Image processor for processing source pixel intensity values

• US 5,170,443 A
• Filed: 12/11/1990
• Issued: 12/08/1992
• Est. Priority Date: 03/07/1990
• Status: Expired due to Fees

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Todd, Stephen J. P.
• Primary Examiner(s): Mancuso, Joseph
• Application Number:
  US07/625,512
• Time in Patent Office: 728 Days
• Field of Search: 382/50, 382/53, 382/54, 382/51, 382/6, 358/465, 358/466, 358/315, 358/27, 358/80, 358/111, 340/793
• US Class Current: 382/274
• CPC Class Codes: G06T 5/40 using histogram techniques
• CPC Class Codes: G06T 5/92 based on global image properties

Abstract

Saturation problems are alleviated, when displaying image data on a device capable only of displaying pixel intensity values between a minimum value (Imin) and a maximum value (Imax), by smoothly mapping source intensity values greater than a threshold value (T), which is intermediate the minimum and maximum values, to output intensity values (Io) in a range between the threshold value (T) and the maximum value (Imax). Intensity values darker than the threshold value are not altered by the image generator with the result that image detail in the lower range of values is not lost. In order to maintain the correct color balance where separate color component intensities are defined in the source image data, each color component is mapped in accordance with a mapping determined for the largest color component value.

Claims

1. An image processor for processing pixel data defining the intensity values of respective pixels of a source image, said image processor comprising image processing logic for modifying source pixel data for display on output means capable of displaying pixel intensity values between a minimum value (Imin) and a maximum value (Imax), the image processing logic comprising:

means for applying a linear mapping of source pixel intensity values (Is) ranging between said minimum value (Imin) and a predetermined threshold value (T) intermediate said minimum and maximum values to give output pixel intensity values (Io) in a range between the minimum value (Imin) and the threshold value (T); and

means for applying a non-linear mapping of source pixel intensity values (IS) ranging between said threshold value (T) and an upper limit greater than said maximum value (Imax) to give output pixel intensity values (Io) in a range between the threshold value (T) and the maximum value (Imax), said non-linear mapping having a decreasing slope that approaches zero as said source pixel intensity value increases beyond said threshold.

2. An image processor as claimed in claim 1 wherein the image processing logic maps a source pixel intensity value (Is) greater than the threshold value into said range to give an output pixel intensity value (Io) such that:

space="preserve" listing-type="equation">Io=Imax-(Imax-T).sup.2 (Tmax-T))/(Is+Imax-2T)

3. An image processor as claimed in claim 1 wherein a plurality of component source intensity values (Isi=Is1, Is2, etc.) are defined for each pixel in the source image, the image processing logic determining whether the largest component source intensity value for a pixel in the source image is greater than the threshold value (T), and if so modifying each component source intensity value for that pixel to give component output intensity values (Ioi=Io1, Io2, etc.) for the pixel such that:

space="preserve" listing-type="equation">Ioi=Isi * Vo/Vs;

where

space="preserve" listing-type="equation">Vo=Imax-(Imax-T).sup.2 (Imax-T))/(vs+Imax-2T);

and

space="preserve" listing-type="equation">Vs=the largest of Is1, Is2, etc.

4. An image processor as claimed in claim 1 wherein the threshold value is approximately 95% of the maximum value.

5. An image processor as claimed in claim 1 comprising a visual display monitor as the output means.

6. An image processor as claimed in claim 1 comprising storage for the output intensity values.

7. An image processor as claimed in claim 1 comprising image rendering logic for producing the source image.

8. An image processor as claimed in claim 1 comprising an image capture device for producing the source image.

9. An image processor as claimed in claim 1 in the form of a computer workstation incorporating the image processing logic.

10. An image processor as claimed in claim 1 in the form of a camera.

11. An image processor as claimed in claim 1 in which a plurality of component source intensity values (Isi) are defined for each pixel of the source image, said second mapping means determing whether the largest component source intensity value for a pixel is greater than the threshold value (T) and, if so, modifying each component source pixel intensity value proportionately.

12. In an image processor for processing pixel data defining the intensity values of respective pixels of a source image, a method for modifying source pixel data for display on output means capable of displaying pixel intensity values between a minimum value (Imin) and a maximum value (Imax), said method including the steps of:

applying a linear mapping of source pixel intensity values (Is) ranging between said minimum value (Imin) and a predetermined threshold value (T) intermediate said minimum and maximum values to give output pixel intensity values (Io) ranging between the minimum value (Imin) and the threshold value (T); and

applying a non-linear mapping of source pixel intensity values (Is) ranging between said threshold value (T) and an upper limit greater than said maximum value (Imax) to give output intensity values (Io) ranging between the threshold value (T) and the maximum value (Imax), said non-linear mapping having a decreasing slope that approaches zero as said source pixel intensity value increases beyond said threshold.

13. A method as claimed in claim 12 in which the output pixel intensity value (Io) asymptotically approaches the maximum value (Imax) as the source pixel intensity value (Is) increases beyond said threshold value (T).

14. A method as claimed in claim 12 in which a plurality of component source intensity values (Isi) are defined for each pixel of the source image, said second mapping step including the steps of determining whether the largest component source intensity value for a pixel is greater than the threshold value (T) and, is so, modifying each component source pixel intensity value proportionately.

15. In an image processor for processing source pixel data defining the intensity values of respect pixels of a source image, a method for modifying said data for display on an output device capable of displaying pixel intensity values within a predetermined range, said method comprising the steps of:

providing a linear mapping between source pixel intensity values below a threshold intensity value and output pixel intensity values within a first portion of said range;

providing a non-linear mapping between source pixel intensity values above said threshold intensity value and output pixel intensity values within a second portion of said range, said non-linear mapping having a decreasing slope that approaches zero as said source intensity value increases beyond said threshold.

16. A method as in claim 15 in which the slope of said non-linear mapping approaches the slope of said linear mapping as said source intensity value approaches said threshold.

17. A method as in claim 15 in which a plurality of source pixel intensity values are defined for each pixel of the source image and are mapped to corresponding output pixel intensity values, said method comprising the steps of:

(a) if the maximum source pixel intensity value for a given pixel is below said threshold;

(1) providing said linear mapping between each source pixel intensity value for said pixel and the corresponding output pixel intensity value; and

(b) if the maximum source pixel intensity value for a given pixel is above said threshold;

(1) providing said non-linear mapping between said maximum source pixel intensity value and the corresponding output pixel intensity value; and

(2) providing a mapping proportionate to said non-linear mapping between each remaining source pixel intensity value and the corresponding output pixel intensity value.

18. A method as in claim 15 in which said linear mapping maps each source pixel intensity value to itself as an output pixel intensity value.

19. In an image processor for processing pixel data defining the intensity values of respective pixels of a source image, a method for modifying source pixel data for display on output means capable of displaying pixel intensity values between a minimum value (Imin) and a maxium value (Imax), said method including the steps of:

mapping source pixel intensity values (Is) ranging between said minimum value (Imin) and a predetermined threshold value (T) intermediate said minimum and maximum values to output pixel intensity values (Io) ranging between the minimum value (Imin) and the threshold value (T); and

mapping source pixel intensity values (Is) ranging between said threshold value (T) and an upper limit greater than said maximum value (Imax) to output intensity values (Io) ranging between the threshold value (T) and the maximum value (Imax) to give an output pixel intensity value (Io) such that;

space="preserve" listing-type="equation">Io=Imax-(Imax-T).sup.2 /(Is+Imax-2T)

20. In an image processor for processing pixel data defining the intensity values of respective pixels of a source image, a plurality of component source pixel intensity values (Isi) being defined for each pixel of the source image, a method for modifying source pixel data for display on output means capable of displaying pixel intensity values between a minimum value (Imin) and a maximum value (Imax), said method including the steps of:

(a) determining whether the largest component source pixel intensity value (Isi) for a pixel is greater than a predetermined threshold value (T) intermediate said minimum and maximum values;

(b) if said largest component source pixel intensity value for a pixel is greater than said threshold, linearly mapping said source pixel intensity values (Isi) to output pixel intensity values (Ioi) ranging between the minimum value (Imin) and the threshold value (T); and

(c) if said largest component source pixel intensity value for a pixel is greater than said threshold;

(A) non-linearly mapping said largest component source pixel intensity value (Isi) to an output intensity value (Io) ranging between the threshold value (T) and the maximum value (Imax); and

(B) modifying the remaining source pixel intensity values (Isi) for said source pixel proportionately.

21. In an image processor for processing source pixel data defining the intensity values of respective pixels of a source image, a plurality of source pixel intensity values being defined for each pixel of the source image and being mapped to corresponding output pixel intensity values, a method for modifying said data for display on an output device capable of displaying pixel intensity values within a predetermined range, said method comprising the steps of:

(a) if the maximum source pixel intensity value for a given pixel is below a predetermined threshold;

(1) providing a linear mapping between the source pixel intensity values for said pixel and output pixel intensity values within a first portion of said range; and

(b) if the maximum source pixel intensity value for a given pixel is above said threshold;

(1) providing a non-linear mapping between said maximum source pixel intensity value and an output pixel intensity value within a second portion of said range; and

(2) providing a mapping proportionate to said non-linear mapping between each remaining source pixel intensity value and the corresponding output pixel intensity value.