# Brightness and geometric transformations

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1 Brightness and geometric transformations Václav Hlaváč Czech Technical University in Prague Center for Machine Perception (bridging groups of the) Czech Institute of Informatics, Robotics and Cybernetics and Faculty of Electrical Engineering, Department of Cybernetics Brightness scale transformation. Brightness corrections. Geometric transformations. Outline of the talk:

2 Image preprocessing, the intro 2/32 The input is an image, the output is an image too. The image is not interpreted. The aim Contrast enhancement (which is useful only if the human looks at the image). To suppress the distortion (e.g., correction of the geometric distortion caused by spherical shape of the Earth taken from a satellite). Noise suppression. Enhancement of some image features needed for further image processing, e.g., edge finding.

3 Taxonomy of the image preprocessing methods 3/32 Taxonomy according to the neighborhood size of the current pixel. Neighborhood Example of the operation Actually processed neighborhood Brightness scale transform The same for all pixels Single pixel Brightness corrections The current pixel Geometric transforms A current pixel (theoretically), a small neighborhood (practically) Local Local filtration A small neighborhood Global Fourier transform The whole image Image restauration Point Local Global input input input output output output

4 Brightness histogram 4/32 The brightness histogram is the estimate of the probability distribution of the phenomenon that a pixel has a certain brightness input image brighness histogram

5 Transformation of the gray scale 5/32 g(i) input gray scale, f(i) output gray scale, f 255 i = 0,..., imax (maximal brightness). Contrast enhancement Threshold Negative g Notes: Some transformations are implemented in HW, e.g., in the display card (e.g. in the VGA mode). Thresholding: converts a gray scale image into a binary image. It yields two regions. How to chose an appropriate threshold?

6 Nonlinearity in the intensity, γ correction 6/32 Why cameras perform nonlinear brightness transformation, the γ-correction? In the cathode-ray tube (vacuum) displays, brightness U γ, where U is the grid voltage and γ is usually 2.2. The effect is compensated in cameras by the inverse function to preserve the linear transfer characteristic in the whole transmission chain. Having input irradiance E and the input voltage of the camera U, the inverse function is U E (1/γ), usually U E 0,45. Although the brightness of LCD monitors depends on the input voltage linearly, the γ-correction is still in use to secure the backward compatibility.

7 Example of two thresholds 7/

8 The aim is: Histogram equalization To enhance the contrast for a human observer by utilizing the gray scale fully; To normalize the brightness of the image for the automatic brightness comparison. 8/ histogram of the input image histogram after equalization

9 Increased contrast after the histogram equalization 9/32 original image increased contrast

10 Derivation of the histogram equalization 10/32 Input: The histogram H(p) of the input image with the gray scale p = p 0, p k. Aim: to find a monotonic transform of the gray scale q = T (p), for which the output histogram G(q) will be uniform for the entire input brightness domain q = q 0, q k. k k G(q i ) = H(p i ). i=0 i=0 The equalized histogram the uniform distribution. f = N 2 q k q 0.

11 Derivation of the histogram equalization (2) The ideal continuous histogram is available only in the continuous case. 11/32 N 2 q q q 0 G(s) ds = q 0 1 q k q 0 ds = N 2 (q q 0 ) q k q 0 = p p 0 H(s) ds. p p 0 H(s) ds. p p 0 H(s) ds. q = T (p) = q k q 0 N 2 p p 0 H(s) ds + q 0. Discrete case, cumulative histogram q = T (p) = q k q 0 N 2 p i=p 0 H(i) + q 0.

12 Pseudocolor 12/32 Unchanged System (Windows) Spectrum Black body Pseudocolor displays a grayscale image as a color image by mapping each intensity value to a color according to a lookup table or a function. The mapping table (function) is called the palette. Pseudocolor is often used to display a single channel data as temperature, distance, etc.

13 Brightness corrections The new brightness f(i, j) depends on the position in the input image i, j of the input image g(i, j). The multiplicative model of the disturbance is often considered: f(i, j) = e(i, j) g(i, j). 13/32 Two approaches: 1. The corrective coefficients are obtained by capturing some etalon area with the known brightness c. This is used to compensate the irregular illumination. (The AGC in the camera has to be switched off when capturing the etalon area). The captured image is f c (i, j) = e(i, j) c e(i, j) = f c(i, j) c 2. The approximation of the background by some analytic surface and its subtraction from the original image.

14 Geometric transform, a motivation in 2D 14/32 y y T(x,y) The vector transform T can be split into two components x x x = T x (x, y), y = T y (x, y). What are the conditions ensuring a one-to-one mapping? Geometric transforms are used for Size change, shift, rotation, slanting according to the transformation known in advance. Undoing geometric distortions. The geometric distortions are often given by examples, images. Similar techniques are used in theoretical mechanics, robotics, and computer graphics.

15 Topographic view of the digital intensity image 15/32 Recall that we considered the image f(x, y) as a landscape in the introduction lecture. y x The image intensity (brightness) f(x, y) corresponds to the height at the position x, y. f(4,5) A digital image provides only samples on a discrete grid illustrated as the orange arrows y 1 x

16 Topographic view, the geom. transformed image The digital image provides samples in the discrete grid only. See the top right picture. Consider the example: The image is shifted half a pixel in the x-axis direction. See the bottom right picture. The issue: There is no image function f(x + 0.5, y) value available. The solution: Interpolate the missing value from the neighboring image function values. Q: Why neighboring values only? y y 1 1 x x 2 2 f(4,5) 3 3? /32

17 Artistic rendering; maybe of a sampled image Photos are from the exhibition Design Block in Prague in October /32 The artistic piece of work illustrates our topographic view at the image function f (x, y), which can be represented by a collection of poles.

18 Two needed steps due to the discrete raster 18/32 1. Transformation of pixel coordinates provides new positions of a pixel, which is calculated in continuous coordinates (real numbers) because the outcome can be off the original raster. Issues: Part of the new transformed image lies off the image. Transformations are not invertible because some approximations were needed. 2. Approximation of the brightness function seeks the integer brightness value for the integer coordinate, which matches best the newly calculated position x, y given as real numbers.

19 Transformation of pixel coordinates The geometric transformation x = Tx(x, y), y = Ty(x, y) is often approximated by a polynomial of m-th order. 19/32 x = m m r a rk x r y k, y = m m r b rk x r y k. r=0 k=0 r=0 k=0 The formula is linear with respect to the coefficients ark, brk. The unknown coefficients can be obtained by solving a system of linear equations, in which pairs of corresponding control points (x, y) and (x, y ) are known. If the system of linear equations were solved deterministically, only a few pairs of control points corresponding to the number of unknown parameters would suffice. However, very many control point pairs are used in practice, which leads to an overdetermined system of linear equations. The unknown coefficients a rk, b rk are estimated using the least-squares method.

20 A basic set of 2D planar transformations 20/32 y translation Euclidean projective original similarity affine x Inspiration R. Szeliski.

21 Bilinear, affine transformation of coordinates 21/32 If the geometric transformation changes slowly with respect to the position then approximating polynomials of lower order m = 2 or m = 3 suffice. Bilinear transformation x = a 0 + a 1 x + a 2 y + a 3 xy, y = b 0 + b 1 x + b 2 y + b 3 xy. Affine transformation, which is even more special, comprises practically needed rotation, translation and slant. x = a 0 + a 1 x + a 2 y, y = b 0 + b 1 x + b 2 y.

22 Homogeneous coordinates matrix notation 22/32 Homogeneous coordinates are common in theoretical mechanics, projective geometry, computer graphics, robotics, etc. The key idea is to represent the point in the space of the dimension increased by one. The point [x, y] is expressed in the 3D vector space as [λx, λy, λ], where λ 0. To simplify expressions, often only one of the infinitely many forms is used [x, y, 1].

23 Affine transformation expressed in a matrix form 23/32 The affine mapping is expressed after introducing homogeneous coordinates as x y 1 = a 1 a 2 a 0 b 1 b 2 b x y 1. Notes: Relation to the PostScript language. The complicated geometric transforms (deformations) can be approximated by tiling the image into smaller rectangular areas. The simpler transform is used to express distortions in each tile, maybe estimated from the control points.

24 Applying the geometric transformation Dual expression 24/32 The input image has to be mapped by the transform T to the output image. T Two dual approaches: Forward mapping: (x, y ) = T (x, y). Backward mapping: (x, y) = T 1 (x, y ). T -1 The difference between the two expressions is due to the needed brightness interpolation.

25 Forward, backward mapping comparison 25/32 Forward mapping Output coordinates (x, y ) can lie off the raster. Two or more input pixels can be mapped to the same output pixel. Some output pixels might not get a value assigned. Gaps occur. Backward mapping For each pixel in the output image (x, y ), the position in the input image (x, y), where the gray value should be taken from is computed using T 1. No gaps will appear. Problem: T 1 might not always exist.

26 Image function approximation 26/32 The principally correct answer provides the approximation theory. The continuous 2D function is estimated from available samples. Often, the estimated 2D function is approximated locally by a 2D polynomial. Q: Why is the approximation by polymials popular? A 1 : A polynomial has a few parametrs only. The best values of the parameters are sought using an optimization method. Polynomials have continuous derivatives. Consequently, the optimum can be derived analytically or can be computed numerically using the gradient method. A 2 : In the case that the approximation polynomial parameters are estimated experimentally, there are usually many more examples than parameters. Having in mind A 1, the needed derivatives can be expressed analytically.

27 Approximation as 2D convolution 27/32 Instead of the originally continuous image function f(x, y), its sampled variant f s (l, k) is available. The outcome of the approximation (interpolation) is the brightness fn(x, y), where index n denotes individual interpolation methods. The brightness can be expressed as a 2D convolution f n (x, y) = f s (l, k) h n (x l, y k). l= k= Function hn is the interpolation kernel. Often, the local approximation kernel is used, which covers only a small neighborhood of the current pixel, to save computations. The values of the kernel h n = 0 outside the kernel domain.

28 The nearest neighbor approximation 28/32 The point (x, y) is assigned the value of the nearest point g s in the discrete raster. h 1 (x, y) = g s (round (x), round (y)).

29 Linear interpolation 29/32 Explores 4 points in the neighborhood of (x, y) and combines them linearly. The influence of each point in the linear combination depends on the proximity to the current point. f 2 (x, y) = (1 a)(1 b) g s (l, k) + a(1 b) g s (l + 1, k) +b(1 a) g s (l, k + 1) + ab g s (l + 1, k + 1), kde l = ceil (x), a = x l, k = ceil (y), b = y k.

30 Bilinear interpolation 30/32

31 Bicubic interpolation 31/32 h 3 = Interpolates locally the image function by the bicubic polynomial from sixteen points in the neighborhood. 1D interpolation kernel (for simplicity). 1 2 x 2 + x 3 pro 0 x < 1, 4 8 x + 5 x 2 x 3 pro 1 x < 2, 0 jinde.

32 Image enlarding, three methods comparison 32/32 Courtesy: Richard Alan Peters II, Vanderbild University

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