Image rate increasing

FIELD OF THE INVENTION

The invention relates to image rate increasing.

BACKGROUND OF THE INVENTION LCD displays typically suffer from motion blur, caused by a slow response time of the liquid crystal material and the hold time of the picture. In order to reduce motion blur, the hold time of the display can be reduced. Typically, increasing the refresh rate reduces the hold time of a display. Various techniques for generating the extra frames to achieve a higher refresh rate have been disclosed, such as black frame insertion, grey frame insertion and dynamic frame insertion. The latter one (DFI) has been disclosed by Han-Feng Chen, et. al., Smooth Frame Insertion Method for Motion-Blur Reduction in LCDs, Samsung Electronics Co., Ltd., EuroDisplay 2005, using the name Smooth Frame Insertion.

The DFI method doubles the frame rate of a video stream by alternatingly showing blurred and peaked pictures derived from the input pictures. The blurred and peaked pictures are created such that the average of the peaked and blurred image is equal to the input picture. As such DFI reduces the hold time of spatial details by a factor of 2, resulting in less motion blur, while the large areas remain at the original hold time, preventing flicker. The DFI solution has the disadvantage that alternating blurred and peaked frames increases the frame-by- frame picture differences, which has a negative influence on the LCD panel behavior. This leads to a decrease of the LCD panel performance and the visibility of artifacts for near black and/or near white picture details. These artifacts are often more visible and annoying than motion blur.

SUMMARY OF THE INVENTION It is, inter alia, an object of the invention to provide an improved image rate increasing. The invention is defined by the independent claims. Advantageous embodiments are defined in the dependent claims.

In one embodiment, the invention reduces the visibility of artifacts by reducing the alternating frame differences for near black and/or near white picture details.

This is done by mixing the input picture with the DFI result based on the local luminance of the input picture. By defining a proper mixing function the optimal trade-off between artifacts and motion blur visibility can be obtained.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a block diagram of a basic dynamic frame insertion; Fig. 2 shows a block diagram of a luminance-dependent dynamic frame insertion in accordance with an embodiment of the present invention; and

Fig. 3 shows a mixing profile for use in the embodiment of Fig. 2.

DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a basic implementation of the DFI algorithm. An input picture I is applied to a low-pass filter LPF and to a non-inverting input, multiplying by 2, of a subtracter ∑l. An output of the subtracter ∑l is a first output signal A. An output of the low- pass filter LPF is a second output signal B, and is applied to an inverting input of the subtracter ∑l . As a result, each input image I is converted to a peaked picture A and a blurred image B, which are displayed one after the other so as to obtain an image rate doubling. Note that A = 2 x I - B, so that the average output signal equals the input signal: (A+B)/2 = I. By subtracting a low-pass filtered signal from an input signal, the subtracter ∑l acts as a sharpening filter.

The embodiment of Fig. 2 differs from the circuit of Fig. 1 in that the output of the low-pass filter LPF is applied to a first input, multiplying by a factor (1- α), of an adder σ2, which also has a second input, multiplying by a factor α, receiving the input signal I. The output of the adder σ2 is then connected as the output of the low-pass filter LPF was connected in Fig. 1. Another expression for an adder with weighed inputs is a mixer.

The embodiment of Fig. 2 adds a controlled mixing of the input picture I with A and B where the mixing factor α is a function of the luminance of the input picture I such that

A = (2-α) x I - (1- α) x LP and

B = α x I + (l- α) x LP, with LP being the low-pass filtered input picture.

The mixing function is chosen such that the signal range of the input picture maps to a value for α for each input pixel between 0 and 1, where α = 0 results in an output signal identical to Fig. 1 and α = 1 results in A = I and B = I. Typically the mixing function is chosen as sketched in Fig. 3, where n and m are LCD panel-specific parameters, n is chosen as low as possible and m is chosen as high as possible such that DFI performance is maximized, while LCD panel artifacts are not visible. Clearly, alternative mixing functions are possible, which preferably follow the principle that for intermediate values of the input signal the known dynamic frame insertion method is given more weight, while for extreme values of the input signal, the input signal is given more weight. On the basis of the selected mixing function, a skilled person can easily program a look-up table so as to obtain the desired result.

In sum, one aspect of the invention relates to improvements on the Dynamic Frame Insertion method that increases the picture rate of a video stream, in order to reduce the motion blur due to the LCD's sample and hold principle. In an embodiment, the improvements include a luminance-dependent mixing of input and the low-pass filtered image. The invention can be applied in video processing pipelines for TV systems, e.g. backend-scaler ICs on the small-signal board of an LCD-TV or TCON ICs on the LCD panel itself.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. It is possible that one input image is used to obtain more than two (e.g. 3) output images, and that two input images are used to obtain less than four (e.g. 3 in a 50 to 75 Hz conversion) output images. In the claims, the first output signal B is not necessarily displayed before the second output signal A; the reverse order is equally possible. While in the preceding embodiments a subtracter ∑l is used to obtain the peaked signal A, it is alternatively possible to use other sharpening filters, such as a high-pass filter. Preferably, such alternative sharpening filter is controlled by α so that the relation (A+B)/2 = I still holds. The invention is preferably applied to RGB signals, but may alternatively be applied to a composite video signal, to YUV signals or to the Y signal only. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an

element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and/or by means of a suitably programmed processor. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.