TWI414180B - Apparatus and method for contrast enhancement - Google Patents

Abstract

Apparatus and method for contrast enhancement are disclosed. An adjustable blending unit is utilized for adjustably blending a linear contrast function with a nonlinear contrast function, thereby resulting in an adjustable contrast curve. The nonlinear contrast function is adjustable by a blending parameter such that distance of the contrast curve to linear contrast curve may be changed.

Description

Contrast enhancement device and method

The present invention relates to contrast enhancement, particularly contrast enhancement using an adjustable and adaptable transfer function.

Contrast enhancement is an important digital image processing that is often used to improve the visual quality of images with poor intensity distribution. For example, a faded image whose intensity distribution is limited to a portion of the grayscale value.

Traditional chromatographic equalization methods are often used to redistribute or equalize the chromatographic curve of the entire image. However, because it requires a lot of calculations, it is a slow and time-consuming operation. Binary contrast enhancement or thresholding is also one of the traditional contrast enhancement methods. In this method, all luminance values less than the threshold are set to black (0), and all luminance values greater than the threshold are set to white (255). Because the binary contrast enhancement method produces extremely high contrast images, its application is limited, such as the wheel used to identify objects in the image. Profile or shape. In view of this, the general contrast enhancement uses a less steep transition curve.

The lookup table method is also one of the common ways of traditional contrast enhancement. However, this method of looking up requires a considerable amount of memory space, and it takes a lot of read cycles to read data from the memory. Piecewise linear approximation is another common way of traditional contrast enhancement. However, this approach requires a lot of registers to store the end points of the segments and is prone to approximation errors.

Therefore, there is a need for a quick and easy way to perform contrast enhancement.

In view of the above, it is an object of the present invention to provide a quick and easy way of contrast enhancement. The calculations required for such contrast enhancement methods and their conversion curves are much less than those of conventional methods. Furthermore, the user can conveniently scale the shape of the conversion curve, adjust its strength, and change its critical point (or transitional point).

In accordance with an embodiment of the invention, an adjustable mixing unit is used to mix the linear contrast function with the nonlinear contrast function, thereby forming an adjustable contrast curve. The nonlinear contrast function can be adjusted by mixing parameters such that the distance from the contrast curve to the linear contrast curve is changed. The contrast curve can also be adjusted by the intensity parameter such that the transition point or critical point of the contrast curve is changed.

The block diagram of the first diagram shows a contrast enhancement device 100 in accordance with one embodiment of the present invention, and the second diagram shows a contrast enhancement method 200 in accordance with an embodiment of the present invention. The apparatus 100 and method 200 of the present embodiment are used to improve the visual quality of images with poor intensity distribution. This embodiment uses 8-bit to represent the luminance value, so there are a total of 256 gray levels (i.e., 0 to 255). However, other bit numbers can also be used depending on the specific system design requirements.

In this embodiment, the contrast enhancement conversion function is defined as follows: Y"=Y+(Y'-Y)*b (2) a=round(avgBrightness*p) (3) where Y represents the luminance value (or luma value) of the input pixel, and Y' represents the base contrast curve. The output value, Y" represents the luminance value of the output pixel, p is the strength parameter defining the contrast enhancement, and b is the parameter defining the closeness of the contrast enhancement curve to the linear contrast enhancement curve.

The avgBrightness in the equation (3) represents the average luminance value of the current image. In this embodiment, in step 20 (second figure), the pixels of the entire image frame or the image window portion to be contrast-enhanced are obtained by an adaptable brightness value unit 10 (first image). Average brightness value. In this specification, the term "unit" can be used to refer to a circuit, a program segment, or a combination thereof. The average luminance value is influenced by the luminance value "a" to affect the output value Y' of the equation (1), and further affects the Y" of the equation (2). The apparatus 100 and the method 200 adaptively (adaptable) the average luminance for the variation The value is automatically changed by its contrast enhancement. This suitability is especially useful when the device 100 is input images of different average brightness from different imaging devices.

The luminance value "a" is further adjusted by steps 21 to 22. Wherein, in step 21, when it is necessary to adjust the intensity of the contrast enhancement, the intensity parameter p is obtained in step 22 or input by the user, and transmitted to the appropriate (adaptable) luminance value unit 10. In the present embodiment, the average luminance value (avgBrightness) is multiplied by the intensity parameter p using equation (3). The operator "round" in equation (3) performs a mathematical rounding operation. However, if the device 100 is a non-integer system, the rounding operation can be omitted. The effect of the intensity parameter p on contrast enhancement will be detailed later.

The Y' function in equation (1) represents a base contrast curve (steps 23 and 12), which corresponds to the contrast curve of b=1 in the third graph. The Y" function in the formula (2) represents a general contrast curve which is combined or blended by the linear portion Y and the nonlinear portion (Y'-Y). The mixing of the function Y" (step 25) is The non-linear portion (Y'-Y) is multiplied by a mixing parameter b, which can be input (step 24) by the user into the adjustable mixing unit 14. When b = 1, the general comparison curve Y" becomes the basic contrast curve Y'; when b = 0, the general comparison curve Y" becomes the linear contrast curve. As shown in the third figure, the distance from the comparison curve Y"(b≠0) to the linear contrast curve (b=0, that is, without any contrast enhancement) increases with the increase of the mixing parameter b, and vice versa. The mixing parameter b decreases and decreases. The third graph also shows that each comparison curve converges to both ends (ie, 0 and 255 in the legend), and all of the comparison curves have the same transition point "a". The comparison curve includes a first curve having a first direction facing the linear contrast curve; and including A second curve having a second direction facing the linear contrast curve, wherein the second direction is substantially opposite to the first direction.

As previously mentioned, the basic contrast curve is a function of the luminance value "a", and when the intensity adjustment is selected, the luminance value "a" is again a function of the intensity parameter p. The fourth graph shows the contrast curves corresponding to the various intensity parameters p. The intensity of the contrast curve Y" increases as the intensity parameter p increases, whereas the intensity decreases as the intensity parameter p decreases. In the present embodiment, the parameter p greater than 1 makes the contrast enhancement become more aggressive (aggressive). Or a larger intensity, while a parameter p of less than 1 makes the contrast enhancement less intense or less intense.

Accordingly, the embodiments of the present invention provide an apparatus and method that are fast and simple to perform contrast enhancement. The amount of calculation required for the contrast enhancement and its contrast curve disclosed in the embodiments of the present invention is much less than that of the conventional contrast enhancement method. Furthermore, the user can conveniently scale and adjust the intensity of the contrast curve. In addition, in the present embodiment, the use of a single 6-bit register is sufficient to simultaneously store the mixing parameter b and the intensity parameter p. The apparatus and method of an embodiment of the present invention adaptably adjusts the contrast enhancement for the varying average brightness value.

The above description is only the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention; all other equivalent changes or modifications which are not departing from the spirit of the invention should be included in the following Within the scope of the patent application.

100‧‧‧ contrast enhancement device

10‧‧‧adaptable brightness value unit

12‧‧‧Basic comparison curve

14‧‧‧Adjustable mixing

200‧‧‧ contrast enhancement method

20-25‧‧‧Steps to compare enhancement methods

The block diagram of the first figure shows a contrast enhancement apparatus according to one embodiment of the present invention.

The second figure shows a contrast enhancement method of an embodiment of the present invention.

The third graph shows the corresponding curves for the various mixing parameters.

The fourth graph shows the contrast curves for the various intensity parameters.

100‧‧‧ contrast enhancement device

10‧‧‧adaptable brightness value unit

12‧‧‧Basic comparison curve

14‧‧‧Adjustable mixing

Claims (16)

A contrast enhancement device comprising an adjustable mixing circuit for mixing a linear contrast function and a nonlinear contrast function to form an adjustable contrast curve; wherein the nonlinear contrast function comprises a basic contrast function and the linear contrast The difference between the functions, wherein the basic contrast function defines a basic contrast curve; wherein the basic contrast function is a function of the average brightness value of the image. The contrast enhancement device of claim 1, wherein the nonlinear contrast function described above is adjustable. The contrast enhancement device of claim 2, wherein the nonlinear contrast function is adjusted by mixing parameters such that the distance from the contrast curve to the linear contrast curve is changed, wherein the linear contrast curve represents an unacceptable Any contrast enhancement function. The contrast enhancement device of claim 3, wherein the contrast function defines a first curve having a first direction facing the linear contrast curve; and a second curve having a linear orientation A second direction of the contrast curve, wherein the second direction is substantially opposite to the first direction. The contrast enhancement device of claim 1, wherein the image average brightness value is adjustable. The contrast enhancement device of claim 5, wherein the image average brightness value is adjusted by an intensity parameter such that a transition point of the comparison curve is changed. The contrast enhancement device of claim 1, wherein the basic comparison function is: Where Y' represents the output value of the basic contrast curve, Y represents the brightness value of the input pixel, where a=round(avgBrightness*p), avgBrightness represents the average brightness value of the image, and round is a mathematical rounding operation. And p represents the intensity parameter of the contrast enhancement. The contrast enhancement device of claim 7, wherein the above comparison curve can be expressed as: Y" = Y + (Y' - Y) * b Wherein Y" represents the luminance value of the output pixel, and b is a parameter defining a closeness of the contrast curve to the linear contrast curve, wherein the linear contrast curve represents a function that is not subjected to any contrast enhancement; wherein the above Y represents The linear contrast function, and (Y'-Y) represents the nonlinear contrast function. A contrast enhancement method is provided in a circuit for performing the following steps: forming a tunable contrast curve by mixing a linear contrast function and a nonlinear contrast function; wherein the nonlinear contrast function includes a basic contrast The difference between the function and the linear contrast function, wherein the basic contrast function defines a basic contrast curve; wherein the basic contrast function is a function of the average brightness value of the image. The contrast enhancement method of claim 9, wherein the nonlinear comparison function is adjustable. The contrast enhancement method according to claim 10, further comprising a step of adjusting the nonlinear contrast function by mixing parameters such that the distance from the contrast curve to the linear contrast curve is changed, wherein the linear contrast curve represents a A function that is enhanced by any contrast. The contrast enhancement method of claim 11, wherein the contrast function defines a first curve having a first direction facing the linear contrast curve; and a second curve having a linear orientation A second direction of the contrast curve, wherein the second direction is substantially opposite to the first direction. The contrast enhancement method of claim 9, wherein the image average brightness value is adjustable. The contrast enhancement method according to claim 13 further includes a step of adjusting an average brightness value of the image by an intensity parameter such that a conversion point of the comparison curve is changed. A contrast enhancement method as described in claim 9 wherein the basic comparison function is: Where Y' represents the output value of the basic contrast curve, Y represents the brightness value of the input pixel, where a=round(avgBrightness*p), avgBrightness represents the average brightness value of the image, and round is a mathematical rounding operation. And p represents the intensity parameter of the contrast enhancement. The contrast enhancement method according to claim 15, wherein the comparison curve can be expressed as: Y"=Y+(Y'-Y)*b, where Y" represents the luminance value of the output pixel, and b is a definition a parameter comparing the curve to the closeness of the linear contrast curve, wherein the linear contrast curve represents a function that is not subjected to any contrast enhancement; wherein Y represents the linear contrast function, and (Y'-Y) represents the nonlinearity Comparison function.