DE3919979C2 - - Google Patents

Description

The invention relates to a method for converting the Gradation or gradation of a digital image and a circuit for this, whereby original image data with m gradation levels converted into data with n gradation levels will.

If a digital image with gradations on a display unit is displayed or printed by a printer, so it is sometimes necessary that the number of gradations the original image data in a smaller number of increments transformations. For example, a 256 Gradation (8-bit data) converted into a 64 gradation (6-bit data). In such a case, conventionally se the process of simply converting four levels performed on the original image data in a gradation. This means that data processing is easy is performed that the least significant two bits of the 8-bit gradation data of the original image is neglected the.  

Table 1 shows the relationship of the gradation conversion in this case.

Table 1

On the other hand, in a printer in which the heat transfer sublimation of the ink on the printing paper takes place in accordance with the Japanese laid-open specification 62-1 52 763 proposed a technique to give an accurate To create gradation printouts of the image data.

This invention has an arrangement for controlling the Im pulse width for the application of the heat generating element depending on the gradation data of each image mentions. This means that the relationship between the Energy that is applied to the thermal head and that Print density in black and white is obtained by corre primary colors yellow, magenta and cobalt blue, whereby the energy applied to the density scale of each color is controlled by this result.

However, in the case where digital image data is used by users the simple gradation wall described above method to a printer with such a configuration  ration are printed, the problem occurs that the gradations of the part with low gradation, that is, the weakly colored parts appear visually rough. this will thereby causing human eyes to see the difference in color density stronger in the slightly colored part perceive as that regarding the strongly colored part (the Partly with high gradation).

From DE-A 25 32 232 it is known to convert an input signal with 2 ⁿ input levels into an output signal which should have a larger number of output levels (2 ^m ).

From the article "Monochrome digital image enhancement" by Harry Andrews in "Applied Optics", Feb. 1976, Volume 15, No. 2, page 495-503, it is known to have several different Formation gradation curves with which z. B. dark or light areas of the template are emphasized can.

In the book "Digital Image Processing", 2nd edition, Munich, Carl Hanser Verlag, 1987, pp. 104-122 by Peter Haberäcker are generally modifications of the gray value distribution discussed and also addressed the possibilities contrast in dark areas of the image more than bright or vice versa.

From the book "Digital Image Processing", New York, John Wiley u. Sons, 1978, pp. 307-311 by William Pratt the possibility known, possibly also in digital form convert the present template image into a reproduction image. The number of playback cones is at least the same or larger than that of the input cone.

In contrast, the object of the invention is that Process for converting the gradation of a digital image and to improve the circuit for this so that the Gradation visually from the faintly colored part (Part with low gradation) to the strongly colored part (Part with high gradation) in the case where the number changes the gradations of the digital image in a smaller number of gradations is converted.

This object is achieved in that the number of gradation levels m of the original image data is larger than that Number of gradation levels n of the converted data that the n gradation levels into several groups depending on the Gradation can be divided, and that the gradation levels the template image data thus assigned to the n gradation levels be that the number of each one gradation level the gradation levels associated with the converted data the template image is the same within a group and starting from the groups of lower gradation to the Groups of higher gradation increases.

The circuit according to the invention for solving the above described Edition is described in more detail in claim 2.

Further advantageous embodiments of the invention are described in claims 3-6.

An embodiment of the invention is based on the drawings described in more detail. It shows  

Fig. 1 is a block diagram showing an arrangement of a pressure density control device for a printer with a circuit for converting the gradation of a digital image accordingly to the invention;

Fig. 2 is a schematic representation with the data contents of the gradation control table thereof;

Fig. 3 is a schematic diagram showing the data contents of the conversion table;

Fig. 4 is a flowchart showing the control of the CPU and

Fig. 5 is a flowchart showing the control of the gradation control circuit sequencer.

The conversion of Original image data or template image data with 256 gradations or gradation levels (8-bit data) in 64 gradations or gradation levels (6-bit data).

First, the range of gradations or gradation levels after conversion divided into four groups, continuously from one low density (part with low gradation), he most part with the lightest parts, a second part with light most digits, a middle part and a shadow part. Then 256 gradations or gradation levels correspond to the original image data according to the four groups described above. In In this case, the number of gradations will convert the original image data in a gradation different from Group to group set. In the following this one Point explained in more detail.  

In the first part with the brightest parts there is a gradation of the converted data in such a way that the graduation corresponds to the original image data in a ratio of 1: 1. This Part thus includes the four gradations of the original image data from the first to the fourth gradation.

In the second part with the brightest parts there is a gradation of the converted data corresponding to two levels of Original image data set. This part thus includes fifth to 44th gradation of the original image data.

In the middle part there is a gradation of the converted data made so that they have four gradations of the original image data corresponds. The middle section thus shows the 45th to 152nd Gradation of the original image data.

In the shadow part there is a gradation of the converted data made so that they have eight gradations of the original image data corresponds. So the shadow part covers the 153rd to 256th Gradation of the original image data.

Table 2 below shows the relationship of the grada order between the original image data and the conversion data.  

Table 2

Table 3 shows the density range in each color group and the range of the gradation order of the original image data according to the gradation order of the converted data.

Table 3

If the original image data with 256 gradations according to Ta belle 2 and table 3 transformed into data with 64 gradations delt and, for example, printed by a printer, the parts with the brightest areas are visually soft gradation. On the other hand, the shadow part has one rougher gradation than the original image data, but what is hardly perceived visually. Accordingly, if the original Image data with 256 gradations converted into 64 gradations and printed using the method described above, an image quality is achieved that is visually equivalent to a 256 gradation.

In the following, a gradation conversion circuit who use the inventive method described above det, a print density control device for a printer and a Printer described using this circuit.

Fig. 1 shows a block diagram with an inventive gradation conversion circuit and a printing density control device for a printer, in which this circuit is used ver.

In Fig. 1, a thermal head 10 of a large number of heat generating elements 11 and driver circuits 12 , shift register 13 , latch circuits 14 and the like. formed for each of the heat generating elements 11 . The shift register 13 stores the serial output data from a comparator 26 , as will be described later, and outputs it as parallel data. The latch circuit 14 temporarily stores the parallel data output by the shift register 13 and outputs it to the driver circuits 12 in each case.

A gradation control circuit 20 has a line buffer 21 , a gradation control circuit sequencer 22 , a gradation counter 23 , a gradation control table 24 , a pulse generation circuit 25 , the comparator 26 described above, and the like.

In the gradation control circuit 20 , the density information of a line of the image stored in the line buffer 21 , that is, the data of the gradation (6 bits in this embodiment) is converted into a 63-fold loading of the thermal head 10 , whereby a density expression of 64 gradations is possible.

The gradation counter 23 is temporarily reset to "0" at the start of printing a line and it counts each time an excitation pulse is applied in accordance with the respective gradation and indicates the gradation order in accordance with the next pulse to be applied.

The gradation control table 24 is a memory that stores the data of the length of the excitation pulse to be applied to the thermal head 10 for each gradation. The exact data contents thereof are shown in Fig. 2.

In Fig. 2 the value "1" of the "data" shows a pulse duration of 16 µsec. Accordingly, this means that to print the density of a first gradation only one pulse with a time duration of

255 × 16 µsec = 4.08 msec

must be created.  

This also means that to print the density of a second Gradation an additional period of time

56 × 16 µsec = 896 µsec

is needed and that to print the density of a third Gradation another additional period of time

11 × 16 µsec = 176 µsec

is needed.

The pulse generation circuit 25 generates a single pulse with a length corresponding to the data output from the gradation control table 24 in synchronism with a signal applied to another terminal (not shown).

The line buffer 21 stores the density information of a line.

The comparator 26 compares the density information output from the line buffer 21 at each point, that is, the data corresponding to the gradation with the gradation level currently to be printed, and converts it into binary data indicating whether the next gradation level is to be printed or Not.

The gradation control circuit sequencer 22 is a control circuit for controlling the operation sequence of each of the components described above.

A gradation data conversion control circuit 30 is constituted by a CPU 31 which controls the start of the work sequence and monitors the end of the work sequence of the gradation control circuit sequencer 22 , I / O ports (ports) 32 and 33 , a ROM 34 for storing the in Fig. 3 Darge presented table corresponding to the program of the CPU 31 and the above-described inventive method, a RAM 35 , etc.

The gradation data conversion control circuit 30 receives, from an external device such as a computer system, image data with density information of 256 gradations (8-bit data), converts it into data with 64 gradations (6-bit data) and transmits it to the line buffer 21 of the gradation control circuit 20 .

In the following, the gradation conversion circuit and the printing density control device for a printer according to the invention will be described based on the gradation control circuit 20 and the control methods of the CPU 31 .

Fig. 4 shows a flow chart for explaining the operation of the CPU 31st

First, the CPU 31 writes the system from an external computer or the like via the I / O port 32 (step S 1) Original image data input to the RAM 35th Then, the conversion table from 256 gradations to 64 gradations as shown in FIG. 3 is stored in the ROM 34 . The CPU sequentially reads the original image data from the RAM 35 , converts it into 64-gradation data (hereinafter referred to as "C data") according to the table stored in the ROM 34 (step S 2 ), and writes it to the line buffer 21 of the gradation control circuit 20 (step S 3 ). The CPU 31 repeats the above steps for one line until it is complete (step S 4 ) and then outputs a line print start signal to the gradation control circuit sequencer 22 (step S 5 ). Since, as described above, one line is printed by the gradation control circuit sequencer 22 . After completion of the printing of a line, the gradation control circuit sequencer 22 outputs a line printing end signal to the CPU 31 , and the CPU 31 ends processing of a line upon receipt of the line printing end signal (step S 6 ).

Fig. 5 shows a flow chart of the tung Gradationssteuerschal sequence generator 22 of the gradation control 20th

First, upon reception of the line print start signal from the CPU 11 clears the gradation counter in step S by the Gradationssteuerschaltungs sequence timer 22 a line 31 to the start of printing, as described above. Thereafter, the gradation control circuit sequencer 22 reads the data of one line from the line buffer 21 , sequentially transfers it to the comparator 26 , so that the information on each dot as a result of the processing by the comparator 26 regarding printing / non-printing in the gradation at that time in the shift register 13 of the thermal head 10 (step S 12 ) is written. Thereafter, the gradation control circuit sequencer 22 outputs a latch signal to the latch circuit 14 so that it latches the contents of the shift register 13 (step S 13 ), and then outputs a pulse start signal to the latch circuit 14 (step S 14 ). Thereafter, a signal from each latch circuit 14 constituting a printing / non-printing signal and a pulse for printing a gradation from the pulse generating circuit 25 of the driver circuit 12 are supplied from each heat generating element 11, and therefore, a printing operation with a density equivalent is performed to a gradation.

The gradation control circuit sequencer 22 repeats the above-described process 63 times (steps S 15 , S 16 , S 17 , S 13 ...), And thereby printing one line is completed.

Thereafter, the gradation control circuit sequencer 22 outputs the line print end signal to the CPU 31 as described above.

As already described above, the Invention, for example, in the case of the original image data with 256 gradations as image data with 64 gradations be printed, the part with low gradation visually recognized so that it changes gently with regard to gradation different. Conversely, the high gradation area is related the gradation is rough, but this is hardly perceived visually becomes. This will even in the case where the original picture data in data with a gradation lower than converted the number of gradations of the original image data high image quality equivalent to that of the Preserved original image.

Changes and configurations of the described from leadership are easily possible for the expert and fall within the scope of the invention.

Claims (6)

1. A method for converting the gradation of a digital image, in which original image data with m gradation levels are converted into data with n gradation levels, characterized in that
that m <n,
that the n gradation levels are divided into several groups depending on the gradation, and
that the gradation levels of the template image data are assigned to the n gradation levels in such a way that the number of gradation levels of the template image assigned to a gradation level of the converted data is the same within a group and increases from the groups of lower gradation to the groups of higher gradation. 2. A digital image gradation conversion circuit which converts the original image data having m gradation levels into data having n gradation levels using a conversion table storage means, characterized in that
that a first storage device ( 35 ) is provided for storing the original image data,
that m <n,
that conversion data is stored in the conversion table storage device ( 34 ) in such a way that the number of gradation levels of the original image assigned to a gradation level of the converted data is the same within a group and increases from the group of lower gradation to the group of higher gradation,
that means ( 31 ) are provided for converting the original image data stored in the first storage means ( 35 ) in accordance with the conversion table ( 34 ), and
that a second storage device ( 21 ) for storing the image data converted by the device ( 31 ) is present. 3. A circuit according to claim 2 for use in a printing density control device for a printer, wherein a gradation control circuit sequencer ( 22 ) is provided for controlling the gradation in accordance with the data stored in the second storage device ( 21 ). The circuit of claim 3, wherein the means ( 31 ) converts the gradation of the original image data stored in the first storage means ( 35 ) according to the conversion table ( 34 ) simultaneously for one line. 5. A circuit according to claim 2 for use in a printer, wherein a gradation control circuit sequencer ( 22 ) is provided for controlling the gradation in accordance with the data stored in the second storage device ( 21 ). A circuit according to claim 5, wherein the means ( 31 ) converts the gradation of the original image data stored in the first storage means ( 35 ) according to the conversion table ( 34 ) simultaneously for one line.