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EP-4459380-B1 - PICTURE DISTORTION

EP4459380B1EP 4459380 B1EP4459380 B1EP 4459380B1EP-4459380-B1

Inventors

  • CRESPEL, Thomas
  • Cooney, Rory

Dates

Publication Date
20260506
Application Date
20240412

Claims (15)

  1. A method of calculating a map in real-time, the map being for distorting a target picture to be projected by a holographic projector and to compensate for changes in the current temperature of the holographic projector, the method comprising the steps of: receiving a calibrated map comprising a plurality of mappings, each mapping for transforming a respective two-dimensional coordinate of an array of two-dimensional coordinates to compensate for distortion at a predetermined temperature, each two-dimensional coordinate corresponding to one or more image points of a target picture; receiving an array of vectors comprising a vector for each two-dimensional coordinate, each vector representing a calibrated change of each respective two-dimensional coordinate over a predetermined temperature range; receiving a current temperature of the holographic projector; determining a scaling factor based on the difference between the current temperature and the predetermined temperature; calculating a modified map based on the current temperature by, for each coordinate of the array of two-dimensional coordinates: multiplying the vector that relates to the respective coordinate of the array of two-dimensional coordinates by the scaling factor to output a scaled vector; and applying the scaled vector to the respective mapping of the calibrated map; outputting the modified map.
  2. A method as claimed in claim 1, wherein the scaling factor has a linear dependence on temperature.
  3. A method as claimed in claim 2, wherein the step of determining the scaling factor comprises determining the difference between the current temperature and the predetermined temperature and dividing that difference by the predetermined temperature range.
  4. A method as claimed in any one of the preceding claims, wherein the scaling factor is equal to (T - T0) / (Tmax - Tmin); wherein T is the current temperature, T0 is the predetermined temperature, Tmax is a maximum temperature of the predetermined temperature range and Tmin is a minimum temperature of the predetermined temperature range.
  5. A method as claimed in any one of the preceding claims, further comprising the step of receiving the array of two-dimensional coordinates.
  6. A method as claimed in any one of the preceding claims, further comprising the step of applying the modified map to the array of two-dimensional coordinates to output a modified array of two-dimensional coordinates.
  7. A method as claimed in claim 6, further comprising: receiving a target picture comprising a plurality of image points, wherein each two-dimensional coordinate of the array of two-dimensional coordinates corresponds to one or more image points of the target picture; and pre-distorting the target picture based on the modified array of two-dimensional coordinates.
  8. A method as claimed in claim 7, further comprising calculating a hologram of the pre-distorted target picture.
  9. A method claimed in any one of the preceding claims, wherein the target picture is a first target picture, the array of two-dimensional coordinates is a first array of two-dimensional coordinates, the calibrated map is a first calibrated map, the array of vectors is a first array of vectors and the modified map is a first modified map; wherein the method further comprises calculating a second map for distorting a second target picture to be projected by the holographic projector by: receiving a second calibrated map comprising a plurality of second mappings, each second mapping for transforming a respective two-dimensional coordinate of a second array of two-dimensional coordinates to compensate for distortion at a predetermined temperature, each two-dimensional coordinate corresponding to one or more image points of a target picture; receiving a second array of vectors comprising a vector for each two-dimensional coordinate, each vector of the second array representing a calibrated change of each respective two-dimensional coordinate over a predetermined temperature range; calculating a second modified map based on the current temperature by, for each coordinate of the second array of two-dimensional coordinates: multiplying the vector that relates to the respective coordinate of the second array of two-dimensional coordinates by the scaling factor to output a scaled vector; and applying the scaled vector to the respective mapping of the second calibrated map; and outputting the second modified map.
  10. A method as claimed in claim 9, wherein the mapping of the first calibrated map and the vectors of the first vector array have been determined for when a first wavelength is used in the holographic projection of the first target picture.
  11. A method as claimed in claim 10, wherein the mapping of the second calibrated map and the vectors of the second vector array have been determined for when a second wavelength is used in the holographic projection of the second target picture.
  12. A method as claimed in any one of claims 9 to 11, further comprising applying the second modified map to the second array of two-dimensional coordinates to output a second modified array of two-dimensional coordinates.
  13. A method as claimed in claim 12, further comprising distorting the second target picture based on the modified map.
  14. A method claim in claim 13, further comprising calculating a second hologram of the distorted second target picture.
  15. A holographic projector comprising a display device arranged to display a hologram of a picture and to spatially modulate light incident thereon in accordance with the hologram, wherein the holographic projector is arranged to form a holographic reconstruction of the picture at a replay plane; wherein the holographic projector further comprises a controller comprising a memory in which is stored: a calibrated map comprising a plurality of mappings, each mapping for transforming a respective two-dimensional coordinate of an array of two-dimensional coordinates to compensate for a distortion at a predetermined temperature, each two-dimensional coordinate corresponding to one or more image points of a target picture; and an array of vectors comprising a vector for each two-dimensional coordinate, each vector representing a calibrated change of each respective two-dimensional coordinate over a predetermined temperature range; wherein the controller is arranged to: determine a current temperature of the holographic projector; determine a scaling factor based on the difference between the current temperature and the predetermined temperature; and calculate a modified map based on the current temperature by, for each coordinate of the array of two-dimensional coordinates: multiplying the vector that relates to the respective coordinate of the array of two-dimensional coordinates by the scaling factor to output a scaled vector; and applying the scaled vector to the respective mapping of the calibrated map.

Description

FIELD The present disclosure relates to holographic projection of a target image. More specifically, the present disclosure relates to a method of calculating a distortion map for pre-distorting the target image to compensate for distortions caused by the holographic projector. Even more specifically, the present disclosure relates to said method being a method for performing in real-time. Some embodiments relate to a holographic projector, picture generating unit or head-up display. BACKGROUND AND INTRODUCTION Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or "hologram", comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object. Computer-generated holography may numerically simulate the interference process. A computer-generated hologram may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel/Fourier transform holograms or simply Fresnel/Fourier holograms. A Fourier hologram may be considered a Fourier domain/plane representation of the object or a frequency domain/plane representation of the object. A computer-generated hologram may also be calculated by coherent ray tracing or a point cloud technique, for example. A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and/or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micro-mirrors, for example. A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission. A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, "HUD". EP 3 839 638 A1 discloses a method for calculating a map in real-time, the map being adapted to distort a target picture to be projected by a holographic projector and to compensate for changes of the holographic projector. SUMMARY The invention is directed to a method of calculating a map in real-time as recited in appended independent claim 1, and to a holographic projector as recited in appended independent claim 15. Other aspects of the invention are recited in the appended dependent claims. In general terms, there is provided a real-time method of calculating a map for processing, e.g. (pre-) distorting, a target picture to be projected by a holographic projector, the map being suitable for pre-distorting the target picture to compensate for temperature such as the current temperature of the holographic projector. A holographic projector may comprise a display device such as a spatial light modulator. The display device may be arranged to display a hologram of a target picture (for example, a hologram of a pre-distorted picture). The hologram may be a computer-generated hologram. The display device may be arranged to spatially modulate light incident thereon in accordance with the hologram displayed on the display device. The holographic projector may be arranged to direct or relay the spatially modulated light (spatially modulated in accordance with the hologram) towards a viewing plane or eye-box. The holographic projector may be arranged such that the spatially modulated light forms a holographic reconstruction of the picture on a replay plane. The holographic projector may be arranged such that an image (such as a virtual image) of the holographic reconstruction (of the target picture) is visible (i.e. viewable) from the viewing plane or eye-box. The image of the holographic reconstruction of the target picture may be distorted relative to the holographic reconstruction itself. Conceptually, it is possible to consider a plurality of points (e.g. pixels or groups of pixels) of the holographic reconstruction and the corresponding plurality of points of the image of the holographic reconstruction. A distribution of points, spots, pixels or features in the (image of the) holographic reconstruction may be positionally different to the respective points, spots, pixels or features of the target picture (encoded in the hologram). For ex