Image-dependent exposure enhancement

An image processing system for preparing a color image for printing, and including an exposure adjustment system altering the exposure in the reproduction of an electronically encoded natural scene image, comprising:

• a) a memory receiving and storing an electronically encoded natural scene image, where the encoding is in terms of color density signals;
• b) a color space conversion circuit, converting the color density signals to an image intensity signal format, wherein at least one signal represents image intensity I and is defined for L levels of intensity;
• c) a thresholding circuit, receiving as inputs a pair of preset threshold signals T.sub.light, T.sub.dark, and the intensity signals, and providing at least a multi state output signal respectively indicating satisfactory brightness and darkness of the image;
• d) a logic controller, responsive to the state signal, and controlling transfer of the electronically encoded natural scene image directly to a subsequent image processing element if the signal state indicates that the intensity signal has dark or light values exceeding both threshold signals T.sub.light, T.sub.dark or neither threshold signals T.sub.light, T.sub.dark and responsive to a signal state indicating that the signal state indicates that the intensity signals has light or dark values exceeding one of threshold signals T.sub.light, T.sub.dark generating an evaluation signal;
• e) a .gamma. calculating circuit evaluating:
• ##EQU8## f) means for adjusting the tone reproduction characteristics of a destination reproduction device by the exponential function .gamma..

A method for reproducing a natural scene image from electronically encoded signals, while altering the exposure, comprising the steps of:

• a) receiving signals for an electronically encoded natural scene image, where the encoding is in terms of red-green-blue color density signals;
• b) converting the color density signals to a luminance-chrominance signal format, wherein at least one signal represents image intensity I;
• c) comparing the intensity signal to a pair of preset threshold signals T.sub.light, T.sub.dark, respectively indicating satisfactory brightness and darkness of the image and generating a multi state signal responsive to the comparison;
• d) storing the signals to an output buffer if the signal state indicates that the intensity signal has dark and light values exceeding both threshold signals T.sub.light, T.sub.dark, or neither threshold signals T.sub.light, T.sub.dark, thereby directing the electronically encoded image to a destination printer without exposure alteration;
• e) responsive to a signal state indicating that one of the pair of preset thresholds T.sub.light, T.sub.dark is exceeded, using the value of I.sub.light, I.sub.dark as an address to device memory to retrieve a stored value of .gamma. for the predetermined function of ##EQU9## f) adjusting the tone reproduction characteristics of the destination reproduction device by an exponential function defined by the retrieved value .gamma. for printing portions of the electronically encoded intensity signals representing the natural scene image, in which the intensity signal has dark or light values exceeding one of threshold signals T.sub.light, T.sub.dark.

An image processing system for preparing a color image for printing, and including an exposure adjustment system altering the exposure in the reproduction of an electronically encoded natural scene image, comprising:

• a) a page buffer memory receiving and storing an electronically encoded natural scene image, where the encoding is in terms of red-green-blue color density signals;
• b) a color space conversion circuit, converting the color density signals to a luminance-chrominance signal format, wherein at least one signal represents image intensity I;
• c) a thresholding circuit, receiving as inputs a pair of preset threshold signals T.sub.light, T.sub.dark, and the intensity signals I, and providing a four state output signal respectively indicating satisfactory brightness and darkness of the image;
• d) a logic controller, responsive to the four state signal, and controlling transfer of the electronically encoded natural scene image directly to a subsequent image processing element if the signal state indicates that the intensity signal has dark or light values exceeding both threshold signals T.sub.light, T.sub.dark or neither threshold signals T.sub.light, T.sub.dark and responsive to a signal state indicating the intensity signal I has dark or light values exceeding one of threshold signals T.sub.light, T.sub.dark generating an evaluation signal;
• e) a device memory, operative in response to the evaluation signals, and receiving as an address the dark or light intensity values and returning from the address location in memory a stored value of .gamma. for the predetermined function of ##EQU10## f) tone reproduction correction generator adapted for communication with a destination reproduction device to vary the toner reproduction response thereof by the exponential curve defined by the retrieved value .gamma..

A method for reproducing a natural scene image from electronically encoded intensity signals, while altering the exposure, comprising the steps of:

• a) receiving an electronically encoded natural scene image, where the encoding is in terms of red-green-blue color density signals;
• b) converting the color density signals to an L level luminance-chrominance signal format, wherein at least one signal represents image intensity I;
• c) comparing the intensity signal to a pair of preset threshold signals T.sub.light, T.sub.dark, respectively indicating satisfactory brightness and darkness of the image and generating a four state signal responsive to the comparison;
• d) storing the signals to an output buffer if the signal state indicates that the intensity signal has dark or light values exceeding both threshold signals T.sub.light, T.sub.dark or neither threshold signals T.sub.light, T.sub.dark thereby directing the electronically encoded image to a destination reproduction device without exposure alteration;
• e) responsive to a signal state indicating that the intensity signal has dark or light values exceeding one of threshold signals T.sub.dark, T.sub.light, evaluating ##EQU11## f) repeating steps c), d) and e) for intensity signal I, subtracting those portions of the signal in preselected outer bands of intensity to derive a signal I' and using threshold signals T'.sub.dark, T'.sub.light to obtain .gamma.';
• g) averaging .gamma. and .gamma.' to obtain .gamma..sub.ave ;
• h) adjusting the tone reproduction characteristics of a destination reproduction device by the exponential function .gamma..sub.ave for printing portions of electronically encoded intensity signals in which the intensity signal has dark or light values exceeding one of threshold signals T.sub.light, T.sub.dark.

An image processing system for preparing a color image for printing, and including an exposure adjustment system altering the exposure in the reproduction of an electronically encoded natural scene image, comprising:

• a) a system input receiving an electronically encoded natural scene image, where the encoding is in terms of red-green-blue color density signals;
• b) a color space conversion circuit, converting the color density signals to an L level luminance-chrominance signal format, wherein at least one signal represents image intensity I;
• c) a thresholding circuit, receiving as inputs a pair of preset threshold signals T.sub.light, T.sub.dark, and the intensity signals I, and providing a four state output signal respectively indicating satisfactory brightness and darkness of the image;
• d) a logic controller, responsive to the four state signal, and controlling transfer of the electronically encoded natural scene image directly to a subsequent image processing element if the signal state indicates that the intensity signal has dark or light values exceeding both threshold signals T.sub.dark, T.sub.light or neither threshold signals T.sub.dark, T.sub.light and responsive to a signal state indicating that the signal state indicates that the intensity signals has dark or light values exceeding one of threshold signals T.sub.dark, T.sub.light generating an evaluation signal;
• e) a .gamma. calculating circuit evaluating:
• ##EQU12## f) a thresholding circuit, receiving as inputs a pair of preset threshold signals T.sub.light, T.sub.dark, and the intensity signals, subtracting those portions of the intensity signal in preselected outer bands of intensity, and providing a four state output signal respectively indicating satisfactory brightness and darkness of the image;
• g) a logic controller, responsive to the four state signal, and controlling transfer of the electronically encoded natural scene image directly to a subsequent image processing element if the signal state indicates that the intensity signal has dark or light values exceeding both threshold signals T'.sub.dark, T'.sub.light or neither threshold signals T'.sub.dark, T'.sub.light and responsive to a signal state indicating that the signal state indicates that the intensity signals has dark or light values exceeding one of threshold signals T'.sub.dark, T'.sub.light generating an evaluation signal;
• h) a .gamma.' calculating circuit evaluating:
• ##EQU13## i) an averaging circuit for averaging .gamma.' and .gamma. to obtain .gamma..sub.ave ;
• j) a destination reproduction device having a tone reproduction relating input signals to output response thereof, and responsive to the averaging circuit to vary the output response by the exponential function .gamma..sub.ave.

A color reproduction system acquiring and printing a color image, and including an exposure adjustment circuit altering the exposure in the reproduction of an natural scene image, comprising:

• a) a scanner suitable for generating a digitally encoded natural scene image from an original scene, in terms of color density signals;
• b) a multicolor printer, responsive to signals directed thereto to print an image, and provided with a tone reproduction control relating input signals to printer response;
• c) a color space conversion circuit, converting the color density signals to an L level luminance-chrominance signal format, wherein at least one signal represents image intensity I;
• d) an exposure adjustment circuit including
• i) a thresholding circuit, receiving as inputs a pair of preset threshold signals T.sub.light, T.sub.dark, and the intensity signals I, and providing a multi state output signal respectively indicating satisfactory brightness and darkness of the image;
• ii) a logic controller, responsive to the multi state signal, and controlling transfer of the electronically encoded natural scene image directly to a subsequent image processing element if the signal state indicates that the intensity signal has dark or light values exceeding both of threshold signals T.sub.light, T.sub.dark, or neither of threshold signals T.sub.light, T.sub.dark, and responsive to a signal state indicating that the intensity signals has dark or light values exceeding one of threshold signals T.sub.light, T.sub.dark, to generate an evaluation signal;
• iii) a .gamma. calculating circuit, responsive to the evaluation signals to evaluate the function:
• ##EQU14## e) said toner reproduction control responsive to said calculated .gamma. to correct the printer response to the a digitally encoded natural scene image as an exponential function of .gamma..