Patent Description:
A projector display system typically includes a light source that illuminates a screen with an image that is modulated by some optical system within or on the projector. A projector includes a light source, which may include one or more light emitting elements such as lasers, Xenon lamps, arc lamps, light emitting diodes, etc. Light from the light source may be directed along an optical path which may include one or more optical components, such as lenses, modulators, beam expanders, beam splitters, irises, filters, and the like.

Light exiting the projector display system may be directed to a two-dimensional screen, such as a movie theater screen. The one or more modulators which may be included in the projector display systems selectively direct light to particular portions of the screen to generate an image, which may include comparatively bright portions and comparatively dark portions. The contrast ratio of a projector is a measurement of the capabilities of the projector display system, and is defined as the ratio between the peak brightness to the dark level of the display. Because an infinite contrast ratio is not technologically feasible, it may be technically difficult for a projector to simultaneously display a very bright image portion and an absolute black image portion.

Backlight control of projectors based on data from ambient light sensors and luminance metadata is disclosed in <CIT>.

Projector or other display systems including or relating to global dimming have been described in commonly-owned patents and patent applications, including:.

Various aspects of the present disclosure relate to devices, systems, and methods for global dimming in a projector to reduce the perceptibility of visual artifacts.

In one exemplary aspect of the present disclosure, there is provided a projection display system comprising a light source configured to emit a light in response to a content data; an optical modulator configured to modulate the light; and a controller configured to adjust a light level of the projection display system based on the content data and a metadata relating to a future frame, thereby to reduce a perceptibility of a visual artifact.

In another exemplary aspect of the present disclosure, there is provided a non-transitory computer-readable-medium storing instructions that, when executed by a processor of a projection display system comprising a light source and an optical modulator, cause the projection display system to perform operations comprising receiving a content data; emitting a light, by the light source, in response to the content data; modulating the light, by the optical modulator; and adjusting a light level of the projection display system based on the content data and a metadata relating to a future frame, thereby to reduce a perceptibility of a visual artifact.

In this manner, various aspects of the present disclosure provide for global dimming in a projector display system using psycho-visual masking, and effect improvements in at least the technical fields of image projection, signal processing, and the like.

These and other more detailed and specific features of various aspects are more fully disclosed in the following description, reference being had to the accompanying drawings, in which:.

This disclosure and aspects thereof can be embodied in various forms, including hardware or circuits controlled by computer-implemented methods, computer program products, computer systems and networks, user interfaces, and application programming interfaces; as well as hardware-implemented methods, signal processing circuits, memory arrays, application specific integrated circuits, field programmable gate arrays, and the like. The foregoing summary is intended solely to give a general idea of various aspects of the present disclosure, and does not limit the scope of the disclosure in any way.

In the following description, numerous details are set forth, such as circuit configurations, waveform timings, circuit operations, and the like, in order to provide an understanding of one or more aspects of the present disclosure. It will be readily apparent to one skilled in the art that these specific details are merely exemplary and not intended to limit the scope of this application.

Moreover, while the present disclosure focuses mainly on examples in which the various circuits are used in digital projection systems, it will be understood that this is merely one example of an implementation. It will further be understood that the disclosed systems and methods can be used in any device in which there is a need to reduce or attenuate the perceptibility of a visual artifact; for example, microscopy, image sensing, telecommunications, non-projection image display, and so on.

As used herein, "global dimming" refers to a technique where the overall light level of a light source for a projector is changed depending upon the content. For bright scenes, the light level of the source is increased, and for dim scenes the level is decreased. This can be done in a variety of ways including via an iris, variable density filters, and modulation of the light source itself. Furthermore, this may be done using hardware components, software components, firmware, or a combination thereof. In one example, global dimming (such as any one or more of the following techniques) may be implemented by a variable multi-input analog or digital low pass filter function using metadata parameters as the input to this function.

Global dimming systems may suffer from undesirable visual artifacts. For example, consider a very dark scene where a relatively small bright light such as a stop light blinks slowly. In this case, an undesirable artifact due to pumping of the dark level may be present. If the modulator has limited dynamic range, the dark level in the scene will increase when the light is on, and decrease when the light goes dim; in other words, the projector dark level is proportional the source level. Since the bright light is small, it does not provide significant masking for the rest of the scene, and the projector dark level changes are readily visible. The artifacts become less visible with a projector with a higher base (simultaneous) contrast, but may still be readily seen. A projector typically used in Digital Cinema (DCI) projectors is around <NUM>:<NUM>, while enhanced DCI projectors may have <NUM>:<NUM> or higher. For example, a projector with a base simultaneous contrast ratio over <NUM>,<NUM>,<NUM>:<NUM> may benefit less from global dimming, and artifacts from it may not be visible, but projectors with contrast ratios (CRs) around <NUM>,<NUM>:<NUM> may benefit from global dimming, and the projector dark level pumping may still be perceived.

However, it is possible to mask these visual artifacts so that they are not perceived by an observer, or are only slightly perceived. This masking may be achieved through the use of various techniques based on the human visual system thereby to reduce the visibility of these effects.

According to a "slow dimming" technique, when a bright light appears, the source light is increased to allow the brightest part of the scene to be at the correct or desired brightness. <FIG> illustrates a slow dimming technique. Specifically, <FIG> illustrates a brightness level of a projector display system as a function of time. In <FIG>, a series of peak levels <NUM> are shown, which correspond to the peak brightness level in an image in a given frame. Curve <NUM> illustrates a capacity of a projector display system; that is, a peak white level a projector display system is able to display at a given time. Curve <NUM> illustrates a dark level the projector display system is able to display at the given time. In the particular example illustrated in <FIG>, the projector display system has a <NUM>: <NUM> base contrast ratio; thus, the dark level curve <NUM> is <NUM>/<NUM> of the peak white level curve <NUM>. When the light disappears, the level of the source light slowly decreases. When the bright light in the scene re-appears, the source light is quickly increased to allow the bright object to be shown at its correct level. In this manner, the source light is adjusted such that the rate-of-change of the brightness is smoothed. The projector dark level pumping may be still visible, especially on the faster dark to light transition, but it is less perceivable because humans do not notice slow changes as readily as faster changes. Also, one need not dim the source such that the brightest part of the scene is just covered by the light source. For example, if the scene jumps from having a bright area at <NUM> nits to <NUM> nits, one need not reduce the source light by a factor of <NUM>,<NUM>. A smaller reduction (partial dimming) can be used and it will reduce the pumping effect; however, partial dimming may have lower effectiveness in reducing the projector dark level.

If the bright areas are small, it may be advantageous to have a lower source light level than that which would be used to reproduce these areas in full detail. This will result in saturation and loss of detail on the bright objects, but result in a lower projector dark level. In many cases this is a desirable tradeoff, as detail in small bright objects is not easily visible to observers. Based upon analysis of the image and knowledge of the human visual system, one can determine the proper level to use. This is called small bright object (SBO) compensation.

Another consideration is that light from the screen reflects from the walls of the auditorium and back to the screen. This results in a dark level that is proportional to the total energy of the image on the screen. It is not strictly necessary to keep the dark level of the projector much below the room reflectance dark level, so adjustments of the algorithm to determine the source level can take this into consideration. This is called "room refection compensation. " Because the amount of light reflected from the walls of the auditorium is based, at least in part, on the amount of light reflected from the screen, the algorithm for room reflection compensation may also incorporate an adjustment based on the screen gain.

The speed of the dimming may be variable depend on one or more internal or external factors. For example, in instances where there is a small bright object plus a high average pixel level (APL) and/or a high level of room reflection or ambient light, it may be preferable to provide comparatively "fast" dimming as compared to instances where there is a low APL and/or a low level of room reflection or ambient light.

In addition to room reflection compensation, it is also possible to compensate for the dark level from the projector, which is dependent on the total energy of the image being produced by the projector. This level is produced by, for example, scattering in optical surfaces within the projector (for example, by the projection lens). This may be referred to as "lens veiling glare" or "veiling glare dark light. " Because both the room reflection dark light and the veiling glare dark light are dependent upon the APL, the effects may be combined into a single term for purposes of analysis and compensation.

Furthermore, it is also possible to compensate for the effects of ambient light in the theater, or "room ambient dark light. " If the ambient light level is high, there is a reduced need to provide a large degree of global dimming at least because the ambient light effect would dominate. The degree of dimming may also be affected by the contrast ratio of the image being projected. That is, there are diminishing returns when producing an image from the projector/room system that has a significantly higher contrast than what is specified in the image data. From the image data, it is possible to calculate the darkest pixel in a scene to determine the dark level that would be in fact exhibited on the screen. The result of this calculation may be added to the data used for calculating the amount of dimming.

The global dimming algorithm may also be reset or reinitialized at the time of a scene change in the image data. For example, the source level may be immediately reset to the peak level of the image upon scene change, and various parameters of the low pass filter may change as appropriate to the new image sequence. The time of scene change may be determined dynamically (for example, by monitoring the image data in a current or future frame) or may be flagged by metadata included with the image data.

The perceptibility of visual artifacts may also be influenced by audio. For example, higher sound pressures (for example, a scene with a loud explosion) may reduce the ability of the human visual system to perceive low level dark areas; therefore, a smaller degree of global dimming may be used for periods during which the soundtrack has a high volume. In this manner, an additional parameter used to influence global dimming may be the sound pressure level.

Preferably, many or all of the above components should be considered when determining the particular degree and type of global dimming to be applied. That is, the degree and type of global dimming should be calculated so as to minimize the amount of dimming utilized to only that which is necessary to match the sum of all the dark levels produced by the system. <FIG> illustrates an example of this for a particular (and arbitrary) frame sequence. In <FIG>, the upper line <NUM> represents the total light on the screen; that is, the APL. The lower line <NUM> represents the light reflected from the room and scattered from the projection lens onto the screen. As illustrated in <FIG>, the light reflected provides a contribution of approximately <NUM>% as representative of typical auditorium conditions.

In a "slow dimming with look-ahead" technique, the projector has information about the future peak level of frames. This information may be contained in metadata, or determined by pre-reading the frames in advance of playback. Similar to the general slow dimming technique, the source light changes slowly. Because the projector has information on future frames, this allows the source light to increase slowly before the bright light appears, further reducing the visibility of the dark pumping. Additionally, the slope or speed of the change from one state to another could be different for different levels and content, depending upon measurements and observations from viewers to determine the correct transition rate and shape. In some cases the transitions could be very slow, on the order of minutes, and track the accommodation rate of the human visual system. Partial dimming, SBO compensation, and room reflection compensation can also be used in this technique.

<FIG> illustrates a brightness level of a projector display system that incorporates partial diming and SBO clipping, as a function of time. In <FIG>, a series of peak levels <NUM> are shown, which correspond to the peak brightness level in an image in a given frame. Curve <NUM> illustrates a capacity of a projector display system; that is, a peak white level a projector display system is able to display at a given time. Curve <NUM> illustrates a dark level the projector display system is able to display at the given time. In the particular example illustrated in <FIG>, the projector display system has a <NUM>:<NUM> base contrast ratio; thus, the dark level curve <NUM> is <NUM>/<NUM> of the peak white level curve <NUM>. As compared with the technique illustrated in <FIG>, <FIG> shows that the brightness level of the projector display system may be increased in advance of a comparatively bright frame or frame portion. Such an increase may allow for a slower change and thus a less-perceivable degree of projector dark level pumping. In <FIG>, two of the peak levels <NUM> correspond to SBOs. Using the future frame data, the projector display system clips the SBOs such that a peak white level for the frame containing the SBOs is below the actual peak levels <NUM>. This may allow for a lower dark level and thus a frame reproduction that appears to the viewer to contain fewer visual artifacts.

Techniques <NUM> and <NUM>, both directed to "slow dimming," result in single images that during the steady state condition have the dynamic range of the base projector (except when partial dimming is used). In a dimming technique with "dynamic range compression," especially during the transitions, the dynamic range is decreased. This results in images where the image dark detail comes and goes depending upon the global dimming level. With bright scenes where the source level is high, image dark detail is obscured by the projector dark level. At lower levels, when the bright light is not present and the source level and projector dark level decreases, the image dark detail re-appears. One solution to alleviate this effect is to change the shape of the tone curve dynamically during the transitions, and with different steady state condition. <FIG> illustrates such a tone curve. In <FIG>, the brightness (that is, the output level) is shown as a function of the image input level for a single frame at full brightness. Dashed line <NUM> corresponds to the projector dark level and dashed line <NUM> corresponds to the projector peak level. If an unmodified tone curve <NUM> were implemented, the brightness would simply correspond to the image input level in a linear fashion. By providing a modified tone curve <NUM>, however, it is possible to allow enhanced dark image detail (for low image input levels) and/or allow SBO detail (for high image input levels).

That is, an example would be to increase the levels of the dark parts of the image which would normally be obscured by the projector dark level when there is (small area) bright content on screen and a high source level. The tone curve would revert to linear when there is no bright content, and the source level (and projector dark level) is low. Variations with different shaped tone curves, including shapes that affect the bright content could also be advantageous. For example, if there are small bright objects, it might be advantageous to have a sigmoid shape to allow for detail on the small bright objects (at a lower modulation depth), while using a lower source level similar to what was described in technique <NUM>, but without the loss of detail.

It is also possible to incorporate "human tuned" dimming. This may be achieved through the any of the above techniques as well as human tuning to generate metadata in the mastering or color grading process that explicitly controls the source levels. Alternatively, one may use any of the techniques above and human tuning to provide metadata that will guide an algorithm in the projector that determines the appropriate level for the source. The provided metadata may include information for the use of multiple techniques so as to be utilized by projectors which have different features. For example, the provided meta data may include a first set of information for use in projectors which support look-ahead, a second set of information for use in projectors which support partial dimming, a third set of information for use in projectors with no additional feature support, etc..

The above techniques may be used together. For example, it is possible to use a slow dimming technique (with or without look-ahead) in combination with dynamic range compression and/or human tuning. The combined technique may be further refined with the inclusion of one or more of SBO compensation, partial dimming, or room reflection compensation.

Moreover, while <FIG> and <FIG> illustrate a tone curve showing the illumination levels of an image compared with the peak and dark levels of a projector, it is possible to vary the color components of the image individually in a hue-preserving manner. For example, if a small bright object is red, it is possible to apply a tone curve as illustrated in <FIG> and <FIG> to red light in the projector while not applying such a curve to blue or green light.

In the above techniques, various parameters (for example, characteristics relating to the room reflection, ambient light, and the like) may be measured and input to the algorithm for calculation of the dimming in real time. Alternatively, the various parameters may be measured and input to the algorithm manually at the time of calibration. In some aspects of the present disclosure, some parameters may be measured and input at the time of calibration while other parameters are measured and input in real time.

<FIG> illustrate examples of dark level contributions for a series of particular projectors and rooms. Details of the projectors and room configurations are set forth in the following Table <NUM>:.

In Table <NUM>, "MPPL" refers to the maximum projector peak level in nits; PBCR refers to the projector base contrast ratio (<NUM>:<NUM> for DCI <NUM>); RRR refers to the room reflectance ratio (APL : room reflectance dark level), and includes the veiling glare term; ADL refers to the ambient dark level; and SFRR refers to a safety factor for room reflection and ambient light. The safety factor is a margin that the particular global dimming algorithm uses to determine how far down from the combined dark level it puts the projector dark light. This defines how much more dark light (of the combined dark light) will be contributed by the projector. The above values are merely exemplary.

<FIG> illustrates all dark level contributions for Example <NUM>, which is a high contrast (<NUM>) projector. Specifically, <FIG> illustrates the room reflection / veiling glare dark level contribution (dashed line <NUM>), the projector dark level contribution (dash-dot line <NUM>), the room ambient dark level contribution (long-dashed line <NUM>), the image dark level contribution (short-dashed line <NUM>), and the sum of all dark level contributions (solid line <NUM>).

<FIG> illustrates the effect of basic global dimming on Example <NUM>. In the particular illustration of <FIG>, the global dimming algorithm corresponds to an instantaneous method in which the source level directly tracks the peak level of the image for each frame. This may be referred to as instantaneous global dimming or "max peak" global dimming. The instantaneous method was chosen for ease of explanation. <FIG> illustrates the projector peak level and image peak level (solid line <NUM>), the APL (short-dashed line <NUM>), the room reflection / veiling glare dark level contribution (dotted line <NUM>), the image dark level contribution (dash-dot line <NUM>), the ambient dark level contribution (dash-dot-dot line <NUM>), the projector dark level contribution (dash-dash-dot line <NUM>), and the sum of all dark level contributions (dashed line <NUM>).

As can be seen from <FIG>, the projector system provides an image that is close to what could be obtained in that room for most frames. The projector dark level contribution <NUM> does not significantly contribute to the sum <NUM>. The room reflection / veiling glare dark level contribution <NUM> dominates the system for the most part. During frames <NUM>-<NUM>, the image dark level contribution <NUM> dominates. In this illustration, the global dimming algorithm provides a large amount of change to the projector peak level <NUM>. For example, the ratio of frame <NUM> to frame <NUM> is <NUM>: <NUM>. This may cause visible dark level pumping in the darker scenes.

For comparison, <FIG> illustrates the effect of using compensation for the room reflection / veiling glare, ambient, and image dark level contributions in the global dimming algorithm in Example <NUM>. In <FIG>, the projector peak level and image peak level (solid line <NUM>), the APL (short-dashed line <NUM>), the room reflection / veiling glare dark level contribution (dotted line <NUM>), the image dark level contribution (dash-dot line <NUM>), the ambient dark level contribution (dash-dot-dot line <NUM>), the projector dark level contribution (dash-dash-dot line <NUM>), and the sum of all dark level contributions (dashed line <NUM>) are shown. Comparing APL <NUM> of <FIG> to APL <NUM> of <FIG>, it can be seen that the results are substantially the same. That is, the peak levels of the image are approximately the same in <FIG> and <FIG>; however, in the case of <FIG> the projector did not implement as large a degree of source dimming to achieve the same goal. This may be referred to as the "least dimming" principle. The idea underlying the least dimming principle is to only dim the projector by the smallest amount in order to have substantially the same results as could be obtained by full dimming, taking into consideration the other sources of dark light on the screen.

<FIG> illustrate simplified versions of the dark level contributions, instantaneous global dimming, and compensated global dimming ("least dimming") for Example <NUM>, which is a comparatively-lower contrast (<NUM>) projector as compared to Example <NUM>. <FIG> may be considered simplified analogues to <FIG>, respectively.

<FIG> illustrates the desired dark level in the image (dash-dot line <NUM>), the actual dark level on the screen (dashed line <NUM>), and the ambient dark level (dash-dot-dot line <NUM>). In <FIG>, no global dimming is performed. <FIG> illustrates the desired dark level in the image (dash-dot line <NUM>), the actual dark level on the screen (dashed line <NUM>), the ambient dark level (dash-dot-dot line <NUM>), and the projector peak level (solid line <NUM>). In <FIG>, instantaneous global dimming is performed. <FIG> illustrates the desired dark level in the image (dash-dot line <NUM>), the actual dark level on the screen (dashed line <NUM>), the ambient dark level (dash-dot-dot line <NUM>), and the projector peak level (solid line <NUM>). In <FIG>, least global dimming is performed.

Collectively, <FIG> illustrate that least global dimming performs about as well as max peak global dimming, but may utilize less dimming of the source. With the lower contrast ratio projector (Example <NUM>), however, the least global dimming algorithm includes more modulation than with the higher contrast ratio projector (Example <NUM>). In situations where the room is very dark with high contrast content, and where the projector base contrast ratio is low, the least dimming algorithm becomes more and more like the max peak dimming algorithm; moreover, in situations where the room is not dark or the image content is low contrast with a high base contrast ratio projector, the least dimming algorithm stops modulating altogether, as it may not be needed.

<FIG> illustrates an exemplary projection system according to various aspects of the present disclosure. Specifically, <FIG> illustrates a projector <NUM> which includes a light source <NUM>, an optical modulator <NUM>, a controller <NUM> operatively connected to the light source <NUM> and the optical modulator <NUM>, and projection optics <NUM>. The projector <NUM> projects light toward a screen <NUM>. In practice, the projector <NUM> may include additional components such as a memory, input/output ports, communication circuitry, a power supply, and the like. Furthermore, the projector <NUM> may include additional optical components such as mirrors, lenses, waveguides, optical fibers, beam splitters, diffusers, additional spatial light modulators (SLMs), and the like. For ease of explanation, these additional components are not illustrated here.

The light source <NUM> may be, for example, a laser light source, a high-pressure discharge lamp, an LED, and the like. In some aspects of the present disclosure, the light source <NUM> may comprise multiple light sources <NUM>, each corresponding to a different wavelength or wavelength band. The light source <NUM> emits light in response to an image signal provided by the controller <NUM>. The controller <NUM> may be, for example, a processor such as a central processing unit (CPU) of the projector <NUM>. In one example, the optical modulator <NUM> may be an SLM, including a reflective SLM or a transmissive SLM. The optical modulator <NUM> may be a liquid-crystal-on-silicon (LCOS) SLM, a digital micromirror device (DMD), a light valve, and the like. The controller <NUM> also controls the optical modulator <NUM>, which receives light from the light source <NUM>. The optical modulator <NUM> imparts a spatially-varying modulation, such as a phase modulation, to the light, and redirects the modulated light toward the projection optics <NUM>. The projection optics <NUM> may include one or more lenses and/or other optical components, thereby to cause light from the light source <NUM> to form an image on the screen <NUM>.

In some aspects of the present disclosure, the projector <NUM> may include one or more sensors in order to determine various parameters (for example, characteristics relating to the room reflection, ambient light, and the like) in real time. In other aspects of the present disclosure, one or more sensor may be provided external to the projector <NUM>, and the projector may include components (for example, the input/output ports mentioned above) to receive parameter data from the sensor(s) in real time. A parameter sensed in real time may be stored in a memory, such as a RAM. Where a parameter is not sensed or determined in real time, the data may be manually entered at a time of calibration and stored in memory, such as a hard disk. Different parameters may be sensed or determined in different ways, such that some parameters are determined by internal sensors, others are determined by external sensors, and others still are manually entered at a time of calibration.

An exemplary projector display method is illustrated in <FIG>. The exemplary method may be performed or caused to be performed using a non-transitory computer-readable-medium storing instructions that, when executed by an electronic processor, cause one or more of the operations described in <FIG> to be performed. A non-transitory computer-readable-medium includes any element configured to temporarily, permanently, or semi-permanently store data; for example, a RAM, a hard disk, a removable storage device such as flash memory, an optical disk, and the like. The exemplary method may be performed in or by a projector display device, such as the projector <NUM> described above with regard to <FIG>.

At step <NUM>, the projector display device receives content data. The content data may be received from an external data source by, for example, a wired or wireless connection. The content data may also be received from an internal data source, such as an internal storage device or a removable storage medium. In some aspects of the present disclosure, the content data is received by a controller of the projector display device, such as the controller <NUM> described above with regard to <FIG>. After receiving the content data, at step <NUM>, the projector display device emits light in response to the content data. In one example, the controller <NUM> causes the light source <NUM> to emit light with a brightness that is determined by the content data.

At step <NUM>, the projector display device modulates the emitted light. In one example, the controller <NUM> causes the optical modulator <NUM> to perform a spatially varying modulation on light emitted by the light source <NUM>. At step <NUM>, the projector display device adjusts a light level thereof based on the content data and based on a metadata relating to a future frame. In this manner, the projection display device reduces a perceptibility of a visual artifact. For example, the controller <NUM> may receive the metadata from an internal or external data source and adjust the light source <NUM> accordingly.

The controller <NUM> may be configured to adjust the light source <NUM> in a number of ways. For example, the controller may adjust a light level emitted by the light source <NUM> by performing a partial dimming, by performing an SBO compensation, by compressing a dynamic range, and the like. In adjusting the light level, the controller <NUM> may perform a pulse skipping on the light source <NUM> itself, an amplitude modulation on the light source <NUM> itself, an attenuation on light emitted from the light source <NUM>, and the like. Where the light source <NUM> is made up of multiple individual light sources, the controller <NUM> may adjust light levels for the light sources collectively or independently. In some examples, steps <NUM>-<NUM> may be performed repeatedly on a frame-by-frame basis to thereby display a moving (video) image. The particular adjustment may be implemented using any one or more of the dimming techniques described above.

With regard to the processes, systems, methods, heuristics, etc. described herein, it should be understood that, although the steps of such processes, etc. have been described as occurring according to a certain ordered sequence, such processes could be practiced with the described steps performed in an order other than the order described herein. It further should be understood that certain steps could be performed simultaneously, that other steps could be added, or that certain steps described herein could be omitted. In other words, the descriptions of processes herein are provided for the purpose of illustrating certain aspects of the present disclosure, and should in no way be construed so as to limit the claims.

Claim 1:
A projection display system, comprising:
a light source configured to emit a light in response to a content data;
an optical modulator configured to modulate the light; and
a controller configured to adjust a light level emitted by the light source of the projection display system based on the content data and a metadata relating to a future frame, thereby to reduce a perceptibility of a visual artifact, wherein the content data includes an image data indicating the peak brightness level in an image in a current frame and an ambient condition data,
characterized in that
the metadata relating to a future frame comprises information about the peak brightness level of said future frame, and wherein adjusting the light level includes adjusting a rate-of-change of the brightness of the light source depending on the ambient condition data.