Image based learning correction for mitigating thermal ghosting in a digital printer
An image based correction system compensates for the image quality artifacts induced by thermal ghosting on evolving imaging member surfaces. With thermal ghosting directly tied to previous image content, a feed forward system determines thermal ghosting artifacts based on images previously rendered and generates an open loop gray-level correction to a current image that mitigates undesirable ghosting. For example, the correction system compensates for the thermal ghosting by making the current image “lighter” in areas that will be imaged onto warmer blanket regions, thereby cancelling out TRC differences between different temperature regions. A temperature sensor is used to measure the temperature of the imaging blanket due to the stresses induced by the image. This data is used to learn the parameters of the temperature model periodically during operation, and used in subsequent corrections to mitigate thermal ghosting in spite of changes in blanket properties over use and time.
1. A method for mitigating thermal ghosting on a printed image with an ink-based image forming device having a rotatable imaging member with a reimageable surface in an environment having an ambient temperature, the method comprising:
a) after an imaging of a prior image on the reimageable surface, measuring temperatures of the reimageable surface with a temperature sensor adjacent the reimageable surface, the temperatures being affected by stress induced by the imaging;
b) aligning information of a current image with a temperature map of the reimageable surface where the current image will be rendered, the information including a pixel-wise gray level of the current image, the temperature map associated with the measured temperatures by the temperature sensor;
c) determining a corrected current image including a gray level modification of the current image based on the pixel-wise gray level of the current image and the temperature map to decrease marking material density at reimageable surface discrete coordinates that are warmer than a reference temperature and prevent ghosting of the prior image on a printing of the current image; and
d) rendering the corrected current image on the reimageable surface for a printing by the ink-based image forming device, the printing of the rendered corrected current image appearing not as the corrected current image, but as the current image.
2. The method of claim 1 , further comprising before step b), creating the temperature map of the reimageable surface based on the measured temperature increases on the reimageable surface caused by the imaging of the prior image on the reimageable surface.
3. The method of claim 1 , further comprising before step b), storing the information of the current image.
4. The method of claim 1 , the step c) further comprising determining the gray level modification of the current image via a correction LUT.
5. The method of claim 1 , wherein the pixel-wise gray level of the current image is a pixel-wise contone gray level of the current image, and the corrected current image includes a contone gray level modification of the current image.
6. The method of claim 1 , the step c) further comprising determining the corrected current image with a contone image modifier that makes the current image lighter at reimageable surface discrete coordinates that are warmer than the background area.
7. The method of claim 1 , the step c) further comprising determining the corrected current image including the gray level modification of the current image further adjusted for lateral thermal diffusion.
8. The method of claim 1 , wherein the rotatable imaging member includes a conformable blanket around a central drum with the conformable blanket including the reimageable surface, and the measured temperature increases are based on previous imaging rendered on the reimageable surface, temporal 3D thermal conduction/diffusion, a temperature of the central drum chilling system, and the ambient temperature.
9. The method of claim 1 , step a) further comprising:
d) determining temperature increases above a reference temperature for a plurality of reimageable surface discrete coordinates on the reimageable surface with the temperature sensor, the reference temperature associated with a temperature at a background area of the reimageable surface different than where the prior image was imaged on the reimageable surface, the temperature increases for the plurality of reimageable surface discrete coordinates resulting from imaging of the prior image on the reimageable surface; and
e) creating the temperature map of the reimageable surface based on the determined temperature increases at the plurality of reimageable surface discrete coordinates.
10. The method of claim 9 , wherein the rotatable imaging member includes a conformable blanket around a central drum with the conformable blanket including the reimageable surface, and the step d) determining temperature increases above the reference temperature includes measuring the conformable blanket at the plurality of reimageable surface discrete coordinates on the reimageable surface according to the equation
Δ
T
(
n
,
x
d
,
y
d
)
=
PSF
(
[
Δ
T
(
n
-
1
,
x
d
,
y
d
)
e
-
t
d
/
τ
d
+
Δ
T
0
(
g
d
(
n
,
x
d
,
y
d
)
)
]
;
w
)
,
where ΔT(n,x d ,y d ) is the temperature rise at location (x d ,y d ) on the reimageable surface at drum rotation ID n, PSF is a gaussian point spread function used to account for lateral thermal diffusion at the reimageable surface, e is Euler's number (e=2.718281828 . . . ), t d is the time for one drum rotation, τ d is the time constant of temperature decay on the reimageable surface, ΔT 0 (g d ) is the expected temperature rise immediately after imaging a gray scale image of level g d at location (x d ,y d ) on the reimageable surface at drum rotation n, and w is the width of the gaussian PSF.
11. The method of claim 1 , wherein the rotatable imaging member includes a conformable blanket around a central drum with the conformable blanket including the reimageable surface, and the gray level modification is determined according to the equation
Δ
g
p
(
k
,
x
p
,
y
p
)
=
-
∂
f
∂
Δ
T
|
g
=
g
p
0
,
Δ
T
=
0
∂
f
∂
g
❘
g
=
g
p
0
,
Δ
T
=
0
Δ
T
(
n
-
1
,
x
d
,
y
d
)
g
p
(
k
,
x
p
,
y
p
)
=
g
p
0
(
k
,
x
p
,
y
p
)
+
Δ
g
p
(
k
,
x
p
,
y
p
)
for an engine response curve of the form deltaE=f(g,ΔT) where g p 0 is an original uncorrected contone gray level, g p (k,x p ,y p ) is a corrected contone gray level for page k (=1 . . . K), at pixel location x p , y p , and each pixel location (x p ,y p ) on a page k uniquely maps to a location (x d ,y d ) on the reimageable surface at drum rotation n.
12. A non-transitory computer-readable medium having stored thereon computer executable instructions that, when executed on a controller of an image forming device, cause the controller to perform the method of claim 1 .
13. An image formation apparatus for mitigating thermal ghosting on a printed image with an ink-based image forming device having a rotatable imaging member with a reimageable surface in an environment having an ambient temperature, comprising:
a controller;
a temperature sensor adjacent the reimageable surface; and
a storage device coupled to the controller, the storage device having instructions operative on the controller to
a) measure temperatures of the reimageable surface with the temperature sensor adjacent the reimageable surface after an imaging of a prior image on the reimageable surface, the temperatures being affected by stress induced by the imaging;
b) align information of a current image with the temperature map of the reimageable surface where the current image will be rendered, the information including a pixel-wise gray level of the current image, the temperature map associated with the measured temperatures by the temperature sensor;
c) determine a corrected current image including a gray level modification of the current image based on the pixel-wise gray level of the current image and the aligned temperature map to decrease marking material density at reimageable surface discrete coordinates that are warmer than the background area and prevent ghosting of the prior image on a printing of the current image; and
d) instruct the ink-based image forming device to render the corrected current image on the reimageable surface for a printing by the ink-based image forming device, the printing of the rendered corrected current image appearing not as the corrected current image, but as the current image.
14. The image formation apparatus of claim 13 , the storage device having further instructions operative on the controller to create the temperature map of the reimageable surface based on the measured temperature increases on the reimageable surface caused by the imaging of the prior image on the reimageable surface.
15. The image formation apparatus of claim 13 , the storage device having further instructions operative on the controller to:
d) determine temperature increases above a reference temperature for a plurality of reimageable surface discrete coordinates on the reimageable surface with the temperature sensor, the reference temperature associated with a temperature at a background area of the reimageable surface different than where the prior image was imaged on the reimageable surface, the temperature increases for the plurality of reimageable surface discrete coordinates resulting from imaging of the prior image on the reimageable surface; and
e) create the temperature map of the reimageable surface based on the determined temperature increases at the plurality of reimageable surface discrete coordinates.
16. The image formation apparatus of claim 13 , the storage device storing the information of the current image and the corrected current image.
17. The image formation apparatus of claim 13 , the storage device storing a lookup table having corrective information, and the controller determines the gray level modification of the current image via the corrective information.
18. The image formation apparatus of claim 13 , wherein the controller determines the corrected current image with a contone image modifier that makes the current image lighter at reimageable surface discrete coordinates that are warmer than the background area.
19. The image formation apparatus of claim 13 , wherein the controller determines the corrected current image including the gray level modification of the current image further adjusted for lateral thermal diffusion.
20. A method for mitigating thermal ghosting on a printed image with an ink-based image forming device having a rotatable imaging member with a reimageable surface in an environment having an ambient temperature, the method comprising:
a) creating a temperature map of the reimageable surface based on measured temperature increases above a reference temperature for a plurality of reimageable surface discrete coordinates on the reimageable surface, the reference temperature associated with a temperature setpoint of a central drum chilling system, the temperature increases for the plurality of reimageable surface discrete coordinates resulting from imaging of prior images on the reimageable surface;
b) aligning information of a current image with the temperature map of the reimageable surface where the current image will be rendered, the information including a pixel-wise gray level of the current image;
c) determining a corrected current image including a gray level modification of the current image based on the pixel-wise gray level of the current image and the aligned temperature map to decrease marking material density at reimageable surface discrete coordinates that are warmer than the background area and prevent ghosting of the prior images on a printing of the current image; and
d) rendering the corrected current image on the reimageable surface for a printing by the ink-based image forming device, the printing of the rendered corrected current image appearing not as the corrected current image, but as the current image.