CN102523386B - Auto Exposure Method Based on Histogram Equalization - Google Patents

Abstract

The invention relates to an automatic exposure method based on histogram equalization, which comprises the steps of collecting a frame of image, recording the exposure time of the frame, calculating the brightness mean value of the collected image, carrying out histogram equalization processing on the collected image, calculating the gray mean value of the image after the histogram equalization processing, calculating the exposure time of the next frame according to the principle that the exposure time is in direct proportion to the brightness mean value of the image, setting the exposure adjustment tolerance and the like. The invention solves the technical problem that the shooting subject has underexposure or overexposure in the prior art, and adopts the gray level mean value of the image after histogram equalization as the optimal mean value. The optimal mean value can fully reflect the brightness characteristics of the image, and the optimal mean value is dynamic and can adapt to various brightness environments.

Description

Auto Exposure Method Based on Histogram Equalization

technical field

The invention relates to an automatic exposure method based on histogram equalization.

Background technique

The exposure method adopted by the camera can be either metered manual exposure or automatic exposure.

Metering manual exposure is an exposure method in which the photographer chooses the aperture factor and shutter speed, and uses the camera's metering system to determine whether the exposure is appropriate.

Automatic exposure is an exposure method in which the camera automatically selects the aperture factor and shutter speed according to the metering results of its own metering system to obtain accurate exposure. Automatic exposure can be divided into aperture priority automatic exposure, shutter priority automatic exposure, program automatic exposure and other forms.

Aperture-priority automatic exposure means that the aperture factor is selected artificially, and the camera automatically selects the shutter speed.

Shutter-priority auto exposure means that the shutter speed is selected artificially, and the camera automatically selects the aperture factor.

Programmed automatic exposure means that the aperture factor and shutter speed are automatically adjusted by the camera's built-in program. Its advantage is that it does not require the photographer to set the aperture factor and shutter speed.

In the traditional automatic exposure control methods, some use a specific microprocessor hardware circuit to control the exposure; some use the overall brightness of the image to control the exposure, but if the brightness difference between the subject and the background is too large, it will make the subject The phenomenon of underexposure or overexposure occurs; some use the reference brightness value to divide the image into blocks, and the brightness of each sub-image is used to set the reference brightness value. The reference brightness value can be obtained by adjusting the size of the aperture, of course, it can also be The reference brightness value is obtained by setting the shutter speed; others control the exposure by studying the relationship between the brightness and the exposure value under different lighting conditions.

Contents of the invention

In order to solve the technical problem of underexposure or overexposure of the subject in the prior art, the present invention provides an automatic exposure method based on histogram equalization.

Technical solution of the present invention:

The automatic exposure method based on histogram equalization is special in that it includes the following steps:

1] Collect a frame of image, and record the exposure time t _old of the frame;

2] Calculate the brightness mean value m(Y) of the collected image;

3] Perform histogram equalization processing on the collected images;

4) Calculating the gray mean value Yt of the image processed by the histogram equalization;

5] Calculate the exposure time t _new of the next frame, according to the formula t _new =t _old *(Y _t /m(Y)) (the exposure time is proportional to the average brightness of the image)

6] If |t _new -t _old |<δ (exposure adjustment tolerance), it means that the exposure time of the next frame does not need to be adjusted, t _new =t _old , continue to step 1];

If |t _new -t _old |＞δ, it means that the exposure time of the next frame needs to be adjusted according to the new exposure time t _new , and proceed to step 1].

The advantages that the present invention has:

1. The image after the histogram equalization of the present invention has the advantage of covering the gray level value of the entire gray scale range, that is, the contrast of the image is enhanced, so we can use the average gray value of the image after the histogram equalization as the optimal mean. The optimal mean value can fully reflect the brightness characteristics of the image, and the optimal mean value is dynamic and can adapt to various brightness environments.

2. The present invention does not need to manually set the optimal average brightness value.

3. The data processing capacity of the present invention is small.

Description of drawings

Fig. 1 is a flowchart of the present invention.

Detailed ways

As shown in Figure 1, the automatic exposure method based on histogram equalization includes the following steps:

1] Collect a frame of image, and record the exposure time t _old of the frame;

2] Calculate the brightness mean value m(Y) of the collected image;

3] Perform histogram equalization processing on the collected images;

4) Calculating the brightness mean value Yt of the image processed by the histogram equalization;

5] Calculate the exposure time t _new of the next frame, according to the formula t _new =t _old *(Y _t /m(Y)) (the exposure time is proportional to the average brightness of the image)

6] If |t _new -t _old |<δ (exposure adjustment tolerance, the value is based on the subjective perception of image brightness as the selection criterion), it means that the exposure time of the two frames before and after is basically the same, and the exposure time of the next frame does not need to be adjusted. t _new ＝t _old , proceed to step 1】;

If |t _new -t _old |>δ, it means that the exposure time of the two frames before and after is quite different, and the exposure time of the next frame needs to be adjusted according to the new exposure time t _new , and continue to step 1];

The principle of the present invention: the exposure adjustment of the camera is based on the principle that the image brightness is proportional to the exposure time, that is, the longer the exposure time, the brighter the image and the higher the image brightness; the shorter the exposure time, the darker the image and the lower the image brightness. When the captured image is underexposed, it is reflected in the image that the average brightness value is small, and the image after using the histogram equalization will cover the gray value of the entire gray scale range, that is, the average value of the image after the histogram equalization is larger, according to t _new =t _old *(Y _t /m(Y)) is adjusted, and the calculated t _new will be longer than t _old , thus achieving the purpose of prolonging the exposure time when the exposure is insufficient. vice versa.

Claims (1)

1. the automatic explosion method based on histogram equalization, is characterized in that: comprise the following steps:

1] gather a two field picture, and record the time for exposure t of this frame _old;

2] calculate the brightness average m (Y) of the image that gathers;

3] gathered image being done to histogram equalization processes;

4] calculate the gray average Y of the image after histogram equalization is processed _t;

5] principle that is directly proportional to the brightness average of image according to the time for exposure, calculates the time for exposure t of next frame _new, according to formula t _new=t _old* (Y _t/ m (Y));

6] setting exposure adjustment tolerance limit is δ;

If | t _new-t _old| < δ, the real exposure time of next frame is t _old, continue execution step 1];

If | t _new-t _old| > δ, the real exposure time of next frame is t _new, continue execution step 1].

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