GB2265518A

GB2265518A - Avoiding the effects of misregistration in producing colour separations

Info

Publication number: GB2265518A
Application number: GB9306051A
Authority: GB
Inventors: David Malone
Original assignee: MPC DATA SYSTEMS Ltd
Current assignee: MPC DATA SYSTEMS Ltd
Priority date: 1992-03-24
Filing date: 1993-03-24
Publication date: 1993-09-29
Anticipated expiration: 2013-03-24
Also published as: GB9306051D0; GB2265518B

Abstract

An original keyline image is scanned by raster scanner 2 and the resulting data is used to generate a high resolution keyline file and a separate low resolution raster file. The raster file is coloured in by an image processor 5. A computer 6 carries out post-processing of raster file by identifying keyline pixel fields, amending the data in these fields so that the colour of a keyline pixel is the same as that of the nearest pixel bordering on the keyline. This generally involves changing the colour separation value from 0 to the relevant value. An imagesetter 7 then exposes a film for the high resolution keyline file, and a set of file colour separation films, one for each of the colour separations. When these are superimposed the colour separations overlap onto the keylines so that any mis-registration in the printing process is compensated for. <IMAGE>

Description

"A method of producing printing films The invention relates to a method of producing printing films, each for a particular colour separation, for example, red, blue and green or alternatively cyan, magenta and yellow. More particularly, the invention relates to production of printing films for images where there are keylines, such as for comic books.

One of the major problems in printing of documents such as comics having images with keylines, is that the various printing films do not align perfectly for printing. This causes gaps to arise between the in-fill colours and the keylines. Irrespective of how carefully one sets up the printing presses, changes in density of paper and film may cause mis-registration between the printing films.

The invention is directed towards providing a method of producing printing films which overcomes these problems.

According to the invention, there is provided a method of producing printing films comprising the steps of: inserting keyline indicators in colour data of an image to indicate pixels which are for a keyline; changing the colour data so that a keyline pixel has the same colour as that of a pixel adjacent to the keyline; and exposing a printing film for each of the colour separations of the colour data, and exposing a separate printing film for a separate set of keyline data.

In one embodiment, the colour data is in a colour separation raster file having a field for each pixel, each field having a channel for each colour and an additional channel having a keyline indicator.

Preferably, one bit position of the keyline-associated channel is provided for indicating if the pixel is for a keyline.

In another embodiment, the keyline data and the colour data are produced by scanning an original keyline image, generating two separate sets of keyline data and carrying out image processing of one set of keyline data to produce the set of colour data.

The invention will be more clearly understood from the following description of some preferred embodiments thereof, given by way of example only with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic view showing an image processor operating according to the invention, in which the arrows at the various components indicate data flow; Fig. 2 is a flow chart illustrating the method of the invention; and Fig. 3 is a diagram showing how pixel colours are changed according to the invention.

Referring to the drawings, and initially to Fig. 1 an image processor 1 is shown. This comprises a raster scanner 2 which is connected to a local area network 3, in this embodiment of the "Ethernetna" type. A pre-processing computer 4 is connected to the network 3 for carrying out pre-processing of the output of the scanner 2. An image processor 5 is also connected to the network 3 for carrying out image processing of the pre-processed data.

There is also a post-processing computer 6 connected to the network 3 for carrying out post-processing on the file after image processing. Finally, an imagesetter 7 is connected to the network 3 and this exposes colour separation films according to data received from the network.

Referring now to both Figs. 1 and 2, the method of the invention is now described. Initially, in step 11, the raster scanner 2 scans an original keyline image. The resolution setting of the scanner 2 is 1168 dpi as it relates to final page size.

The "raw" output scan file is transmitted on the network 3 and is retrieved by the pre-processing computer 4. This carries out steps 12, 13 and 14 of the method 10 shown in Fig. 2. Step 12 involves generation of a high-resolution keyline file which is basically the scanner output converted to a Postscript file format. This is to be subsequently used as a black separation of a page. In step 13, the pre-processing computer 4 generates a low resolution file which is a down-sampled version of the high resolution file. The low resolution file is a raster file 32 bits deep. The first 24 bits are used for the three colours cyan, magenta and yellow (CMY) with each colour assigned an 8-bit channel for 256 discrete values.

The remaining 8 bits are referred to as the alpha channel, in which one of the bits is assigned for keyline data.

This is referred to as the keybit. The high resolution file is approximately 12 Mbytes, whereas the low resolution file is approximately 6.5 Mbytes in size for a single comic book page.

In summary, therefore, after operation of the preprocessing computer 4, there is a high resolution keyline file which contains keyline data only. There is also a separate raster file which is 32 bits deep having four channels, one channel for each of the colours and an alpha channel which contains keybits.

In step 15, the low resolution raster file is retrieved by the image processor 5 from the network 3. The image processor carries out image processing of the data according to user instructions. This involves "painting in" of colours between the keylines in conventional manner. The "coloured" raster file is then transmitted back to the network 3 where it is subsequently retrieved by the post-processing computer 6. In step 16, the postprocessing computer 6 searches through the 32-bit field associated with each pixel. When it locates a keybit in the alpha channel, the computer 6 determines the values for the fields of neighbouring pixels. The computer 6 is pre-programmed to insert values into the colour channels of the relevant pixel 32-bit field so that the pixel has the same colour as the pixel bordering on the keyline, at the same side of the centre line of the keyline.Thus, if the keyline is six pixels wide, six successive pixel fields would be located which have keybits in the alpha channel. The first three fields will have data inserted in the colour channels so that the pixel has the same colour as the pixel next to the keyline on that side, whereas the second three fields will have data inserted to make the pixels the same colour as the bordering pixel on the other side of the keyline.

This operation is indicated by steps 17 to 22 in the method 10 whereby in step 17 a keybit may or may not be located. If a keybit is present, in step 18 the computer 6 identifies the colours of the neighbouring pixels. If it is the first keyline pixel, then there will be a colour on one side of it, whereas the colour channels for the pixel field on the other side will be zero. The computer 6 then inserts the same colour values into the field as indicated by steps 19 and 20. This usually involves changing the colour separation channel values from zero value to the relevant colour value. As indicated by step 21, this is repeated for each pixel field of the image in succession until all of them have been post-processed. In this particular embodiment, the computer 6 has been preprogrammed with the width of the keyline so that it "knows" how to change the pixel field values.This is indicated by the diagram in Fig. 3 in which "x" indicates a colour on one side of a keyline and "0" indicates the colour on the other side. As will be seen, these colours merge at the centre of the keyline. The computer 6 temporarily uses the free bits of the alpha channel to carry out some of the post-processing operations When all fields have been searched, in step 22 the postprocessing computer 6 outputs a modified raster file to the network 3. Both the original high resolution keyline file and the post processed raster file are retrieved by the imagesetter 7. This prints a black film according to the high resolution keyline file, and a colour film for each of the colour separations of the raster file. In this embodiment, there is thus a keyline film printed and a film printed for each of the cyan, magenta and yellow colour channels of the raster file. It will be appreciated that changes in densities of the printing films and any other causes of mis-registration will not result in colour gaps because the colours overlie the keyline file which has been printed separately from the high resolution file. In the embodiment illustrated, it will be necessary for mis-registration of the order of four pixels to cause a noticeable dis-improvement in the printed page.

The invention is not limited to the embodiments hereinbefore described. For example, it is not essential that the keyline file be generated by scanning as, for example, it could be computer generated. Further, it is not essential that a raster file which is 32-bits deep be used as there could be a separate file for each colour separation, the files being linked. Further, the manner in which pixel field colours are changed may vary according to design requirements and the nature of the colours.

Claims

1. A method of producing printing films comprising the steps of: inserting keyline indicators in colour data of an image to indicate pixels which are for a keyline; changing the colour data so that a keyline pixel has the same colour as that of a pixel adjacent to the keyline; and exposing a printing film for each of the colour separations of the colour data, and exposing a separate printing film for a separate set of keyline data.

2. A method as claimed in claim 1, wherein the colour data is in a colour separation raster file having a field for each pixel, each field having a channel for each colour and an additional channel having a keyline indicator.

3. A method as claimed in claim 2, wherein a bit position of the keyline-associated channel is provided for indicating if the pixel is for a keyline.

4. A method as claimed in any of claims 1 to 3, wherein the keyline data and the colour data are produced by scanning an original keyline image, generating two separate sets of keyline data, and carrying out image processing of one set of keyline data to produce the set of colour data.

5. A method substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.