中提的:

 %计算当前的shape与mean shape之间的相似性变换         Data{i}.tf2meanshape{1} = fitgeotrans(bsxfun(@minus, Data{i}.intermediate_shapes{1}(1:end,:, 1), mean(Data{i}.intermediate_shapes{1}(1:end,:, 1))),(bsxfun(@minus, meanshape_resize(1:end, :), mean(meanshape_resize(1:end, :)))), 'NonreflectiveSimilarity');Data{i}.meanshape2tf{1} = fitgeotrans((bsxfun(@minus, meanshape_resize(1:end, :), mean(meanshape_resize(1:end, :)))),bsxfun(@minus, Data{i}.intermediate_shapes{1}(1:end,:, 1), mean(Data{i}.intermediate_shapes{1}(1:end,:, 1))), 'NonreflectiveSimilarity');% calculate the residual shape from initial shape to groundtruth shape under normalization scaleshape_residual = bsxfun(@rdivide, Data{i}.shape_gt - Data{i}.intermediate_shapes{1}(:,:, 1), [Data{i}.intermediate_bboxes{1}(1, 3) Data{i}.intermediate_bboxes{1}(1, 4)]);% transform the shape residual in the image coordinate to the mean shape coordinate[u, v] = transformPointsForward(Data{i}.tf2meanshape{1}, shape_residual(:, 1)', shape_residual(:, 2)');  Data{i}.shapes_residual(:, 1, 1) = u'; Data{i}.shapes_residual(:, 2, 1) = v';