Halftone dot-growth technique using a dot edge-detection scheme

1. A method for color connecting an original halftone bitmap image by a predefined color correction function to produce a color corrected halftone bitmap image comprising:

providing an original halftone bitmap image;

estimating the dot area percentage of the original halftone bitmap image in a set of sub-image blocks;

calculating an aim dot area percentage, based on a predefined color correction function, for each sub-image block in said original halftone bitmap image;

calculating the number of halftone bitmap image pixels to convert to on or off states to produce a modified original halftone bitmap image that has the aim dot area percentage where said value is designated by N, for each sub-image block in said original halftone bitmap image, and

converting N pixels in said original halftone bitmap image to either on or off states depending on whether the aim dot area percentage is greater or less than the dot area percentage of said original halftone bitmap image respectively, for each sub-image block in said original halftone bitmap image.

2. The method of claim 1 wherein the process of estimating the dot area percentage of the original halftone bitmap image in a set of sub-image blocks includes recursively lowpass filtering, sub-sampling, and then interpolating said original halftone bitmap image until it possesses the desired number of sub-image blocks.

3. The method of claim 1 wherein the process of calculating the number of halftone bitmap image pixels (N) to convert to on or off states to produce a modified original halftone bitmap image that has the aim dot area percentage includes:

first computing the difference between the aim dot area percentage and the estimated dot area percentage of said original halftone bitmap image, for a given sub-image block of the color corrected halftone bitmap image and a corresponding block of said original halftone bitmap image; and

converting this difference to the number of halftone bitmap image pixels to modify, in the current block of said original halftone bitmap image, by dividing said difference by the dot area percentage of a single halftone bitmap image pixel and rounding the result of this division to the nearest integer value.

4. The method of claim 1 wherein the process of converting |N| on pixels (where |N| represents the absolute value of the value of N), in a given block of said original halftone bitmap image, to |N| off pixels, in a corresponding block of the color corrected halftone bitmap image, includes:

calculating an edge mask of the interior-boundary pixels of the halftone bitmap image contained in a given block;

counting the number of interior-boundary pixels in the current block, where said value is designated by k,

converting |N| on pixels present in the edge mask to off pixels, for the case where k≧|N|, or converting all pixels of the edge mask to off pixels, for the case where k<|N|,

updating the number of pixels to convert in the current block by creating a modified N value equal to −(|N|−k), and

recursively repeating said calculating, counting, converting and updating until the modified value of N is zero for the current block.

5. The method of claim 1 wherein the process of converting N off pixels, in a given block of said original halftone bitmap image, to Non pixels, in a corresponding block of the color corrected halftone bitmap image includes:

calculating an edge mask of the exterior-boundary pixels of the halftone bitmap image contained in a given block

counting the number of exterior-boundary pixels in the current block, where said value is designated by k,

converting N off pixels present in the edge mask to on pixels, for the case where k≧N, or converting all pixels of the edge mask to on pixels, for the case where k<N,

updating the number of pixels to convert in the current block by creating a modified N value equal to N−k, and

recursively repeating said calculating, counting, converting, and updating until the modified value of N is zero for the current block.

6. The method of claim 4 wherein the process of calculating the interior-boundary pixels of the halftone bitmap image contained in a given block includes:

eroding the halftone bitmap image in the current block by a symmetrical morphological structuring element that removes one layer of on pixels from the perimeter of the halftone bitmap image in the current block; and

calculating the interior-boundary image of the halftone bitmap image, for the current block, by computing the difference between the halftone bitmap image for the current block and the eroded halftone bitmap image for the current block.

7. The method of claim 5 wherein the process of calculating the exterior-boundary pixels of the halftone bitmap image contained in a given block includes

first dilating the halftone bitmap image in the current block by a symmetrical morphological structuring element that adds one layer of on pixels to the perimeter of the halftone bitmap image in the current block; and

calculating the exterior-boundary image of the halftone bitmap image, for the current block, by computing the difference between the dilated halftone bitmap image for the current block and halftone bitmap image for the current block.

8. The method of claim 4 for converting on pixels present in the edge mask to off pixels includes:

recursively converting halftone bitmap image pixels, for the current block, identified in the edge mask, calculated from the interior-boundary calculation, to off pixels according their position in a predefined order-selection matrix.

9. The method of claim 5 for converting off pixels present in the edge mask to on pixels includes:

recursively converting halftone bitmap image pixels, for the current block, identified in the edge mask, calculated from the exterior-boundary calculation, to on pixels according their position in a predefined order-selection matrix.

10. The method of claim 8 wherein the order selection matrix includes:

using an image mask of the same dimensions as the sub-image blocks of the original halftone bitmap image whose elements contain numbers from one to the number of pixels in the sub-image blocks, where these numbers are randomly positioned within the image mask and correspond to the order in which pixels are converted from on-to-off or from off-to-on.

11. The method of claim 8 where the order selection matrix includes:

using an image mask of the same dimensions as the sub-image blocks of the original halftone bitmap image whose elements contain numbers from one to the number of pixels in the sub-image blocks, where these numbers are ordered, according to a desired dot growth pattern, within the image mask and correspond to the order in which pixels are converted from on-to-off or from off-to-on.

12. The method of claim 9 wherein the order selection matrix includes:

using an image mask of the same dimensions as the sub-image blocks of the original halftone bitmap image whose elements contain numbers from one to the number of pixels in the sub-image blocks, where these numbers are randomly positioned within the image mask and correspond to the order in which pixels are converted from on-to-off or from off-to-on.

13. The method of claim 9 wherein the order selection matrix includes:

using an image mask of the same dimensions as the sub-image blocks of the original halftone bitmap image whose elements contain numbers from one to the number of pixels in the sub-image blocks, where these numbers are ordered, according to a desired dot growth pattern, within the image mask and correspond to the order in which pixels are converted from on-to-off or from off-to-on.

14. The method of claim 1 wherein said original halftone bitmap image includes:

one of the halftone bitmap images of a multiple separation halftone bitmap image.

15. The method of claim 1 wherein the color correction function includes:

sets of channel independent color correction functions.

16. The method of claim 1 wherein the color correction function consists of

sets of channel independent color correction functions and multi-dimensional color correction functions that consider interactions between the channels of a multiple separation halftone bitmap image.