S22 Advanced: Black Level Adjustment
Projectors tend to leak a tiny bit of light, when displaying a black image. This tiny bit of light becomes very noticeable, in the areas where the projectors overlap.
To hide this noticeable light leak in the projector overlap areas, then it’s needed to raise the light level, in the areas where the projectors don’t overlap.
When setting up the black level adjustment, it is critical that the Warping and Blending has been correctly set, and the projectors have been turned on for at least 30mins.
This warm up period is critical, as both the projector’s mechanical and electrical parameters will change during this warm up period.
Some of the issues you might encounter are:
Misaligned projector overlap areas – due to the mechanical properties of the projector optics.
Misaligned black level values, as the electrical properties of the lamp, DLP or SXRD chips changes.
Pre black level adjustment checklist
Make sure all the projectors are at normal operating temperature
Make sure all projectors are outputting the same luminance, using a bright image
Make sure, all Warp and Blend adjustments are done properly
A: Setup the projector overlap areas
Go to the menu point: Config →Black level adjustment → Black level settings and enable the Black level regioning mode
As using the normal Warp controls, then mark the areas, where the projectors do not overlap.
B: Setup the black level blending
While the Black level regioning mode is enabled, then you can configure the blending, using the normal blending tools. When using the Blending – then by using a small overlap, then it’s possible to hide/smear out the sharp edge between the non-overlapped, and the overlapped area. This is achieved by defining the non-overlapped area a few pixels into the projector overlap area. If the blend area is too wide, then this can cause some unwanted artefacts - within the blend area.
C: Export black level masks
When done, then go to Config → Black level adjustment and select Export Black level masks. When selected then the Warper will export a set of black level masks, used for the black level adjustment.
D: Setup the black level
Go to the menu point: Config →Black level adjustment → Black level settings and enable the Black level regioning mode
Using the Black level adjustment, then turn up the black level, to match the centre of the overlapped area. One thing that might be a bit tricky setting the correct black level – is due to the non white leakage from the projectors. Using the R, G and B trim – then you can adjust the color nuance for the black level uplift. This RGB trim – is only affecting the black level uplift, and therefore not changing the color temperature, when the projector is outputting white.
Depending on the projector setup used in the installation, then you probably need some more black level trimming of the corners of the area towards the overlapped area. This can be done with the Upper left, Lower left, Upper right, Lower right and Right centre compensation. Adjust these values to get the most uniform black uplift, and color.
E: Intersection trim
The Intersection, is the area, where the projector overlap. On a projector then the projected luminance is non linear from black to approx 4 times the black level. To compensate for this non-linearity, then we need to do a few adjustments, to make a gradient look as smooth as possible.
Firstly, make sure: Config →Black level adjustment → Black level settings and enable the Black level regioning mode is now turned off.
Load the image Gradient – Test in the Warper.
Set the Blacklevel width to 0.1
By adjusting the Blacklevel threshold and the Blacklevel width, then make the adjustments to make the gradient look as close, to the gradient displayed, within the non overlapped areas.
F: Intersection black cutoff
If you are encountering some artefacts in the near black area, then you can use the black cutoff – for removing the near black gradient, that cause the visible artefacts. This should be seen as a last resort quick fix, for in situations, where it’s not possible to re-align the black level compensations.
Additional Notes
Configure the Black Level uplift parameters
Go to the menu point: Config →Black level adjustment → Black level settings
Using the Black level adjustment, then turn up the black level to match the centre of the overlapped area.
Depending on the projector setup used in the installation, you maybe need to adjust the amount of uplift by different amounts to match the corners of the overlapped area. This can be done with the Upper left, Lower left, Upper right, Lower right and Right centre compensation. Adjust these values to get the most uniform black uplift.
Configure the Black Level rolloff parameters
At higher luminance values, the adjustment at the non-overlapped regions can overshoot their luminance artificially as a result of the Black level adjustment at low luminance values. The parameters Blacklevel Threshold and Blacklevel Width correct the Black level adjustment for higher luminance values by gradually scaling its value to zero after a threshold luminance value.
The parameter Blacklevel Threshold controls the luminance value after which the Black level adjustment decays to zero. The parameter Blacklevel Width controls how fast the adjustment falls off to zero.
Figure 1 illustrates the shape of the Blacklevel rolloff as a function of image luminance. The x-axis represents the image luminance and the y-axis represents the scaling of the Black level adjustment. The Blacklevel Threshold value is 0.5 and the Blacklevel Width is 0.1. Figure 2 shows the effect of changing the Blacklevel Threshold value to a lower value 0.2.
Advanced mode
By checking the Show advanced the advanced black level settings will be shown.
Easy Mode by checking this checkbox, then some of the more advanced features will be turned off.
Blacklevel intersection trim is used together with the Black Gradient for adjusting the intersections, when not using a gradient.
Monochrome when checked, then the output will be black and white only - this is useful, when adjusting the RGB low level trim.
Influence map The influence map, is used to reduce the blacklevel adjustment, nearby bright pixels within the output. This reduces the artefacts introduced by the blacklevel uplift.
Blur width is how many pixels nearby, that should be affected by a bright pixel in the output. Please note: set the Blur width as small as possible, as this parameter will affect the GPU performance.
Blur mix the amount of blur vs the high level pixels in the image
Multiplier amplifies the influence map, so the slightly lower level pixels will make an higher influence on the black level compensation.
Display influence map displays the influence map on the output
Bypass influence map Disables the influence map
Display blacklevel makeup displays the actual black level appended to the output
Boost output level when set, then the output will be applied heavy amplification. This can be useful when tracking down artefacts in the output
Blackout by checking the checkbox, then the projector output will be black only, and no black uplift, etc will be added to the projector output.
Show projected area by checking this checkbox, then the full projector output will be dimmed blue.
Show overlap by clicking this checkbox, then the projector will output red only – within the defined overlap region.
Blend If this checkbox is unchecked, then the black level blending will be disabled, for the projector.
Left blending trim if needed, then the left hand side of the black level blending can be trimmed.
Right blending trim if needed, then the right hand side of the black level blending can be trimmed.
2 Theory behind the black level adjustment
2.1 Black level adjustment in non overlapped areas
The normal black level adjustment is done by adding an offset to the pixel values. By adding the offset, then the normal black [0,0,0] will then be [blackLevel,blackLevel,blackLevel]. When the offset is added to the current pixel values, then white [1,1,1] also need to have the black level offset added. So to avoid the pixel values to go above 1.0, then we need to scale the pixel range [0.0;1.0] accordingly.
So for a given color, then the black level is done in this manner:
colorOut = color*(1-blackLevel) + blackLevel
2.2 Black level adjustment in overlapped areas
Within the area, where the projectors overlap, then black [0,0,0] equals our black level - so for black we don’t apply the black level, and neither any other compensation.
When the projectors starts to display a tiny dimmed gradient, then this will result in a small luminance bump, and the linearity for the projectors, within the lower gradient range, is not linear - compared to the non overlapped areas, with the black level applied. This troublesome area will be present within the range from near black to 4 times the black level, where the difference is too small to be practically visible.
To compensate for the luminance bump, then we will be applying a non linear Sigmoid function. To compensate for the non-linear from near black, to 4 times the black level, then we are using a Sigmoid function. The output from this function can be set by a Blacklevel threshold and a Blacklevel width. For the overlapped area, then we still need to apply the pixel scaling range - to match the non-overlapped areas. The Blacklevel threshold is the luminance level, where the Sigmoid non-linear function starts, and the Blacklevel width defines the width/range for the non-linear transaction for the Sigmoid function.
2.3 Edge blending areas
The edge blending area is the area, where content projected on two projectors, will be faded together by a gradient, defined by the mask, for a given pixel. During the setup and configuration of the black level compensation, then a mask/image containing the information about the edge blending, is generated for each projector.
Within the edge blending area we won’t apply the black level offset to both projectors at the same time, as with active content, this will give us two times the black level offset.
As the mask will give us a value of 1.0 for non blending areas, and values lower than 1.0 for the blending area, then we will be multiplying the black level offset to the mask value - as this will equal a summed black offset from both projectors.
So for a given color, then the added black level offset will be like this:
colorOut = color*(1-blackLevel) + blackLevel*mask
(c) Igloo Vision Ltd. 2023