Image data processing method and apparatus

1. A medical image processing apparatus comprising:

a memory; and

processing circuitry configured to:

acquire a data volume to be compressed;

acquire a function relating color value to each data value of the data volume;

determine an index value for each data value using the function, wherein the index value is dependent on color value and the index value is determined by obtaining a difference between a combination of the color value and opacity for the data value and a combination of the color value and opacity for an immediately preceding data value, and using the difference to determine an interval gradient visibility for the data value;

store the data values and index values in the memory; and

change a compression rate per region of the data volume based on color values in each region and in dependence on the index values corresponding to the data values within each region.

2. The apparatus according to claim 1 , wherein different regions are rendered using different compression rates.

3. The apparatus according to claim 1 , wherein the processing circuitry is further configured to divide the data volume into a plurality of blocks each comprising a plurality of pixels or voxels; and

wherein the changing of the compression rate is dependent on color values in each of the plurality of blocks.

4. The apparatus according to claim 3 , wherein the index values are calculated in a CIELAB color space.

5. The apparatus according to claim 3 , wherein the processing circuitry is further configured to designate the each of the plurality of blocks as containing a visible gradient, or as not containing a visible gradient, wherein the designating of each block is performed using the index values corresponding to the data values of pixels or voxels within the block.

6. The apparatus according to claim 5 ,

wherein the designating of the each of the plurality of blocks as containing the visible gradient or as not containing the visible gradient comprises:

determining a range of data values that are obtainable by interpolation of the data values of pixels or voxels within the block;

designating the block as not containing the visible gradient if there is no difference in index values within the range of data values; and

designating the block as containing the visible gradient if there is a difference in index values within the range of data values.

7. The apparatus according to claim 1 , wherein using the difference to determine an interval gradient visibility comprises comparing the difference to a first threshold value.

8. The apparatus according to claim 7 , wherein the index value comprises a summed gradient visibility value, wherein the summed gradient visibility value for the data value is obtained by summing interval gradient visibility values from a minimum data value to the data value.

9. The apparatus according to claim 7 , wherein the processing circuitry is further configured to re-compute the index values using a second threshold value that is less than the first threshold value.

10. The apparatus according to claim 9 , wherein the processing circuitry is further configured to render a further image from the data volume in which compression rates are based on index values calculated using the second threshold value.

11. The apparatus according to claim 1 , wherein the processing circuitry is configured to generate a plurality of downsized volumes from the data volume, wherein each of the downsized volumes is generated using a different compression rate.

12. The apparatus according to claim 11 , wherein the processing circuitry is configured to divide each of the downsized volumes into a respective plurality of blocks, and to designate each block of each downsized volume as containing a visible gradient or as not containing a visible gradient.

13. The apparatus according to claim 12 , wherein the processing circuitry is configured to compute a sparse octree indicating, for each region of the data volume, a most-downsized block for that region that is designated as not containing a visible gradient.

14. The apparatus according to claim 13 , wherein the processing circuitry is configured to compute a mixed-resolution data volume using the sparse octree, such that the mixed-resolution data volume comprises blocks from different ones of the downsized volumes.

15. The apparatus according to claim 14 , wherein the processing circuitry is configured to send the mixed-resolution data volume over a network connection to a further apparatus for rendering.

16. The apparatus according to claim 15 , wherein the processing circuitry is configured to update the mixed-resolution data volume based on a threshold value and to send the updated mixed-resolution data volume over the network connection to the further apparatus for rendering.

17. The apparatus according to claim 13 , wherein the processing circuitry is further configured to select a ray step size for a ray traversal based on the sparse octree.

18. A medical image processing method comprising:

acquiring a data volume to be compressed;

acquiring a function relating color value to each data value of the data volume;

determining an index value for each data value using the function, wherein the index value is dependent on color value and the index value is determined by obtaining a difference between a combination of color and opacity for the data value and a combination of color and opacity for an immediately preceding data value, and using the difference to determine an interval gradient visibility for the data value;

storing the data values and index values in a memory; and

changing a compression rate per region of the data volume based on color values in each region and in dependence on the index values corresponding to the data values within each region.

19. A medical image processing apparatus comprising:

processing circuitry configured to:

acquire a data volume to be compressed;

acquire a function relating color value to each data value of the data volume;

divide the data volume into a plurality of blocks each comprising a plurality of pixels or voxels;

change a compression rate per region of the data volume based on color values in each region and in dependence on color values in each of the plurality of blocks; and

designate the each of the plurality of blocks as containing a visible gradient, or as not containing a visible gradient,

wherein the designating of the each of the plurality of blocks as containing the visible gradient or as not containing the visible gradient comprises:

determining a range of data values that are obtainable by interpolation of the data values of pixels or voxels within the block;

designating the block as not containing the visible gradient if there is no difference in index values within the range of data values; and

designating the block as containing the visible gradient if there is a difference in index values within the range of data values.