Shadow brightening image enhancement

A shadow brightening method includes receiving, at a memory device, an original input image, a brightening level, and a threshold pixel intensity. If a pixel intensity is greater than the threshold, then the pixel is considered bright. Otherwise, the pixel is shadowed. The method includes calculating a gamma transformation for each pixel. If the pixel intensity is less than or equal to the threshold, then a gamma transformation equal to the received brightening level is applied. If the pixel intensity is greater than the threshold, then the gamma transformation is scaled to decrease with intensity. For each shadowed pixel, the method includes computing a minimum value. It also includes determining the brightening level to be applied, thus creating a gamma map. The method also includes applying the determined brightening level to the shadowed pixels and outputting a shadow-brightened output image.

BACKGROUND OF THE INVENTION

Field of Invention

This disclosure relates to image processing, and more particularly, to shadow brightening in images.

Description of Related Art

Shadowed regions are common in backlit imagery where the light source is behind the subject. The standard brightening approach is to use a gamma (power-law) transformation. The pixel intensity is raised to a power (gamma) where gamma is less than intensity. This transformation maps a narrow range of dark input values to a wider range of output levels, with the opposite true for higher values of input levels. The opposite is true for gamma values that are greater than one.

The gamma transformation operates over the entire image. That is, the dark regions get brighter and the bright regions get brighter. This can often lead to “blooming” in the brightened image and a washed out result in general.

Another previous method that results in brightening of the image is to use histogram equalization. While not a brightening technique per se, histogram equalization has the effect of spreading the pixel intensities over the entire range. For a very dark image, this means that some areas become brighter, while the darkest stay dark. The result of histogram equalization can be very harsh, especially for a dark image where a relatively small number of levels get spread over the entire range. Contouring is a common objectionable artifact from histogram equalization.

For some applications, shading correction by imaging a target of constant intensity may be used to correct the brightness. In a controlled setting like a manufacturing plant, for example, a reference image can be used to correct the brightness and illumination differences in other images. The reference image could be obtained prior to the start of imaging using a constant intensity target. The reference image may show the illumination function that can be used to correct the rest of the images. This approach can work well, but this approach is obviously not applicable to most situations. A related approach is to use skin-colored pixels as a reference for brightness adjustment. However, this is only valid when the subject is human and of expected skin tone.

Other previous methods sometimes employed are top-hat/bottom-hat corrections for light objects on a dark background and dark objects on a light background, respectively. These techniques work well for certain types of images (e.g. inspection), but have limited usefulness for natural scenes. Local thresholding using moving averages can be used to account for illumination differences, but does not enhance the image for the viewer.

A method to deconstruct an image into illumination and reflectance components called homomorphic processing has been used with success in certain conditions, but it does not work well against a variety of images. This is sometimes referred to as separating the image into low and high frequency luminance components. Tone mapping approaches have also been explored, but similar to gamma transformations, these have been applied to the whole image, causing the same drawbacks. More complicated techniques such as genetic algorithms have been proposed as well, but these are very complex and computationally expensive.

There is a need for a shadow brightening system and method that reduce the chances of blooming or washed out portions, that does not result in contouring artifacts, and that works with non-humans and/or unexpected skin tones. There is further a need for a shadow brightening system and method that are useful for natural scenes.

BRIEF SUMMARY OF INVENTION

The present disclosure provides a shadow brightening method and system. In accordance with one embodiment of the present disclosure, a method for shadow brightening is provided.

The method comprises receiving, at a memory device, an original input image, a brightening level, and a threshold pixel intensity, wherein if a pixel intensity in the original input image is greater than the threshold pixel intensity, then the pixel intensity is a bright pixel, and wherein if a pixel intensity for a pixel in the original input image is less than or equal to the threshold pixel intensity, then the pixel is a shadowed pixel.

The method further includes calculating, via a processor, a gamma transformation for each said pixel, wherein if the pixel intensity is less than or equal to the threshold pixel intensity, then a gamma transformation value equal to the received brightening level is applied. If the pixel intensity is greater than a threshold pixel intensity, then a gamma transformation value that is scaled is applied such that the gamma value decreases with intensity.

Then, for each said shadowed pixel, the method includes computing, via the processor, the minimum of the calculated gamma transformation value and the number one for each said shadowed pixel. The method also includes determining, via the processor, the brightening level to be applied, thus creating a gamma map that indicates brightening levels to be applied to the original input image.

The method also includes applying, via the processor, the determined brightening level to the shadowed pixels; and outputting, via the processor, a shadow-brightened output image.

These, as well as other objects, features and benefits will now become clear from a review of the following detailed description, the illustrative embodiments, and the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

The present system and method for shadow brightening involves enhancing overly dark images by brightening the shadowed portions of an image.

The present system and method are used to brighten the dark, or shadowed, regions of an image while leaving the bright regions of the image intact. This avoids the common problems caused by a standard gamma transformation, such as a washed-out result or blooming of the bright regions. Instead of a single gamma value being applied to every pixel in the original input image, the present system and method let the user divide the image into two regions: shadowed and bright regions. Thereafter, the shadowed and bright regions may be handled separately so that the bright regions are not further brightened, but the shadowed regions are enhanced.

Referring now toFIG. 1, illustrated is a block diagram of a method for shadow brightening in accordance with one embodiment of the present disclosure. At step110, an original input image is provided. This image will have its shadows brightened, if needed. The image may be a still image, or an image from a video or any other image type. It may also be monochrome or color. Along with receiving the original input image, or prior thereto, or even thereafter, the system may also receive a brightening level to be applied. The brightening level may be input by the user or predetermined via the shadow brightening method.

At step120, the method includes receiving input of a threshold pixel intensity, thus allowing the user to set the threshold level (T) for pixel intensities. Pixel intensities below or equal to the threshold are treated as shadowed pixels, and pixel intensities above the threshold are treated as bright pixels.

This value assigned for the threshold pixel intensity will depend on the specific images, but can vary in the range from [0,1] where zero (0) means that all the pixels are above the threshold (all bright) and one (1) means that no pixels are above the threshold (all dark). The image pixels are handled differently depending on whether a pixel is considered to be in the dark/shadowed region or the bright region. If the pixel is in the dark/shadowed region, then at step130, the gamma (γ) value for the power-law transformation is just the user input value, L. This value L determines the level of brightening to be applied and is in the range of [0,1]. The lower the value of L, the more brightening is applied. At zero (0), all the pixels become white and at one (1), no change is applied to the pixels.

If the image pixel intensity value is greater than the threshold T, then at step140, the gamma value used in the power-law transformation is calculated differently than at step130. In this case, at step140, the gamma value is a function that decreases with intensity, e.g., in accordance with the following equation:
γx,y=L*eS(Ix,y−T)(Equation 1)

Specifically, the further the pixel intensity is from the threshold, the less brightening is applied. If the pixel intensity is above, but near the threshold, the term in the exponent S (Ix,Y−T) evaluates to approximately zero (0), so the gamma value is nearly L. As the pixel intensities get farther and farther from the threshold, the term in the exponent increases, which increases gamma. A larger gamma means less brightening. The value of the scale factor S changes the rate at which the exponent increases.

Referring now toFIG. 2, illustrated is an example of pixel mapping for shadow enhancement in accordance with one embodiment of the present disclosure. The identity transformation is the straight line where input equals output. The standard gamma function as shown in the dotted line where it can be seen that even high input values map to even higher output values (e.g. output=0.92 for input=0.8). The pixel mapping for the shadow enhancement is shown by the solid black line. The same value was used for gamma and L (0.4 for this example). In this example, a threshold of 0.3 was chosen for the shadows enhancement and it can be seen fromFIG. 2that this is where the shadow enhanced pixel mapping diverges from the prior art gamma pixel mapping. There is a transition region between the shadowed and bright areas of image from 0.3 to 0.45. The larger the value of the scale factor, the smaller this transition region (that is, the shadows pixel mapping curve diverges from the gamma curve faster). Conversely, the lower the scale factor, the wider the transition region (slower it diverges from gamma curve and the more it resembles the gamma curve). For the shadow enhancement, for input=0.8, the output=0.8 in this example.

The scale factor has the effect of changing how abrupt the transition from shadowed to bright regions is. A higher value of S means a more abrupt transition and a lower value means a less abrupt transition. If S is lowered enough, the transition ceases to exist and the whole image is treated as the shadowed region. Referring back toFIG. 1, after the gamma value is calculated for each pixel in the bright region, at step150, the minimum of the calculated gamma and one (1) is computed. That is, if any gamma value is greater than one (1), the gamma value is reduced to one (1). This is necessary because otherwise, gamma values over one (1) could occur, which have the effect of darkening the bright pixels. The result would end up looking like an image with the intensities inverted. Instead, by limiting the gamma values to one (1), if a pixel has a gamma value of one (1) then that pixel will be unchanged.

After a gamma value is calculated for each pixel in the image, thus creating a gamma map, then at step160, the gamma value is applied to each pixel in the standard power-law transformation. Since the gamma values in the bright region are a function of intensity, but the gamma values in the shadowed region are constant, the shadowed areas are brightened, but the bright values are primarily left untouched (except in the transition region). For these pixels, the gamma value was greater than or equal to one and was reduced to one (1). Raising the pixel intensity to the power of 1 has no effect.

At step170, the shadow brightened image is output. Referring now toFIG. 3A, illustrated is the original input image in contrast to the shadow enhanced image ofFIG. 3Bin accordance with one embodiment of the present disclosure. Shown is the Earth and the International Space Station (ISS). Note that the intensity of the Earth is largely unchanged, but now the details of the ISS are much more evident. For example, in the top and bottom of the image, the details on the ISS are much easier to see. Additionally, we can now see the atmosphere of the Earth on the horizon.

It is clear that only the dark portions of the image were changed when looking at the magnitude of the differences between the shadow enhanced image and the original images. This is evident inFIG. 4which is an illustration of a magnitude of differences between enhanced shadows and the original input image in accordance with an embodiment of the present disclosure. Note that the Earth area is all black since this portion of the image has not changed at all, nor have the brightest portions of the ISS.FIG. 4highlights where the details in the shadows have been enhanced. A gamma map (not shown), as known in the art, may indicate brightening levels to be applied to the original input image. For example, pixels given a value of 1 (white) are changed pixels, and 0 (black) for unchanged pixels.

The advantages of the present method over the old methods are that it brightens the dark, or shadowed, regions of an image while leaving the bright regions of the image intact as shown inFIGS. 3 and 4. This avoids the common problems caused by other methods (e.g. standard gamma transformation) of a washed out result or blooming of the bright regions.

Instead of the blooming that can be caused by typical brightening schemes, the present system and method avoid blooming with the independent adjustment of the threshold level to define the shadows (T), a level of brightening (L), and a scale factor (S) to determine the sharpness of the transition between the shadowed and bright areas.

The washed out look from a prior art gamma correction and the shadows enhanced version may be seen inFIGS. 5A-5C.FIG. 5Aillustrates the original input image.FIG. 5Billustrates a gamma-corrected image forFIG. 5A.FIG. 5Cillustrates a shadow-enhanced version of the image ofFIG. 5Ain accordance with one embodiment of the present disclosure. Note how the Earth in the gamma-corrected image (FIG. 5B) has been made too bright, but this is not the case for the shadow-enhanced image (FIG. 5C).

Another way to highlight the differences between the standard gamma correction and the new shadow enhancement is to examine the histograms shown inFIGS. 6A-6C.FIG. 6Ashows the original histogram,FIG. 6Bshows the gamma-corrected histogram andFIG. 6Cshows the shadow-enhanced histogram.

FIG. 6Ais the original histogram, of the original input image. Clearly, there are many pixels in the lower end of the histogram, which are the shadowed pixels. Using the same value for gamma and L, both the gamma correction and shadow enhancement were applied separately. InFIG. 6B, the histogram of the gamma-corrected image is shown. The large spike from the input histogram has shifted up and the rest of the histogram has been compressed. However, in the histogram from the shadows enhanced image on the right inFIG. 6C, it may be seen that the histogram from about 0.75 to 1 has been left untouched by the enhancement when compared to the original histogram.

Another new feature of this method is the level of fine control over how the shadowed and bright regions are separated. That is, the threshold controls the pixel intensity at which to set the threshold while the scale factor independently determines how sharp that transition is. The level of brightening then affects how bright the shadowed regions will become.

An advantage of the present system and method is the simplicity. It is computationally efficient and though three parameters seem burdensome to adjust, it has been found that the scale factor is nearly always a constant and the other two parameters only need to be varied over a very a small range. In fact, for a wide variety of images, a single set of parameters has been shown to be very effective. The present system and method is much simpler than, say, a genetic algorithm.

Another advantage of the present system and method is that they work across a variety of natural images including images from space (e.g.FIG. 3), images in air, and images underwater. Instead of previous approaches that required a target of constant intensity to be imaged or only worked for certain classes of images (e.g. white objects on a black background or vice versa), this invention does not require any reference images and works for any type of imagery.

One alternative is that the present system and method could be reversed, so to speak, and be used to darken the bright areas of an image while leaving the darker areas untouched. In this case, the level of brightening would take on values greater than one (1) and the pixels below the threshold would be handled differently (instead of those above the threshold for the shadow enhancement).

Another alternative would be that the selection of the threshold and level of brightening could be automated (the value of scale does not seem to vary much) based on histogram metrics or otherwise, instead of having the user select values.

The gamma map may be manipulated to prevent very small regions of dark pixels from being brightened. For example, groups of pixels numbering smaller than ten might be eliminated before the power-law transformation.

Instead of an actual power-law transformation calculated with gamma, a pixel mapping look-up table could be computed to reduce computations and improve speed.

The present system and method can be used for monochrome images or color images by converting the color image to an intensity image prior to processing.

FIG. 7is a system for shadow brightening in accordance with one embodiment of the present disclosure. The system700may include an imaging system710, a personal computer720that is operably coupled to the imaging system710, a processor730that is operably coupled to the imaging system710and a display740that is operably coupled to the imaging system710.

The imaging system710could be any digital imaging system. Digital imaging system710can connect to personal computer720. The original input image may be fed from the imaging system710to the personal computer720. The personal computer720, which may include its own memory and processor, may feed the image to another processor730such as a graphics processing unit.

As an alternative to the system illustrated inFIG. 7, the personal computer720may be removed and the imaging system710and processor730can be connected immediately adjacent to each other. Some processing that was done by the personal computer720may be off-loaded to the imaging system710(which may include a processor) and/or the processor730shown inFIG. 7.

Software (not shown inFIG. 7) may be resident in the memory of personal computer720, which may cause the processor730to perform one or more steps of the method for shadow brightening as set forth herein.

A memory resident on imaging system710and/or personal computer720, as noted hereinabove, is sufficient to hold at least the original input image, a brightening level L and a threshold pixel intensity. A memory resident on imaging system710and/or personal computer720, may also include other elements such as copies of the original input image, as well as processing steps or instructions related to shadow enhancement. Examples of such processing steps are described in the flow block diagram ofFIG. 1.

The speed of the processor730needed may depend on the application in which the processor730is used, as can be appreciated by one of ordinary skill in the art.