Question

我正在尝试在Matlab中实现FAST角点检测算法，我知道已经有一个预建版本。我不确定为什么我的实现似乎检测到甚至不接近边缘或者只是坏的功能。

readImage1 = imread('img1.png');
im1 = rgb2gray(readImage1);
img1 = medfilt2(im1);

rowStart = 0.1 * size(img1, 1);
rowStart = fix(rowStart);
rowEnd = 0.9 * size(img1, 1);
rowEnd = fix(rowEnd);

colStart = 0.1 * size(img1, 2);
colStart = fix(colStart);
colEnd = 0.9 * size(img1, 2);
colEnd = fix(colEnd);

阅读我的图片，执行中位数蓝色。我只希望特征检测从图像边缘开始一定距离。

array = [ ; ];

for c = colStart:colEnd
    for r = rowStart:rowEnd

        %get the intesity of the pixel
        intensity = img1(r, c);

        temp = [r,c];

        intensity1 = img1((r+3), c);
        intensity5 = img1(r+1, c+3);
        intensity9 = img1(r-3, c);
        intensity13 = img1(r+1, c-3);

        threshold = 70;

        count = IntensityCount(intensity1, intensity9, intensity5, intensity13, intensity, threshold);

        if count >= 3
              array = [array; temp];
        end
    end
end

然后，对于每个像素，将像素的强度与其最重要的4个邻居进行比较。如果将其检测为拐角，请将其保存为Nx2阵列。

此后，执行非最大抑制。

[rows, columns] = size(array);

discardArray = [ ; ];

for index = 2:rows
    currX = array(index);
    currY = array(index, 2);

    prevX = array(index - 1);
    prevY = array(index - 1, 2);

    if(adjacencyCheck(currX, currY, prevX, prevY))
        currScore = pixelScore(img1, currX, currY);
        prevScore = pixelScore(img1, prevX, prevY);

        if(currScore > prevScore)
            temp = [prevX, prevY];
            discardArray = [discardArray; temp];
        else
            temp = [currX, currY];
            discardArray = [discardArray; temp];
        end

    end
end

discardArray = unique(discardArray, 'rows');
finalArray = setdiff(array,discardArray, 'rows');

然后显示图像上的点。

[rows, columns] = size(finalArray);

for index = 1:rows
   img1 = insertMarker(img1, [finalArray(index), finalArray(index,2)], 'x'); 
end

imshow(img1);

用于强度检查和非最大抑制的函数。

function number = IntensityCount(int1, int9, int5, int13, origIntensity, thresh)

number = 0;

if abs(int1 - origIntensity) > thresh
    number = number + 1;
end

if abs(int9 - origIntensity) > thresh
    number = number + 1;
end

if abs(int5 - origIntensity) > thresh
    number = number + 1;
end

if abs(int13 - origIntensity) > thresh
    number = number + 1;
end

end

%Get the score by sum of absolute differences between the pixel and its neighbours.
function scoreNumber = pixelScore(img, r, c)

intOriginal = img(r,c);
intensity1 = img((r+3), c);
intensity3 = img(r+3, c-1);
intensity5 = img(r+1, c+3);
intensity7 = img(r-1, c+3);
intensity9 = img(r-3, c);
intensity11 = img(r-3, c-1);
intensity13 = img(r+1, c-3);
intensity15 = img(r-1, c-3);

scoreNumber = abs(intOriginal - intensity1) + abs(intOriginal - intensity3)+ abs(intOriginal - intensity5) + abs(intOriginal - intensity7) + abs(intOriginal - intensity9) + abs(intOriginal - intensity11) + abs(intOriginal - intensity13) + abs(intOriginal - intensity15); 
end

%Check pixel adjacency using Euclidean distance. 
function isAdjacent = adjacencyCheck(x1, y1, x2, y2)
xDist = x1 - x2;
yDist = y1 - y2;

dist = (xDist.^2 + yDist.^2).^0.5;

isAdjacent = dist <= 4;
end

为什么我的算法检测到这么多坏的功能？

Answer 1

这是MATLAB语法的一个不幸的困难，并且经常引起混淆。

如您所知，MATLAB将矩阵索引为[row,column]，这在线性代数中很自然，完全适用于矩阵。但由于MATLAB已超越线性代数，因此矩阵通常用于存储空间数据，其中x沿着矩阵的列增加，y沿着行增加。因此，应该反转空间位置[x,y]以在矩阵中找到索引：[y,x]。

insertMarker是以[x,y]形式获取空间位置而不是矩阵中的索引的函数之一。因此，将[finalArray(index), finalArray(index,2)]切换为[finalArray(index,2), finalArray(index,1)]可以解决此问题。

请注意，[finalArray(index,2), finalArray(index,1)]也可以写为finalArray(index,[2,1])。

MATLAB中的快速角点检测实现查找非角点特征

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