Image processing device

• US 5,748,794 A
• Filed: 05/24/1995
• Issued: 05/05/1998
• Est. Priority Date: 11/24/1992
• Status: Expired due to Term

• Current Assignee: Sharp Electronics Corporation (Hon Hai Precision Industry Co., Ltd.), Sharp Kabushiki Kaisha (Hon Hai Precision Industry Co., Ltd.)
• Original Assignee: Sharp Kabushiki Kaisha (Hon Hai Precision Industry Co., Ltd.)
• Inventors: Itoh, Tatsuya, Nakamura, Masatsugu, Maeda, Hiroshi
• Primary Examiner(s): Coles, Sr., Edward L.
• Assistant Examiner(s): LEE, CHEUKFAN
• Application Number:
  US08/449,542
• Time in Patent Office: 1,077 Days
• Field of Search: 358/448, 358/466, 358/465, 358/455, 358/447, 358/445, 358/296, 358/461, 358/462, 358/464, 358/300, 358/302, 382/270, 382/251, 382/252, 382/274
• US Class Current: 382/251
• CPC Class Codes: H04N 1/40087 Multi-toning, i.e. converting a continuous-tone signal for reproduction with more than two discrete brightnesses or optical densities, e.g. dots of grey and black inks on white paper
• CPC Class Codes: H04N 1/4092 Edge or detail enhancement

Abstract

An image processing apparatus and method which can be selectively used in a variable mode and a fixed mode. In the variable mode, the quantizing values and thresholds for converting high valued image gradation into low valued image gradation can be changed so as to vary the surface gray levels of a printable image at a fixed laser gradation value. In the fixed mode, constant thresholds and quantizing values are used. The use of selectable operation modes enables easy printing operation with increased quality. Also, constant quality of printable images despite environmental changes may be realized by sensing and comparing currently read reference data with initially stored reference data and subsequently correcting quantizing values and thresholds in accordance with the compared result.

Claims

1. An image processing apparatus comprising:

fixed mode setting means for setting fixed thresholds of low gradational level for digital image data of high gradation representing an image and for setting fixed quantizing values on the basis of the fixed thresholds;

variable mode setting means for setting variable thresholds of low gradational level for the digital image data representing the image in accordance with image quality and for setting variable quantizing values on the basis of the variable thresholds;

mode selection means for selecting either a fixed mode or a variable mode for processing the digital image data representing the image;

gray level processing means for (a) quantizing all the digital image data representing the image using the fixed thresholds and the fixed quantizing values when the fixed mode is selected by said mode selection means and quantizing all the digital image data representing the image using the variable thresholds and the variable quantizing values when the variable mode is selected by said mode selection means, to provide quantized digital image data, (b) determining an error value between the digital image data representing the image and the quantized digital image data, and (c) distributing the error value of each attentional pixel of the quantized digital image data representing the image between pixels adjacent thereto; and

scanning means for scanning the image to provide the digital image data representing the image, the digital image data being provided by said scanning means with sampled white image data scanned from a reference white document,

said variable mode setting means determining a difference in a white level of the sampled white image data and a predetermined reference white level and setting the variable thresholds in accordance with the determined difference.

2. The image processing apparatus according to claim 1, further comprising:

gray level sensing means for optically sensing the tonal density of a reference toner image created on a light-sensitive drum of the image processing apparatus;

first control means for controlling a charging voltage of said light-sensitive drum according to the sensed gray levels of the reference toner image created with a fixed bias voltage for toner image development until a specified gray level is obtained and determining a potential difference between the toner image development bias voltage and the charging voltage;

second control means for controlling the absolute values of the charging voltage and the toner image development bias voltage so as to keep the potential difference constant; and

third control means for determining a plurality of thresholds and a plurality of quantizing values of gray level density of the reference toner image.

3. The image processing apparatus according to claim 1, wherein the fixed mode is for images containing characters and the variable mode is for images that are photographs.

4. An image processing apparatus comprising:

variable mode setting means for setting variable thresholds of low gradational level for digital image data of high gradation representing an image in accordance with image quality and for setting variable quantizing values on the basis of the variable thresholds;

gray level processing means for (a) quantizing all the digital image data representing the image using the variable thresholds and the variable quantizing values to provide quantized digital image data, (b) determining an error value between the digital image data representing the image and the quantized digital image data, and (c) distributing the error value of each attentional pixel of the quantized digital image data representing the image between pixels adjacent thereto; and

scanning means for scanning the image to provide the digital image data representing the image, the digital image data being provided by said scanning means with sampled white image data scanned from a reference white document,

said variable mode setting means determining a difference in a white level of the sampled white image data and a predetermined reference white level and setting the variable thresholds in accordance with the determined difference.

5. An image processing method comprising the steps of:

(a) selecting either a fixed mode or a variable mode for processing digital image data of high gradation representing an image;

(b) setting fixed thresholds of low gradational level for the digital image data representing the image and setting fixed quantizing values on the basis of the fixed thresholds upon selection of the fixed mode in said step (a);

(c) setting variable thresholds of low gradational level for the digital image data representing the image in accordance with image quality and setting variable quantizing values on the basis of the variable thresholds upon selection of the variable mode in said step (a);

(d) quantizing all the digital image data representing the image using the fixed thresholds and the fixed quantizing values when the fixed mode is selected in said step (a) and quantizing all the digital image data representing the image using the variable thresholds and the variable quantizing values when the variable mode is selected in said step (a), to provide quantized digital image data;

(e) determining an error value between the digital image data representing the image and the quantized digital image data;

(f) distributing the error value of each attentional pixel of the quantized digital image data representing the image between pixels adjacent thereto; and

(g) scanning the image to provide the digital image data representing the image, the digital image data being provided with sampled white image data scanned from a reference white document,

said step (c) further comprising determining a difference in a white level of the sampled white image data and a predetermined reference white level and setting the variable thresholds in accordance with the determined difference.

6. The image processing method according to claim 5, wherein the fixed mode is selected for images containing characters and the variable mode is selected for images that are photographs.

7. An image processing method comprising the steps of:

(a) setting variable thresholds of low gradational level for digital image data of high gradation representing an image in accordance with image quality and setting variable quantizing values on the basis of the variable thresholds;

(b) quantizing all the digital image data representing the image using the variable thresholds and the variable quantizing values to provide quantized digital image data;

(c) determining an error value between the digital image data representing the image and the quantized digital image data;

(d) distributing the error value of each attentional pixel of the quantized digital image data representing the image between pixels adjacent thereto; and

(e) scanning the image to provide the digital image data representing the image, the digital image data being provided with sampled white image data scanned from a reference white document,

said step (a) further comprising determining a difference in a white level of the sampled white image data and a predetermined reference white level and setting the variable thresholds in accordance with the determined difference.

8. An image processing apparatus comprising:

fixed mode setting means for setting fixed thresholds of low gradational level for digital image data of high gradation representing an image and for setting fixed quantizing values on the basis of the fixed thresholds;

variable mode setting means for setting variable thresholds of low gradational level for the digital image data representing the image in accordance with image quality and for setting variable quantizing values on the basis of the variable thresholds;

mode selection means for selecting either a fixed mode or a variable mode for processing the digital image data representing the image; and

gray level processing means for (a) quantizing all the digital image data representing the image using the fixed thresholds and the fixed quantizing values when the fixed mode is selected by said mode selection means and quantizing all the digital image data representing the image using the variable thresholds and the variable quantizing values when the variable mode is selected by said mode selection means, to provide quantized digital image data, (b) determining an error value between the digital image data representing the image and the quantized digital image data, and (c) distributing the error value of each attentional pixel of the quantized digital image data representing the image between pixels adjacent thereto,

said variable mode setting means including

first reference data storage means for initially reading and storing inputted reference image data as first reference data,

second reference data storage means for reading and storing subsequently input reference data as second reference data,

comparing means for comparing the first reference data stored in said first storage means with the second reference data stored in said second storage means and for outputting a difference between the first and the second reference data as an error, and

discriminating and correcting means for comparing the error output from said comparing means with a reference value and for correcting the variable thresholds and the variable quantizing values in accordance with the comparison.

9. An image processing apparatus comprising:

fixed mode setting means for setting fixed thresholds of low gradational level for digital image data of high gradation representing an image and for setting fixed quantizing values on the basis of the fixed thresholds;

variable mode setting means for setting variable thresholds of low gradational level for the digital image data representing the image in accordance with image quality and for setting variable quantizing values on the basis of the variable thresholds;

mode selection means for selecting either a fixed mode or a variable mode for processing the digital image data representing the image; and

gray level processing means for (a) quantizing all the digital image data representing the image using the fixed thresholds and the fixed quantizing values when the fixed mode is selected by said mode selection means and quantizing all the digital image data representing the image using the variable thresholds and the variable quantizing values when the variable mode is selected by said mode selection means, to provide quantized digital image data, (b) determining an error value between the digital image data representing the image and the quantized digital image data, and (c) distributing the error value of each attentional pixel of a respective line of the quantized digital image data representing the image between adjacent pixels of a previous line adjacent to the respective line and then distributing the error value of each attentional pixel of the adjacent previous line between adjacent pixels of the respective line.

10. An image processing method comprising the steps of:

(a) selecting either a fixed mode or a variable mode for processing digital image data of high gradation representing an image;

(b) setting fixed thresholds of low gradational level for the digital image data representing the image and setting fixed quantizing values on the basis of the fixed thresholds upon selection of the fixed mode in said step (a);

(c) setting variable thresholds of low gradational level for the digital image data representing the image in accordance with image quality and setting variable quantizing values on the basis of the variable thresholds upon selection of the variable mode in said step (a);

(d) quantizing all the digital image data representing the image using the fixed thresholds and the fixed quantizing values when the fixed mode is selected in said step (a) and quantizing all the digital image data representing the image using the variable thresholds and the variable quantizing values when the variable mode is selected in said step (a), to provide quantized digital image data;

(e) determining an error value between the digital image data representing the image and the quantized digital image data; and

(f) distributing the error value of each attentional pixel of a respective line of the quantized digital image data representing the image between adjacent pixels of a previous line adjacent to the respective line and then distributing the error value of each attentional pixel of the adjacent previous line between adjacent pixels of the respective line.