JPH1042136A - Image processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the hardware from being complicated in the case of gradation conversion of an input image and to prevent gradation from being lost due to reduction in substantial output gradation number. SOLUTION: A gradation conversion section 4-2 converts, e.g. 3-bit input data into 5-bit data by using a gamma conversion table so as not to reduce substantially the gradation number and then converts the 5-bit data into the original 3-bit output data and provides a output of the data to an output section 4-4. A gradation table processing section 4-3 reports the gamma conversion table used by the gradation conversion section 4-2 to the output section 4-4 and the output section 4-4 converts and processes the output data in 3-bit from the gradation conversion section 4-2 into 5-bit data based on the gamma conversion table reported from the gradation table processing section 4-3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus which performs a gradation conversion so that a halftone image can be reproduced well.

[0002]

2. Description of the Related Art Generally, a method of performing gradation conversion is as follows.
For example, an input value of 8 bits (256 gradations) is similarly converted to an output value of 8 bits (256 gradations) based on a γ conversion table.

[0003]

However, when the number of output gradations is the same as the number of input gradations, the substantial number of gradations is reduced by gradation conversion, and as a result, gradation is lost. There is a problem. FIG. 4 shows that the number of input gradations and the number of output gradations are both quaternary (“0” to “3” = 2) in order to simplify the description.
Bit; “3” is the brightest value) indicates density conversion, and the conversion characteristic is a characteristic for brightening the input image.

According to such characteristics, the input value "1" is converted into the output value "2", and the input values "2" and "3" are both converted into the output value "3". Is not used,
Also, the output value “3” is duplicated. Therefore, the number of output gradations becomes "3" with respect to the number of input gradations "4", the actual number of gradations decreases, and gradation is lost. Further, as shown in FIG. 5, both the number of input gradations and the number of output gradations are eight values (= “0” to
“7” = 3 bits; “7” is the brightest value), and the output value is “6” or “7” also in the case of the characteristic of converting lighter.
Overlap, the actual output gradation number is “6”.

Here, when the number of output gradations is larger than the number of input gradations as shown in FIG. 5, the substantial number of output gradations does not decrease and the gradation is not lost. FIG. 6 shows an example in which the input device 3-1 outputs 4-bit data to the gradation conversion processing unit 3-2, and the gradation conversion processing unit 3-2 converts the 3-bit data into 5-bit data ( The dither processing unit 3-3 dithers the bit data to convert it to 7-bit data, and outputs the converted data.
4. However, in such a method, when the gradation conversion processing unit 3-2 is constituted by a plurality of stages of circuits, the number of bits per stage is maintained in order to maintain the gradation of the original 3-bit data. Therefore, there is a problem that hardware becomes a huge circuit and becomes complicated.

SUMMARY OF THE INVENTION In view of the above-mentioned conventional problems, the present invention reduces the number of actual output gradations without complicating hardware when performing gradation conversion of an input image, thereby losing gradation. It is an object of the present invention to provide an image processing apparatus capable of preventing the image processing.

[0007]

In order to achieve the above object, the first means is a means for inputting a digital multi-valued image, a means for converting the input digital multi-valued image into gradations, and a means for converting the inputted digital multi-valued image. Means for notifying the gradation table to the next stage;
Means for converting the digital multi-valued image subjected to gradation conversion by the gradation conversion means into a gradation based on the gradation table notified by the notification means, and outputting the converted gradation. .

[0008] The second means is to achieve the above object,
The input data of the first number of bits is gradation-converted into data of the second number of bits in which the number of gradations does not substantially decrease based on the γ conversion table, and then the data of the second number of bits is converted to the first data. Tone converting means for converting the bit number into output data and outputting the output data, notifying means for notifying a γ conversion table used by the tone converting means, and a first bit number output from the tone converting means. Image processing means for performing gradation conversion of output data to data of a second bit number based on the γ conversion table notified from the notification means and processing the data.

The third means is characterized in that the gradation conversion means of the first or second means performs density conversion of input data.

The fourth means is characterized in that the gradation converting means of the first or second means dithers the input data.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an image processing apparatus according to the present invention, FIG. 2 is an explanatory diagram showing a density conversion table, and FIG. 3 is an explanatory diagram showing a gradation conversion table when dither processing is performed.

In FIG. 1, a digital image input unit 4-
1 is 8 gradations as an example (3 bits = “0” to “7”;
The digital image data of “7” is the brightest value) is output to the gradation conversion unit 4-2. The tone conversion unit 4-2 has a gamma conversion table such as a ROM. When the tone conversion unit 4-2 performs density conversion, the gamma conversion table is brightly gamma converted as shown in FIG. And a table for performing gamma conversion to a lower level as shown in FIG. 2, and either one is selected. When the tone conversion unit 4-2 performs dither processing, the dark densities “1” and “2” as shown in FIG.
And bright densities "6" and "7" have a γ-conversion characteristic table, and medium densities "3" to "5" have a sharp-characteristic S-shaped characteristic conversion table.

By the way, when a 3-bit input value is converted into a 3-bit output value based on such a table, the input values "4" and "5" are both equal in the bright characteristic table shown in FIG. Is converted to an output value "6", and
Since both the input values “6” and “7” are converted to the output value “7”, the output values “6” and “7” overlap to form six gradations,
A substantial number of gradations is reduced. In the dark characteristic table as shown in FIG. 2, the input values “0” and “1” are both converted to the output value “0”, and the input values “2” and “3” are output.
Are both converted to the output value “1”, so that the output value “0”,
“1” overlaps with six gradations. In addition, in the S-shaped characteristic table as shown in FIG. 3, the input values “0”, “1”
Are both converted to an output value “0”, and input values “6”,
Since both “7” are converted into the output value “7”, the output values “0” and “7” overlap and have six gradations.

Therefore, the gradation conversion section 4-2 first converts the 3-bit input values "0" to "7" to 5 based on the γ conversion table so that the number of gradations does not substantially decrease. It is converted into bit output values "0" to "28". That is, in the table of the brightness characteristic as shown in FIG. 2, the 3-bit input values "4" and "5" are each converted into the 5-bit output value "23" so that the output values "6" and "7" do not overlap.
Converted to “25” and converted the input values “6” and “7” to 5
The bit output values are converted into “27” and “28”.

In the dark characteristic table shown in FIG. 2, the 3-bit input values "0" and "1" are converted into 5-bit output values "0" and "1", respectively. 2 ”and“ 3 ”are each a 5-bit output value“ 3 ”,
Convert to "5". Further, in the S-shaped characteristic table as shown in FIG. 3, the 3-bit input values “0” and “1” are converted into 5-bit output values “0” and “1”, respectively, and the input value “6”. "And" 7 "are each a 5-bit output value" 2
7 "and" 28 ".

Next, similarly, the gradation conversion unit 4-2 performs gradation conversion on the 5-bit data so as to hold a substantial number of gradations, and then, the values "0" to "2" after the gradation conversion.
8 "is the same as 3-bit data" 0 "to
It is converted to “7” and output to the output unit 4-4. At this time,
The gradation table processing unit 4-3 notifies the table used by the gradation conversion unit 4-2 to the output unit 4-4, and the output unit 4-4 outputs the 3-bit data “0” from the gradation conversion unit 4-2. "To" 7 "
Is converted into 5-bit data “0” to “28” based on the table notified from the gradation table processing unit 4-3 to form a halftone image. As a result, it is possible to maintain the actual gradation while keeping the number of bits of the entire system at 3 bits.

In such a system, when the gradation conversion section 4-2 is configured to be connected in several stages, (1) the input signal is referred to a gradation table, and a gradation capable of holding a substantial gradation is obtained. Return to

(2) Perform gradation conversion that can hold substantial cancellation.

(3) The signal after the elimination conversion is output with the same number of bits as the number of input bits.

The following steps are taken.

That is, in (1), the input 0 to 7 is converted into 0 to 28 gradations with reference to a gradation table as shown in FIG. This table was determined in the previous process,
In this embodiment, the configuration is as shown in the figure, but it goes without saying that it differs depending on the conversion characteristics. In (2), similar to the processing described with reference to FIG. In this case, the data is about 6 bits. Then, this data is divided into three bits having the same
Bits, that is, data of 0 to 7 are output. At this time, the three-bit reference gradation table is also notified to be used for the next stage conversion. With this processing, it is possible to output a halftone image without reducing the number of gradations with the same number of bits.

[0022]

As described above, claims 1 and 2
According to the invention described above, it is possible to prevent loss of gradation due to a substantial decrease in the number of output gradations without complicating hardware when performing gradation conversion of an input image.

According to the third aspect of the present invention, when performing density conversion of input data, it is possible to prevent a substantial decrease in the number of output gradations without complicating hardware, thereby preventing loss of gradation. can do.

According to the fourth aspect of the invention, when dithering input data, it is possible to prevent a substantial decrease in the number of output gradations without complicating hardware, thereby preventing loss of gradation. be able to.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an embodiment of an image processing apparatus according to the present invention.

FIG. 2 is an explanatory diagram showing a density conversion table.

FIG. 3 is an explanatory diagram illustrating a gradation conversion table when a dither process is performed.

FIG. 4 is an explanatory diagram showing the reason why the number of output gradations decreases when performing gradation conversion.

FIG. 5 is an explanatory diagram showing a process of not reducing the number of output gradations when performing gradation conversion.

FIG. 6 is a block diagram showing a conventional image processing apparatus.

[Explanation of symbols]

 4-1 Digital image input unit 4-2 Gradation conversion unit 4-3 Gradation table processing unit 4-4 Output unit

Claims (4)

[Claims] 1. A means for inputting a digital multi-valued image, a means for converting the inputted digital multi-valued image into gradations, a means for notifying a gradation table to be converted to the next stage, and the step of performing the gradation conversion Means for converting the digital multi-valued image whose gradation has been converted by the means into a gradation based on the gradation table notified by the notifying means and outputting the converted gradation;
An image processing device comprising: 2. A method of converting input data of a first number of bits into data of a second number of bits in which the number of tones does not substantially decrease based on a gamma conversion table. Means for converting the data into output data of a first number of bits and outputting the gradation, means for notifying the γ conversion table used by the means for converting the gradation, and output from the means for performing the gradation conversion Means for converting the output data of the first number of bits into data of the second number of bits based on the γ conversion table notified from the notifying means and processing the data. 3. The image processing apparatus according to claim 1, wherein said tone conversion means performs density conversion of the input data. 4. The image processing apparatus according to claim 1, wherein said gradation converting means dithers the input data.