DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The lenticular sheet shown in FIG. 1 comprises a transparent acrylic sheet 1 formed with a flat rear surface and a ridged front surface in which the ridges 2 extend parallel to one another and each ridge 2 forms a cylindrical lens. A printed sheet 3 is attached to the rear surface of the acrylic sheet 1 and carries a lenticular image L comprising multiple equal width, parallel lenticular stripes 4 , each aligned longitudinally with a respective cylindrical lens 2 .

[0027] The stripes 4 are all of equal width and each is composed of multiple image strips 5 , each strip 5 being derived from a corresponding original image, which is to be incorporated into the lenticular image. As shown in FIG. 2 , there are four original contone images A, B, C, D, each divided into seven image strips A 1 to A 7 , B 1 to B 7 , C 1 to C 7 and D 1 to D 7 , and successive ones of the image strips from each original image are incorporated into successive ones of the lenticular stripes 4 . Thus, the first stripe 4 is composed of four image strips A 1 , B 1 , C 1 , D 1 ; the second stripe 4 alongside the first is composed of four image strips A 2 , B 2 , C 2 , D 2 ; and so on for all seven stripes 4 . The arrangement of the strips A to D in relation to each of the cylindrical lenses 2 is such that only the strips A 1 to A 7 are visible to a viewer as seen from the front of the sheet 1 at a first viewing angle, only strips B 1 to B 7 are visible to the viewer as seen from a second viewing angle, and so on for each of the groups of strips C 1 to C 7 and D 1 to D 7 each seen from respective viewing angles. Therefore, the viewer sees each of the original images as a composite of its image strips 5 when viewed at a unique angle. This is the known characteristic of lenticular images, which is exploited to produce certain novelty effects.

[0028] It will be appreciated that the lenticular image L, is composed of a number of contone images, and is itself a contone image which is to be printed on the sheet 3 .

[0029] According to a first embodiment of the invention illustrated in FIG. 3 each of the four original contone images A to D is processed separately as a whole to produce a corresponding halftone image A′, B′, C′ and D′. Various known halftoning algorithms may be used to convert individual or groups of pixels from each original image A to D into halftone dot printing values which are stored as the halftone images A′ to D′. The halftone dot printing values of each halftone image are then sampled to output values from each of seven successive, equal width, parallel strips of the halftone image A′ 1 to A′ 7 , B′ 1 , C′ 1 to C′ 7 , D′ 1 to D′ 7 . These strip values are interleaved as they are sampled so as to produce a halftone lenticular image HL consisting of parallel stripes 4 , each including a corresponding one of the image strips from each of the halftone images; A′ 1 , B′ 1 , C′ 1 , D′ 1 ; A′ 2 , B′ 2 , C′ 2 , D′ 2 ; etc. up to the seventh stripe A′ 7 , B′ 7 , C′ 7 , D′ 7 . This halftone lenticular image HL is printed by a dot printer, such as a jet printer, which takes the sampled interleaved strip values as the driving input.

[0030] According to a second embodiment of the invention illustrated in FIG. 4 , the lenticular contone image L of FIG. 2 is processed to produce a halftone lenticular image HL consisting of the same number of lenticular stripes 4 and strips 5 , pixels from the contone image L being converted into corresponding dot printing values for feeding to a dot printer that prints the halftone lenticular image. The conversion from pixels to dot printing values may proceed on a pixel by pixel basis. Alternatively, multiple pixels may be processed simultaneously to produce corresponding dot printing values, the pixels all being selected from the same strip 5 each time. Multiple pixels are selected on the basis of a halftone cell which is either the same width as the strip or is a sub-multiple of the width of the strip. Standard halftoning algorithms may be used for this conversion.

[0031] FIG. 5 illustrates an arrangement of pixels for two adjacent lenticular stripes 4 , each composed of four strips 5 : A 1 , B 1 , C 1 , D 1 and A 2 , B 2 , C 2 , D 2 . The pixels define a square grid with each strip 4 being two pixels wide. Therefore, a halftone cell two pixels square may be used to convert a square array of pixels across the full width of each strip 5 into corresponding dot printing values. Alternatively, smaller pixels will allow a larger number to be accommodated within the width of each strip 5 so that a halftone cell two pixels square may be applied twice for a strip four pixels wide, or more often for even smaller pixels. The limit in size of pixels will be determined by the resolution of the dot printer.

[0032] This conversion of pixels to dot printing values is conducted in a sequential manner across the whole of the lenticular contone image L so as to produce a continuous output of dot printing values for the printer. This embodiment of the invention therefore runs as an online process to produce a halftone lenticular image from a contone lenticular image.

[0033] A third embodiment of the invention illustrated in FIG. 6 , is similar to that of FIG. 4 in that a halftone lenticular image HL is generated from the lenticular contone image L, but the halftoning process used is different in that it makes use of pixels from different strips 5 of the same original image to produce each set of corresponding dot printing values.

[0034] As shown in FIG. 6 , the strips are two pixels wide, and a halftone cell four pixels square is used to convert adjacent pixels of two strips 5 of the same original image in neighbouring lenticular stripes 4 into dot printing values. For example, FIG. 6 indicates that the adjacent pixels of strips B 1 and B 2 in neighbouring stripes are processed together in the halftone cell, each contributing 2×4 pixels to the 4×4 halftone cell. The physical separation of the two strips B 1 and B 2 is therefore counteracted, and the halftone processing gives improved results because of the large pixel sample whilst avoiding adverse boundary effects where neighbouring strips such as B 1 and C 1 meet. The conversion of pixels to dot printing values is performed in a sequential manner across the whole of the lenticular contone image L to produce a continuous printing output, as in the embodiment of FIG. 6 .

[0035] According to a fourth embodiment of the invention, the halftoning process used in the embodiment of FIG. 6 may be an irregular dispersed halftoning process instead of that of a predefined halftone cell. This alternative halftoning process is well known and involves using pixels in neighbourhoods that may vary in extent and structure with the local image characteristics and may include error diffusion. However, pixels of all neighbourhoods are selected so that they come from the strips 5 of the same original image at any one time to produce corresponding dot printing outputs.

[0036] According to another embodiment of the invention, the halftone square cell used in the second embodiment of FIGS. 4 and 5 , or the third embodiment of FIG. 6 is replaced by a halftone cell which is longer than it is wide so as to make more use of the longitudinally aligned pixels in each strip to give improved resolution in the halftone lenticular image. For example, the square 2×2 halftone cell of the second embodiment may be replaced by a 3×2 halftone cell.

[0037] The same principle of making more use of the longitudinally spaced pixels available in each strip can be applied to the irregular dispersed halftoning process of the fourth embodiment, the neighbourhoods used being more elongated longitudinally. For example, the diffusion kernel of the Floyd and Steinbeck error diffusion technique or other similar techniques can be elongated so as to favour more use of the longitudinally aligned pixels.

[0038] The apparatus necessary to carry out the method may be essentially conventional in form. An essentially conventional computing apparatus 101 (such as a PC) with sufficient computational power in its processor 110 and sufficient memory 111 to handle the necessary degree of image processing can carry out all the steps up to provision of the values necessary for printing the halftone lenticular image. These values are then sent to any printer 102 adapted to printing lenticular images through an appropriate communications infrastructure 103 .