Hough Transform – Basic Shape Detection

The Hough Transform is a method to find shapes in an image. The classical transformation was initially designed to identify lines in the image. Later the transform is extended to identify different kind of shapes such as circles, elipsed and even arbitrary objects. The Hough Transform use a voting scheme in a parameter space in a way that a so called accumulators is populated with present edge points. The voting procedure helps to improve the feature detection accuracy and overcome cases of missing or noisy feature parts. The algorithm can be briefly described by the following steps:

• Extract edges using e.g. Sobel edge detector
• Choose desired transfomation e.g. line transform or circle transform
• Iterate through the image and populate the accumulator using the choosen transform
• Search for the local maximum values inside the accumulator space

Hough Line Transform

A line that pass through a point x,y can be represented using the Polar coordinate system by the following equation:

Hough line transform

The equation above is used in a way that every significant edge pixel (e.g. x0,y0) is processed and plot all possible family of lines. Meaning that for each such point the prepared Polar accumulator (r, theta) is filled with calculated radius values at all possible angles. After the whole image is processed and the accumulator is filled, the last step is to find local maximums and retrieve the resulting lines. It should be noted that the returned lines do not contain any information about length and further processing is required to match the exact parts of the image with corresponding lines.

Hough Circle Transform

The math behind the Hough circle transform is similar to this used for lines detection described above. The voting accumulator cells are determined by the following equation:

x = R*cos(phi);
y = R*sin(phi);

Where R is the radius of the searched circles and phi is angle in radians from 0-2PI.

Hough Transform over Canvas2D

In order to optimize the algorithm execution and reduce needed time we use a pre-calculated lookup table (LUT) with x and y values respectively. The JavaScript code snippet below shows the lookup table creation:

``````
// Compute the circle kernel (LUT)
var kernel = [{x: ~~(r*Math.cos(0)), y: ~~(r*Math.sin(0))}];
for (var i = 1; i < 380; i++){
var phi = i*Math.PI/180;
var x = ~~(r*Math.cos(phi));
var y = ~~(r*Math.sin(phi));
var last_element = kernel[kernel.length - 1];
if (last_element.x != x || last_element.y != y)
kernel.push({x: x, y: y});
}
``````

Next step is to loop through image pixels and execute the voting procedures. At every pixel the overall sum is calculated using the lookup table. See the code fragment below:

``````
// Create buffer using pixels.data ctor
var buff = new pixels.data.constructor(new ArrayBuffer(pixels.data.length));
var data = pixels.data;
var w = pixels.width - r;
var h = pixels.height - r;
var i, j, bpr = pixels.width*4;
for (i = r; i < h; i++){
for (j = r; j < w; j++){
var sum = 0, l = kernel.length;
for (var k = 0; k < l; k++){
var x = j + kernel[k].x;
var y = i + kernel[k].y;
sum += data[bpr*y + x*4]; // Assume it's grayscale
}
const pos = bpr*i + j*4;
buff[pos + 0] = buff[pos + 1] = buff[pos + 2] = ~~(sum / l);
buff[pos + 3] = 255;
}
}

pixels.data.set(buff);
}
``````

Hough Transform using OpenGL (WebGL)

In case of GPU based algorithm, the Hough Circle Transform can be obtained using the following OpenGL fragment shader:

``````
precision mediump float;

// our texture
uniform sampler2D u_image;
uniform vec2 u_textureSize;
uniform float u_r;
#define M_PI 3.1415926535897932384626433832795
#define PHI_STEP M_PI/180.0

void main() {
vec2 onePixel = vec2(1.0, 1.0) / u_textureSize;
vec2 textCoord = gl_FragCoord.xy / u_textureSize;
vec4 sum = vec4(0.0);
float phi = 0.0;
for (int i = 0; i < 360; i++)
{
phi += PHI_STEP;
sum += texture2D(u_image, textCoord + onePixel*vec2(u_r*cos(phi), u_r*sin(phi)));
}

gl_FragColor = vec4(vec3(sum / 360.0), 1.0);
}
``````

Sigma: 3

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Hough Transform – Basic Shape Detection
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