Apparatus and method for forming superimposed images and copying machine equipped with apparatus

Disclosed is an image processing apparatus by which a first image corresponding to first image information and a second image corresponding to second image information are superimposed one on the top of the other, the apparatus comprising a scanning device for scanning a document line by line to output the first image information; a composing device for composing the first and second image information so that the first and second images are superimposed one on the top of the other; a detecting device for detecting changes of the scanning speed in a direction perpendicular to the lines; and a controlling device for so controlling the composing device that the relative position of the second and first images in the direction are adjusted line by line in accordance with the results detected.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention relates to an apparatus and method for image processing by 
which two images are superimposed one on the top of the other, and to an 
image forming apparatus, such as a copying machine, equipped with such an 
apparatus for image processing. 
(2) Description of the Related Art 
Various apparatuses for superimposing two images one on the top of the 
other have been known, one of which is a digital-erasing apparatus, used 
in color copying machines and the like to erase analog images. According 
to this apparatus, a document (an analog image) is reflected onto the 
surface of a photoconductor through an optical system consisting of an 
exposure lamp, mirrors, and a lens in order to form an electrostatic 
latent image. The undesired area of the document is erased by use of a 
light emitting diode (hereinafter referred to as LED) head. 
The above-mentioned digital-erasing apparatus, which supplies black toner 
to black electrostatic latent images formed on the photoconductive drum, 
is suitably used to obtain preferable black images because black made by 
composing yellow, magenta, and cyanine is not natural black. The LED head 
successively erases an area of each electrostatic latent image of yellow, 
magenta, and cyanine, the area turning out to be black when these three 
color images are superimposed altogether. Lighting signals corresponding 
to the black area detected on a document read by an image sensor, or a CCD 
are supplied line by line to the LED head synchronously with latch pulses 
controlling the supply of the light signals. Generally, such latch pulses 
have a certain cycle period; however, the torque fluctuation or the like 
during the scanning of the optical system may cause a scanner motor used 
to form analog images to be driven at an irregular speed. This brings 
about a problem that an analog image and a digital image formed based on 
digital data so as to erase part of the analog image may not coincide both 
in position and size. 
If the scanning speed is two-thirds as low as the regulation speed, an 
analog image Ib shown in FIG. 16 (b) is formed which is 1.5 as long in the 
sub scanning direction as the expected black image Ia shown in FIG. 16 
(a). This is because the scanning speed is low while the rotating speed of 
the photoconductor is regulated. As shown in FIG. 16 (c), a digital image 
Ic corresponds in shape to the analog image Ia because the cycle period of 
the latch pulses is regulated. When both images Ic and Ib are superimposed 
one on the top of the other on the photoconductor in order to erase the 
analog image Ib, they appear as shown in FIG. 16 (d). Thus, the irregular 
scanning speed makes magnification of an analog image and a digital image 
disaccord in the sub scanning direction. Furthermore, in most cases, even 
the record starting points of the images Ib and Ic do not coincide, which 
prevent the analog image from being preferably erased. 
SUMMARY OF THE INVENTION 
The object of this invention is to provide an apparatus and method for 
image processing and an image forming apparatus such as a copying machine, 
equipped with the apparatus for image processing by which an image on a 
document to be scanned and another image to modify it can be superimposed 
one on the top of the other without any disaccord in position or 
magnification. 
The above object can be achieved by constructing these apparatuses so that 
the relative positions of both images are adjusted according to the 
scanning speed or position of a document, or the variation of the scanning 
speed. 
According to the above construction, since the relative positions can be 
varied depending on the scanning speed at every moment, disaccord in 
position or magnification can be prevented. Especially when such a 
construction is applied in a color copying machine in which an analog 
image and a digital image are superimposed one on the top of the other, 
the analog image can be erased without fail, thereby realizing clear 
copying with good quality. 
The scanning speed or position of a document, and the variation of the 
scanning speed are easily detected from the interval or timing of pulses 
outputted from an rotary encoder, or the like. 
Images thus superimposed may be analog images formed by optical projection, 
or digital images based on digital data inputted by an image sensor or the 
like. 
Such a superimposition of images may be carried out on an image carrier 
such as an photoconductive drum or on memory. 
The above-mentioned positional adjustment of both images may be carried out 
by controlling the timing for image generation in such a case that the 
image processing is applied in an image forming apparatus by which images 
are formed by the relative displacement between an image carrier and an 
image generation source.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
[The overall construction of the copying machine] 
As shown in FIG. 1, the copying machine 1 of this invention comprises a 
clockwise rotatable photoconductive drum 2 disposed about the center 
thereof, a cleaning unit 3, a main eraser lamp 4, a charger 5, an LED head 
6, development units 7-10, an AIDC sensor 40, and an intermediate transfer 
medium 11 (hereinafter referred to as transfer belt) to transfer a 
visualized toner image formed on the photosensitive drum 2. 
The photoconductive drum 2 has an evenly charged photoconductive layer 
thereon, onto which images through an optical system are exposed. 
The optical system, so disposed that a document can be scanned below an 
original glass plate 59, consists of an exposure lamp 50, sliding mirrors 
51, 52, and 53, a main lens 54, a set of rotatable mirrors 55, and a fixed 
mirror 57. The rotatable mirrors 55 consist of four mirrors 55-N, 55-B, 
55-G, and 55-R respectively having an ND filter, a blue filter, a green 
filter, a red filter deposited on each surface. The reference number 58 
indicates a light path used during the image forming operation. 
The copying machine 1 further comprises a phomoploidy-type color-image 
sensor 60, or a CCD having the same optical path length as the 
photosensitive drum 2 and a home position sensor 56 detecting the home 
position of the rotatable mirrors 55, so disposed that the mirror 55-N is 
positioned perpendicular to a light path 81 used during the image reading 
operation. Thus, a light reflected from a document is eradiated to the 
color-image sensor 60 by positioning the mirror 55-N perpendicular to the 
light path 81. 
The set of rotatable mirrors 55 is so adjusted to the light path 58 that 
each of the mirrors 55-B, 55-G, and 55-R leads a light to the 
photosensitive drum 2 in order to form images of yellow, magenta, and 
cyanine respectively. This adjustment is driven by an unillustrated 
stepping motor and is accurately positioned by a mechanism (not shown in 
FIG. 1). 
The exposure lamp 50 and the slidable mirror 51 compose a first slider 90 
while the slidable mirrors 52 and 53 compose a second slider 91. Both 
sliders 90 and 91 are driven by a scanning motor 63 to slide at a 
respective speed of v/n and v/2n (v is the peripheral velocity of the 
photoconductive drum 2 and n is a magnification). 
The development units 7-10 disposed closely to the photosensitive drum 2 
use toner of yellow, magenta, cyanine, and black respectively, and 
equipped with a toner density sensor 7-a, 8-a, 9-a, or 10-a respectively. 
Furthermore, a moisture sensor 80 is disposed above the development unit 
7. 
Disposed around the transfer belt 11 are a transfer charger 12 to transfer 
a toner image onto the transfer belt 11, a removable belt-cleaning brush 
13, a transfer charger 14 to transfer a toner image onto paper, a paper 
separation charger 15, and a mark sensor 41 which detects a mark (not 
shown) on the transfer belt 11 to time the multi-transfer thereonto. All 
these components in the vicinity of the transfer belt 11 as well as the 
photoconductive drum 2 are driven by a motor 61. 
The copying machine 1 further comprises two cassettes 29 and 31, respective 
paper feed rollers 25 and 26, respective paper size sensors 30 and 32, 
respective paper shortage sensors 34 and 35, a pair of rollers 27 leading 
paper from the cassette 31 to the next step, another pair of rollers 28 
leading paper from either cassette 29 or 31 to the next step, further 
another pair of rollers 23 so driven that paper is fed in an appropriate 
timing for a toner image formed on the transfer belt 11, a paper sensor 36 
disposed near the rollers 28, and another paper sensor 37 detecting the 
top end of the paper to be fed. 
A toner image on the transfer belt 11 is transferred onto paper by the 
transfer charger 14. The paper is separated by the paper separation 
charger 15, sent to a conveyer unit 16, then to either of a fixing unit 17 
or 18 to be made adhere onto the paper, and finally sent out through a 
pair of paper assist rollers 22. All these systems of paper feeding, 
conveying, and fusing are driven by a motor 62. Another paper sensor 39 is 
disposed in the paper assist unit. The fixing unit 17 includes fixing 
heaters 19 and 20, while the other fixing unit 18 includes an fixing 
heater 21. These heaters 19, 20, and 21 are selectively controlled to 
adjust the temperature of the fixing rollers 17 and 18. 
Other components of the copying machine 1 are a manual paper feed lid 33, a 
paper sensor 38 detecting paper fed from a manual paper feed opening, a 
pair of rollers 24 leading paper to the next step, a sorter 70, and a copy 
receiving tray 71. 
[The construction of the LED head] 
As shown in FIG. 2, the LED head 6 has a number of light emitting diode 
elements 95 whose light-emitting operation is individually controlled, 
arrayed opposite to the photoconductor. 
The process of color-image formation in the copying machine 1 is described 
hereinafter with reference to FIG. 3. 
Prior to the color-image forming process, information on the document is 
eradiated to the color-image sensor 60 through the main lens 54 and the 
rotatable mirror 55-N as a result of the scanning by the sliders 90 and 
91. The information thus read is processed in an image processing unit 100 
so as to be stored in memory. The color-image sensor 60, which is not 
detailed since it is well known, has red, green, and blue filters 
deposited on the front surface thereof in order to photolyze the light 
eradiated from the document into the light components of red, green, and 
blue. As a result, color information can be judged in a later process: as 
shown in FIG. 3, a document having a color picture (X) and some black 
letters (Y) is divided between colored area information and black/white 
area information to be stored as data to the image processing unit 100. In 
the image forming process, latent images are scanned to be erased by the 
LED head 6 according to the information thus read. 
In a first image forming process, the mirror 55-B is selected. An 
electrostatic latent image shown in (a1) corresponding to the amount of 
the blue light is formed onto the photoconductive drum 2 by the scanning 
of the optical system. Of the electrostatic latent image thus formed, the 
area for black letters (Y) is erased as indicated by a dotted line in (b1) 
with the use of the LED head 6 based on the data in the memory. Since this 
process is for yellow development, the development unit 7 for yellow is 
put in movable condition. After this process is completed, as shown in 
(c1), a certain amount of yellow toner is made adhere to the color picture 
(X) depending on the amount of the blue light required. After passed the 
development unit 7, the toner image is transferred onto the transfer belt 
11 as shown in (d1). 
In a second image forming process, the mirror 55-G is selected. An 
electrostatic latent image shown in (a2) corresponding to the green light 
is formed onto the photoconductive drum 2. Of the electrostatic latent 
image thus formed, the area for black letters (Y) is erased as indicated 
by a dotted line in (b2) based on the data in the memory. Since this 
process is for magenta development, the development unit 8 for magenta is 
put in movable condition. After this process is completed, as shown in 
(c2), a certain amount of magenta toner is made adhere to the color 
picture (X) depending on the amount of the green light required. After 
passed the development unit 8, the toner image is transferred onto the 
transfer belt 11 and superimposed on the top of the image formed in the 
first image forming process as shown in (d2). 
In a third image forming process, the mirror 55-R is selected. An 
electrostatic latent image shown in (a3) corresponding to the red light is 
formed onto the photoconductive drum 2. Of the electrostatic latent image 
thus formed, the area for black letters (Y) is erased as indicated by a 
dotted line in (b3) based on the data in the memory. Since this process is 
for cyanine development, the development unit 9 for cyanine is put in 
movable condition. After this process is completed, as shown in (c3), a 
certain amount of cyanine toner is made adhere to the color picture (X) 
depending on the amount of the cyanine light required. After passed the 
development unit 9, the toner image is transferred onto the transfer belt 
11 and further superimposed on the top of the images formed in the first 
and second image forming processes as shown in (d3). 
In a fourth image forming process, the mirror 55-N is selected. An 
electrostatic latent image shown in (a4) corresponding to the reflective 
light is formed onto the photoconductive drum 2. Since this process is a 
process for black development, the color picture (X) is erased as 
indicated by a dotted line in (b4), the development unit 10 for black 
being put in movable condition. After this process is completed, as shown 
in (c4), a certain amount of black toner is made adhere to the black 
letters (Y) depending on the amount of the light required. After passed 
the development unit 10, the toner image is transferred to the transfer 
belt 11 and is further superimposed on the top of the images formed in the 
first, second, and third image forming processes as shown in (d4). The 
visible toner image thus formed onto the transfer belt 11 is transferred 
onto paper as shown in (e), fixed, and sent out to the copy receiving tray 
71. As described hereinbefore, a color picture is formed with toners of 
yellow, magenta, and cyanine while black letters with black toner. 
The construction of the image reading unit and the digital image forming 
unit of the copying machine 1 is described as follows with reference to 
FIG. 4. 
A document scale 100 is disposed on the home position of the first slider 
90. The document scale 100 has a center mark 101 thereon to indicate the 
center of the optical system. The center mark 101 is on the extended line 
of the optical axis of the main lens 54 through which the color image 
sensor 60 reads the mark 101. In an image processing unit 99, the mark 101 
thus read is detected at the start point of the pre-scanning so that an 
effective area in each line is determined and data in the effective area 
is exclusively sent to memory 105. The magnifying process is also carried 
out in this unit. Image data in the memory 105 is read out for image 
formation, the photosensitive drum 2 being exposed by use of the LED head 
6. 
The detailed construction of the input/output of digital images is 
described as follows with reference to FIG. 5. Operated for digital image 
input are the color-image sensor 60, the image processing unit 99, and the 
memory 105 while for digital image output are the memory 105, a 
non-reverse/reverse amplifiers 117 and 118, a parallel/serial converter 
112, and the LED head 6. A CPU 110 controls the operations of all these 
components. The above-mentioned input/output units are constructed the 
same as conventional ones; however, the CPU 110 receives pulses from an 
encoder detecting the rotation of the scanner motor 63, and generates 
latch pulses to supply to the LED head 6. 
As shown in FIG. 11, the encoder consists of a disk 115 with a number of 
slits which is attached to the axis of the scanner motor 63 and a 
transmission type photosensor 116. The scanner motor 63 makes the disk 115 
rotate, by which the slits on the disk 115 cross the detecting position, 
so that the photosensor 116 outputs encoder pulses. The process of forming 
the encoder pulses into latch pulses is described later with reference to 
FIG. 15. 
The operations of inputting and reading out image data is described as 
follows with reference to FIG. 5. 
A copying operation is initiated by a direction inputted from an operation 
panel to the CPU 110. The mirror 55-N of the four mirrors is selected to 
intercept mainly the infrared rays by a shift signal of the CPU 110, all 
the color information on a document being inputted to the color-image 
sensor 60, which outputs unprocessed data of red, green, and blue 
according to the colors on the document. The color-image sensor 60 reads 
data for one line on a document all at once, and outputs data for each 
pixel bit by bit to the image processing unit 99 synchronously with its 
clock signal CLK.phi.. The image processing unit 99, when the data is 
inputted, calculates whether a pixel is in the color area or the 
black/letter area from its neighboring pixels and then outputs color data 
indicating either "0" for black/white area or "1" for color area. The 
image processing unit 99 outputs a ready signal to the CPU 110 every time 
the color data reach a certain number of bit (8 bit in this case), and 
puts 8 bit data onto the data bus at the same time. Receiving a ready 
signal, the CPU 110 transmits the address indicating an area in which the 
data to store and a writing signal WR to the memory 105, by which the data 
transmitted from the image processing unit 99 is written in the memory 
105. 
After the data for one line is written in the memory by repeating the 
above-mentioned operation, the CPU 110 outputs a scanning signal in order 
to read image data for the next line so that the first slider 90 is moved 
by one line. Hereafter, the same operation is repeated until the last line 
so that the entire area for the document is written in the memory 105. 
[Reading out of data] 
The entire data for the document thus inputted by a pre-scanning to the 
memory 105 is repeatedly read out in accordance with each image forming 
process of yellow, magenta, cyanine, or black. 
First, in the process for yellow, the first slider 90 starts scanning in 
response to a scanning start direction from the CPU 110 to form an analog 
latent image onto the photoconductive drum 2. At the same time, the data 
in the memory 105 is read out at a predetermined timing. 
Then, the CPU 110 sets an address, a RD signal, and a Select signal, which 
selects between reverse or non-reverse of data. The Select signal 
indicates "L" to select the amplifier 117 on the non-reverse side in the 
process for yellow, magenta, or cyanine, and indicates "H" to select the 
amplifier 118 on the reverse side in the process for black. Consequently, 
the color area "1" and the letter area "0" in 8 bit data are transferred 
to the parallel/serial converter 112 as they are or in reverse. 
The parallel/serial converter 112 converts 8 bit data transmitted from the 
memory 105 into 1 bit serial data to transfer it as data for one line to 
the LED head 6 synchronously with a clock signal CLK1 transmitted from the 
CPU 110. The LED head 6 receives the color data and one line 
synchronization signal, or a latch pulse signal outputted from the CPU 
110, and the LEDs for the color data "0" emit lights. Repeating this for 
the entire document erases the letter area, the color area being 
visualized by yellow toner. The same operation is carried out in the 
processes for magenta and cyanine. On the other hand, in the process for 
black, the Select signal indicates "H", and the data from the memory 105 
is reversed to erase the color area so that the letter area is visualized 
by black toner only. 
The timing to start reading data from the memory 105 is so adjusted by the 
CPU 110 that the starting position to write a digital image onto the 
photoconductive drum 2 by the light-emission of the LED head 6 coincides 
with the top end of an analog latent image formed onto the photoconductive 
drum 2. The adjusting timing is determined mainly by the relative 
positions between exposure of the analog image and the LED head 6 against 
the photoconductive drum 2. 
After the reading began, the data read out is latched to the LED head 6 by 
latch pulses line by line. Since the latch pulses are formed out of the 
encoder pulses in proportion to the rotating angle of the scanner motor 
63, the digital image is written onto the surface of the photoconductive 
drum 2 synchronously with the rotating angle. Since the rotating angle 
corresponds to the scanning length of the document in the sub scanning 
direction, the digital image is written line by line every time the image 
on the document is scanned in a certain length. Accordingly, the length of 
the digital image and the analog image coincide in the sub scanning 
direction. 
The driving circuit of the LED head 6 is described as follows with 
reference to FIG. 6. 
For example, 1300 of D flip-flop 200 are provided to compose a shift 
register to store serial image data for one line. The serial data are read 
in the pre-scanning, transmitted from the parallel/serial converter 112, 
and successively inputted by the clock signal CLK1. Then, when data for 
one line is inputted, the one-line synchronization signal, a latch pulse 
is inputted and latched for one line by a D flip flop 201. At this moment, 
if an ENABLE signal from the CPU 110 indicates "L", the driving circuit 
allows the LED head 6 to emit lights according to the data for one line 
(the ENABLE signal is set to "L" in the process for yellow, magenta, 
cyanine, and black). Thus, data for one line is latched every time a latch 
pulse is inputted, with the LED head 6 emitting lights. These timing chart 
is shown in FIG. 7. 
As shown in FIGS. 8(a)-8(b) and 9(a)-9(b), a solid on a document is 
detected by the color-image sensor 60 at the pre-scanning and erased by 
use of the LED head 6 in the process of forming an analog image. FIG. 8 
(a) is a developed image when the scanning motor 63 works as designed in a 
conventional apparatus having a regulated latch pulse cycle period, and 
FIG. 8 (b) is a developed image when the scanning speed is two-thirds 
lower than designed in the same conventional apparatus. FIGS. 9(a) and 
9(b) show images developed by the apparatus of this embodiment when the 
scanning speed is higher or lower respectively than designed. 
In any of these figures, the pixel density and system speed of the LED head 
6 are set to 100 dpi and 113 [mm/sec] respectively both in the main and 
sub scanning. In the conventional apparatus shown in FIGS. 8(a) and 8(b) , 
one cycle period of the latch pulse of the LED head is 0.25 [mm]/113 
[mm/sec]=2.2[msec]. Since the first slider 90 is driven at the same speed 
of 113 [mm/sec] as the system speed, if the amount of one step movement of 
the scanning motor (stepping motor) 63 is about 62 [.mu.m], the time 
period of the encoder pulses is found to be 0.062/113=0.55 [ms]. When an 
apparatus works under these conditions, an analog image and a digital 
image coincide both in position and size as shown in FIG. 8 (a). The area 
remains black in the figure is an analog image which has not been erased 
because it was not regarded as black. 
If the scanning speed in the process of forming an analog image is 
two-thirds lower than designed in the same conventional apparatus, the 
analog image is formed 1.5 longer in the sub-scanning direction. However, 
since the latch pulse interval of the LED head 6 is still 2.2 [ms], that 
is, the pixel density in the sub-scanning direction remains 100 dpi, the 
digital image does not coincide in position nor magnification with the 
analog image 1.5 longer in the sub-scanning direction as shown in FIG. 8 
(b). 
On the other hand, in the apparatus of this embodiment, if the scanning 
speed is higher than designed, the analog image is formed into a latent 
image compressed in the sub scanning direction, while a digital image is 
also written compressed in the sub scanning direction because the latch 
pulses change its cycle period according to the scanning speed. Therefore, 
the analog image and the image to be erased coincide both in position and 
size in the sub-scanning direction as shown in FIG. 9 (a). Since the 
rotation of the scanning motor 63 and the latch pulses of the LED head 6 
synchronize with each other, they coincide as shown in FIG. 8 (b) even the 
scanning speed varies. The waves of the encoder pulses and the latch 
pulses of this embodiment shown in FIGS. 10(a) and 10(b) depict that one 
latch pulse generates per four encoder pulses. 
The main control flow of the CPU 110 is described as follows with reference 
to FIG. 12. First, the CPU resister, peripheral I/O device, and the like 
are initialized (S1). Then, copy mode and the like are set according to 
the input signal of each switch (S2). The copy mode and the like thus set 
are displayed (S3). The copying operation is sequentially controlled (S4, 
S5, and S6). At (S4) the paper feeding procedure is carried out. At (S5) 
the image forming procedure around the photoconductive drum 2 is carried 
out. At (S6), the optical scanning procedure is carried out (the scanning 
speed is set in a low speed at the pre-scanning because the CCD is 
required both to read images and to process area discrimination). Finally, 
the end of a timer to regulate the time period between (S2) and (S7) is 
judged (S7), and goes back to (S2) to resume the operation. Although more 
complicated control than the flow shown in this embodiment is carried out 
in the actual copying operation, it is not described here. 
In the sub flow of the image forming process shown in FIG. 13, writing 
to/reading from the memory 105 is controlled (S10) as described later. 
Then, latch pulses are formed (S11). Other procedures concerning image 
formation are carried out (S12), which are not described here. 
The control of the writing to/reading from the memory 105 is described as 
follows with reference to FIG. 14. 
First, whether the pre-scanning is in operation or not is judged from a 
flag set at the scanning process. In case it is, data is written in the 
memory 105 (S30), the WR signal is set to "L" and the RD signal to "H" 
(S31), and addresses for one line are sequentially set to write data 
(S32). 
On the other hand, in case the pre-scanning is not in operation, whether 
either process for yellow, magenta, or cyanine is in operation or not is 
judged (S33). In case it is: data in the memory 105 is being read, the RD 
signal is set to "L" (S34), the address of the area to be read is set 
(S35) at the timing mentioned before, and the Select signal is set to "L" 
(S36) for selecting between the reverse or non-reverse of data. 
In the process for black, data is read and processed in the same manner as 
in the processes for yellow, magenta, and cyanine (S38 and S39). However, 
the Select signal is set to "H" and data is reversed (S40). In the case 
that no process is in operation at (S37), the flow proceeds to (S41) 
without the memory 105 being accessed. This is the end of this routine. 
The process of forming latch pulses is described as follows with reference 
to FIG. 15. First, whether any process for yellow, magenta, cyanine, or 
black is in operation or not is judged (S21). In case it is, the flow 
proceeds to (S24), at which whether the encoder pulses outputted from the 
scanning motor 63 is being counted or not is judged. In case it is not, 
the counting starts (S25). In case it is, the flow proceeds to (S26), at 
which whether the fourth pulse was counted or not is judged. If it was, 
the latch pulse is outputted to the LED head 6 (S27) and resets the count 
value (S28) to return to the original image processing routine. If it was 
not, it directly returns to the original image processing routine. 
On the other hand, if it is judged that none of yellow, magenta, cyanine, 
or black process is in operation at (S21), the flow proceeds to (S22) at 
which whether the encoder pulse is being counted or not is judged, and if 
it is, the counting is stopped (S23), otherwise the flow directly returns 
to the original routine. 
In the above embodiment, the error of scanning speed is amended at the 
regular scanning but not at the pre-scanning because the latter is carried 
out at a lower speed thereby causing less effects of irregular speed. 
However, it is possible to detect and memorize the generation timing of 
encoder pulses at the pre-scanning, so as to determine the lighting timing 
of the LED together with the pulse generation timing at the regular 
scanning. 
Although the above embodiment only shows the case of an analog image and a 
digital image both composed on the photoconductor, this invention can be 
applied to the composition of two digital images which are obtained by, 
for example, reading a document line by line with the use of a 
photoelectric conversion element array such as a CCD sensor. In this case, 
both digital images are composed on the memory by writing them into the 
same memory so that an address in which to write at least one of the 
images can be controlled according to the scanning speed of the document. 
Furthermore, this invention can be applied, besides copying machines, to an 
apparatus to photolyze a document into each color component, write them to 
different memories, and read them out to compose, so that each color image 
is stored according to encoder pulses to be generated at a timing 
corresponding to each scanning speed in order adjust their positions. 
Although the present invention has been fully described by way of examples 
with reference to the accompanying drawings, it is to be noted that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as being 
included therein.