Method of superimposing an image onto another, method of personalizing a data carrier using the image superimposing method and a personalized data carrier

1. A method of superimposing a first image onto a second image for creating a target image, wherein the first image, the second image, and the target image each are defined by a plurality of pixels, each pixel being associated with a pixel value, the method comprising:

changing a pixel value of the target image to be equal to a pixel value of the second image if the corresponding pixel value of the first image is equal to zero;

changing the pixel value of the target image to be equal to the pixel value of the first image if the corresponding pixel value of the first image is not equal to zero, wherein

for each pixel of the target image corresponding to a pixel of the first image, a contrast evaluation is performed between the corresponding pixel of the first image and a neighboring pixel of the second image, and

the pixel value of the target image is increased or decreased based on the evaluation result in order to obtain a visible contrast between the first and second image in all parts of the target image.

2. A method according to claim 1 , wherein the contrast evaluation is made by comparing the corresponding pixel value of the second image with a threshold value, and changing the pixel value of the target image comprises:

increasing or decreasing the pixel value of the target image by a value to obtain a new value, depending on the comparison with the threshold value.

3. A method according to claim 2 , wherein increasing or decreasing the pixel value of the target image by a value to obtain the new value comprises:

increasing the pixel value of the target image by a predetermined value if the corresponding pixel value of the second image is lower than the threshold value;

decreasing the pixel value of the target image by the predetermined value if the corresponding pixel value of the second image is higher than the threshold value; and

either increasing or decreasing the pixel value of the target image if the pixel value of the second image is equal to the threshold value.

4. A method according to claim 1 , wherein the contrast evaluation is made by making the difference of corresponding pixel values between the pixel of the first image and the corresponding pixel of the second image; and

wherein the increasing or decreasing of the pixel value of the target image is made if the result of the difference is lower than a predetermined threshold value.

5. A method according to claim 1 , wherein the contrast evaluation is made by:

comparing an average of the values of the plurality of pixels of the second image with a threshold value; and

increasing or decreasing the pixel value of the target image by a value to obtain a new value, depending on how the average value compares with the threshold value.

6. A method according to claim 5 , wherein increasing or decreasing the pixel value of the target image by a value to obtain the new value comprises:

increasing the pixel value of the target image by a predetermined value if the average value is lower than the threshold value;

decreasing the pixel value of the target image by the predetermined value if the average value is higher that the threshold value; and

either increasing or decreasing the pixel value of the target image if the average value is equal to the threshold value.

7. A method according to claim 5 , wherein the pixel value of at least one pixel of the target image is based on the values of a plurality of pixels of the second image, and increasing or decreasing the pixel value of the target image to a new value comprises:

where possible, setting the new pixel value of the target image to a value that is a predetermined value away or a greatest possible distance away from each of the values of the plurality of pixels of the second image;

otherwise, setting the new pixel value of the target image to a median of the values of the plurality of pixel values of the second image, a minimum value of a pixel or a maximum value of a pixel.

8. A method according to claim 5 , wherein the pixel value of at least one pixel of the target image is based on the values of a plurality of pixels of the second image, and increasing or decreasing the pixel value of the target image to a new value comprises:

setting the pixel value of the target image to a value that is a greatest possible distance away from each of the values of the plurality of pixels of the second image.

9. A method according to claim 1 , wherein the pixel value comprises a grayscale value for a black and white second image, or at least one of the values of the lightness and colour components for a coloured second image.

10. A method of personalizing a data carrier comprising:

realizing a security pattern image at the data carrier;

realizing an identification image at the data carrier;

superimposing, by a processor, the security pattern image onto the identification image according to claim 1 ; and

making the security pattern image and making the identification image by printing, by a printer, the identification image with the security pattern image superimposed on the data carrier.

11. A method of personalizing a data carrier according to claim 10 , wherein the data carrier is one of a batch of data carriers, and wherein a different identification image is used for personalizing each data carrier in the batch.

12. A method of personalizing a data carrier according to claim 10 wherein printing comprises one of laser printing, laser engraving, dye sublimation printing and ink jet printing.

13. A data carrier comprising:

a target image printed thereon, said target image comprising an identification image and a security pattern image at least partially overlapping each other, wherein

the identification image and the security pattern image are each defined by a plurality of pixels, and

each pixel of the security pattern image bears a tonal or color relationship to its respective at least one neighboring pixel of the identification image, and the tonal or color relationship comprises at least a predetermined difference between the security pattern image pixel value and its respective at least one neighboring identification image pixel value, said predetermined difference being representative of a minimal contrast.

14. A data carrier according to claim 13 , wherein the predetermined value is at least 1/15 of the maximum value of a pixel.