OpenFace/matlab_version/face_detection/mtcnn/max_pooling2.m

function [ output_maps ] = max_pooling2( input_maps, kernel_size, stride)
%POOLING Summary of this function goes here
%   Detailed explanation goes here
    
    orig_rows = size(input_maps,1);
    orig_cols = size(input_maps,2);
    
    pooled_rows = round((orig_rows - kernel_size)/stride) + 1;
    pooled_cols = round((orig_cols - kernel_size)/stride) + 1;   
     
    if(exist('vl_nnpool', 'file') == 3)
        % Caffe and MatConvNet do pooling slightly differently, so need to
        % counter for that

        pooled_cols_vl = floor((orig_cols - kernel_size)/stride) + 1;
        pooled_rows_vl = floor((orig_rows - kernel_size)/stride) + 1;

        if(pooled_rows_vl == pooled_rows && pooled_cols_vl == pooled_cols)
            output_maps = vl_nnpool(input_maps, [kernel_size, kernel_size], 'stride', stride);
        else
            % Else need to pad right and bottom with infinities 
            for x=1:kernel_size
                pooled_cols_vl = floor((orig_cols + x - kernel_size)/stride) + 1;
                if(pooled_cols_vl == pooled_cols)
                    break;
                end
            end
            for y=1:kernel_size
                pooled_rows_vl = floor((orig_rows +y - kernel_size)/stride) + 1;
                if(pooled_rows_vl == pooled_rows)
                    break;
                end
            end

            input_maps_new = -inf * ones(size(input_maps,1)+y, size(input_maps,2)+x, size(input_maps,3), size(input_maps,4));
            input_maps_new(1:size(input_maps,1),1:size(input_maps,2),:,:) = input_maps;
            output_maps = vl_nnpool(input_maps_new, [kernel_size, kernel_size], 'stride', stride);
        end
    else
    
        up_to_rows_out = floor((orig_rows - kernel_size)/stride) + 1;
        up_to_cols_out = floor((orig_cols - kernel_size)/stride) + 1;

        % How many full max-pooling steps are there
        up_to_cols = kernel_size + (up_to_cols_out-1) * stride;
        up_to_rows = kernel_size + (up_to_rows_out-1) * stride;

        output_maps = zeros(pooled_rows, pooled_cols, size(input_maps,3), size(input_maps,4));

        % Pick only the striding elements
        [y, x] = meshgrid(1:up_to_cols-kernel_size+1, 1:up_to_rows-kernel_size+1);
        to_keep_map = mod(y, stride) == 1 & mod(x, stride) == 1;
        to_keep = find(to_keep_map);

        inds_pooling = im2col_inds(input_maps(1:up_to_rows,1:up_to_cols,1,1), [kernel_size, kernel_size]);
        inds_pooling = inds_pooling(:, to_keep);
        for m=1:size(input_maps,4)
            for i=1:size(input_maps,3)
    %             temp = im2col(input_maps(1:up_to_rows,1:up_to_cols,i,m), [kernel_size, kernel_size], 'sliding');     
    %             temp = im2col_mine(input_maps(1:up_to_rows,1:up_to_cols,i,m), [kernel_size, kernel_size]);        
    %             temp = temp(:,to_keep);

                temp = input_maps(1:up_to_rows,1:up_to_cols,i,m);
                temp = temp(inds_pooling);

                max_val = max(temp);
                output_maps(1:up_to_rows_out,1:up_to_cols_out,i,m) = reshape(max_val, up_to_rows_out, up_to_cols_out);     
            end
        end
        % A bit of a hack for non-even number of rows or columns
        if(orig_cols ~= up_to_cols)
            span = orig_cols - (up_to_cols - kernel_size + stride);
            inds_pooling = im2col_inds(input_maps(1:up_to_rows,end-span+1:end,i,m), [kernel_size, span]);
            inds_pooling = inds_pooling(:, 1:stride:end);
            for m=1:size(input_maps,4)
                for i=1:size(input_maps,3)
    %                 temp = im2col(input_maps(1:up_to_rows,end-span+1:end,i,m), [kernel_size, span], 'sliding');
    %                 temp = im2col_mine(input_maps(1:up_to_rows,end-span+1:end,i,m), [kernel_size, span]);
    %                 max_val = max(temp(:,1:stride:end));

                    temp = input_maps(1:up_to_rows,end-span+1:end,i,m);
                    max_val = max(temp(inds_pooling));
                    output_maps(1:up_to_rows_out,end,i,m) = max_val;     
                end        
            end
        end

        if(orig_rows ~= up_to_rows)
            span = orig_rows - (up_to_rows - kernel_size + stride);
            inds_pooling = im2col_inds(input_maps(end-span+1:end, 1:up_to_cols,i,m), [span, kernel_size]);
            inds_pooling = inds_pooling(:, 1:stride:end);

            for m=1:size(input_maps,4)
                for i=1:size(input_maps,3)
    %                 temp = im2col(input_maps(end-span+1:end, 1:up_to_cols,i,m), [span, kernel_size], 'sliding');
    %                 temp = im2col_mine(input_maps(end-span+1:end, 1:up_to_cols,i,m), [span, kernel_size]);
    %                 max_val = max(temp(:,1:stride:end));
                    temp = input_maps(end-span+1:end, 1:up_to_cols,i,m);
                    max_val = max(temp(inds_pooling));

                    output_maps(end, 1:up_to_cols_out,i,m) = max_val;     
                end   
            end
        end

        if(orig_cols ~= up_to_cols && orig_rows ~= up_to_rows)
            for m=1:size(input_maps,4)
                for i=1:size(input_maps,3)
                    tmp = input_maps(up_to_rows- kernel_size + stride + 1:end,up_to_cols - kernel_size + stride+1:end,i,m);            
                    output_maps(end,end,i,m) = max(tmp(:));
                end
            end
        end
    
    end
    
end