Detection of and correction for specular reflections in digital image acquisition
First Claim
1. A method for detecting and processing a specular image density level X1 and a highlight image density level X2 in an illuminated scene comprising the following steps:
- exposing a photo-sensitive means by a luminous image from said scene, thereby generating a plurality of digital image signals, each said digital image signal representing a density of a portion of said luminous image;
establishing a cumulative histogram for the image density levels of said digital image signals;
providing a reference specular cumulative frequency Y1 ;
providing a reference highlight cumulative frequency Y2 ;
converting said specular cumulative frequency Y1 via said cumulative histogram, to obtain said specular image density level X1 ;
converting said highlight cumulative frequency Y2 via said cumulative histogram, to obtain said highlight image density level X2 ; and
indicating a presence of specular reflections when a ratio α
=∥
XMAX -X2 ∥
/∥
XMAX -X1 ∥
is smaller than a predetermined value, wherein XMAX is an image density level greater than or equal to a highest density level within said luminous image.
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Abstract
Image processing parameters for gradation correction of a digital image from a scene, which includes specular reflections may be seriously influenced by the presence of those specular reflections. Therefore, a method is developed to detect the presence of specular reflections in a scene, to modify the exposure time of the photo-sensitive means, which converts a luminous image to electronic image signals and to apply a gradation correction to the image data thus acquired. In one embodiment a digital camera is used to capture the image data, a cumulative histogram is built and two characteristic cumulative frequencies are used to determine via the histogram specular densities and highlight densities. The relative position of these density values indicates whether specular reflections are present, and give suitable parameters for setting an optimal exposure time and for computing a gradation correction curve.
153 Citations
18 Claims
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1. A method for detecting and processing a specular image density level X1 and a highlight image density level X2 in an illuminated scene comprising the following steps:
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exposing a photo-sensitive means by a luminous image from said scene, thereby generating a plurality of digital image signals, each said digital image signal representing a density of a portion of said luminous image; establishing a cumulative histogram for the image density levels of said digital image signals; providing a reference specular cumulative frequency Y1 ; providing a reference highlight cumulative frequency Y2 ; converting said specular cumulative frequency Y1 via said cumulative histogram, to obtain said specular image density level X1 ; converting said highlight cumulative frequency Y2 via said cumulative histogram, to obtain said highlight image density level X2 ; and indicating a presence of specular reflections when a ratio α
=∥
XMAX -X2 ∥
/∥
XMAX -X1 ∥
is smaller than a predetermined value, wherein XMAX is an image density level greater than or equal to a highest density level within said luminous image. - View Dependent Claims (2, 3, 5, 7, 8, 9, 10, 11)
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4. The method according to claim 4, wherein:
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D1 ≦
0.2%; and
,D2 ≦
1.0%.
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6. The method according to claim 6, wherein said exposure time is selected to move either X2 or X1 to an extreme image density level.
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12. A device for detecting and processing a specular image density level X1 and a highlight image density level X2 in a reproduction of an illuminated scene comprising:
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means for exposing a photo-sensitive means by a luminous image from said scene, thereby generating a plurality of digital image signals, each said digital image signal representing a density of a portion of said luminous image; means for establishing a cumulative histogram for the image density levels of said digital image signals; means for providing a reference specular cumulative frequency Y1 ; means for providing a reference highlight cumulative frequency Y2 ; means for converting said specular cumulative frequency Y1 via said cumulative histogram, to obtain said specular image density level X1 ; means for converting said highlight cumulative frequency Y2 via said cumulative histogram, to obtain said highlight image density level X2 ; and means for indicating a presence of specular reflections when a ratio α
=∥
XMAX -X2 ∥
/∥
XMAX -X1 ∥
is smaller than a predetermined value, wherein XMAX is an image density level greater than or equal to a highest density level within said luminous image. - View Dependent Claims (13, 14, 15, 16, 17, 18)
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Specification