Digital fulldome test pattern

Written by Paul Bourke
June 2018

v09, 25 June 2018. First release.
v10, 30 June 2018. Added star field, white patch, and bars near pole.
v11, 02 July 2018. Added identifying text at pole.


Introduction

In the digital fulldome world, for example planetariums, there are a number of possible data projection options. Ranked roughly in order of potential resolution these range from single projector and spherical mirror, single projector and fisheye lenses, dual projectors and finally multiple (typically 5 or more) projectors. While single projector systems are simpler, their quality can vary depending on the quality of the first surface mirror (for spherical mirror systems) and the quality of the lens optics in the case of a single fisheye. For multiple projector systems the situation is more complex and since one is generally investing more in such systems the resolution is more important. One cannot simply assume that more projectors are better, for example, will more lower resolution projectors result in a higher resolution result than say a system with fewer higher resolution projectors? To what degree does the optical quality of the lens affect the final dome resolution? What role does the different projection technologies affect the actual result on the dome?

The following attempts to provide a method of estimating the true resolution of a particular digital fulldome system. "True resolution" is taken to mean what the projection system is actually resolving on the dome surface rather than any theoretical resolution. Specifically, at what point does a higher resolution input image not deliver any visual improvement, or indeed can lead to a reduction in quality. There are a number of reason why this is important, some are listed below.

  • The resolution claims of many fulldome suppliers are often more about marketing than based upon anything in reality.

  • The approach presented here can serve to provide a single method that can be used to compare different systems rather than trying to assess the quality using different images supplied by each manufacturer/installer.

  • There can be many parts to the projection pipeline, each can affect the final quality. Employing a standard test pattern allows one to adjust the quality of each stage and see which is the limiting factor. For example, many systems (unfortunately) employ a highly lossy movie encoding codecs; this can be the limiting factor rather than the number of projectors or the resolution of still uncompressed images. [See comment below about creating a movie based upon this test pattern].

  • Movie playback on fulldome systems is often challenging in terms of data transfer from disk and/or decoding requirements. Knowing the maximum resolving capabilities of a dome allows one to choose an appropriate movie resolution. Playing movies that are a higher resolution than the dome projection can resolve is a waste of resources.

  • If you have convinced your funding source to invest in a dome resolution of X then after it is installed you don't want to find out that it is not actually that resolution, or only by a definition that may not stand up to close inspection.

Unfortunately there are not well defined metrics for measuring the actual dome resolution. As such the various dome installation companies have invented their own measures, as is the way with commercial rivals these measures are generally invented to suit the company using them. In particular terms such as "4K", "8K", "True8K" have ended up not meaning much at all but largely just serve as marketing terms. For single projection systems the underlying resolution of the projector does not necessarily match the final perceived resolution on the dome, for example, different lenses have various degrees of chromatic error. There are a number of possible definitions:

  • The worst, but not uncommon, is to imagine that because the movie playback software supports 4K fisheyes then the system is representing that on the dome.

  • Counting the total number of pixels on the dome. So, for example, a dome might be rated as having a resolution of 4K if it places 12.5 million pixels on the dome (pi r^2 where r = 2K).

  • The quite reasonable definition proposed by Evans and Sutherland for True8K is that there are 8K pixels along any half great circle curve on the dome, also known as a meridian.

By themselves none of these can be used to accurately determine what the projection system can actually resolve. There are realities inherent in all multiple projector systems on the market at the moment that makes a theoretical evaluation problematic. These are:

  • Limitations in the optics of the lenses employed. Not all pixels are necessarily of equal quality. Some may be less in focus than others; some may have more chromatic error than others.

  • Not all pixels are necessarily the same size. Pixels directly opposite projectors tend to be a different size than pixels at wider angles.

  • Not all pixels are necessarily the same aspect ratio. Pixels directly opposite projectors tend to be square but those at wider angles less so.

  • And the big one, how does one judge or count pixel resolution across blend zones. Blend zones are notoriously imperfect even after relatively short periods following the last warp/blend calibration.

  • Compression degradation. Most movie playback systems on the market employ some form of data compression. Since this is designed to reduce the data reading and decompression the result is a degraded version of the input fisheye images sequence. This is particularly problematic for mpeg style codecs, but also for the increasingly more popular HAP codec. So, even before the image hits the projectors it is an inferior version of the original.

The approach proposed here is a practical one. A metric for dome quality (resolution) needs to be one that reflects the actual image quality on the dome, sounds obvious but the definitions above do not generally measure that. A better metric for describing the resolving power of a dome is to determine the point at which increasing the resolution of the input image does not improve the perceived quality. The test is easy and similar to the way one determines the actual resolution of lenses for example. This is to photograph, or broadcast, a known image (a test pattern) that has been designed to reveal the resolving limits. The test pattern presented and adapted here for fulldome contains similar elements used to determine the resolution for broadcast or lenses. One defines this test fisheye pattern at "infinite" resolution (continuous not discrete) and one renders that at various discrete resolutions, presenting each on the dome. The true resolution of the dome is defined as the point at which increasing the test pattern resolution does not improve the result on the dome. An important feature of the test pattern will be features at known spatial resolutions; a claimed dome resolution above those spatial features should be able to resolve them.


Test pattern

Downloads
1024 (210)
1080 (eg: 1080 pixel projector)
1200 (eg: 1200 pixel projector)
1536
1600 (eg: 1600 pixel projector)
2048 (211)
2160 (eg: 2160 pixel projector)
2400 (eg: 2400 pixel projector)
2560
3072
3200
3584
4096 (212)
4320 (eg: 4320 pixel projector)
5120
6144
7168
8192 (213)
9216
10240

License

Image may be freely used under the condition it is not modified in any way.
The image may be freely distributed as long as this license statement is included.


How to use this test pattern

The following describes the features of the image and how it might be used.

  • The text characters in all the images irrespective of the resolution are 12 pixels wide. Pixelated for sure but easily enough pixels to represent numbers. On a projection system capable of resolving a particular resolution the numbers should be clearly discernible, good choices for inspection would be "0", "3" and "8".

  • In these images, irrespective of the resolution, the lines making up the polar grid are two pixels wide plus any antialiasing. These white lines are useful for picking up chromatic errors, which are likely to be most obvious towards the extreme angles of any lens, especially wide angle lenses. Noting that chromatic errors are not always due to lenses, some projector technologies can also generate chromatic-like effects and the higher end models should allow that to be corrected.

  • In the top right and bottom left quadrants are variously spaced lines (vertical and horizontal) as well as variously spaced rings and radial lines. The white and dark zones should be resolvable by a projection system of the corresponding (or greater) resolution. In particular the finest rings give the best indication of the resolving power. If these structures blend into a solid grey that is an indication that the resolving capabilities are lower than the test pattern resolution being used. The relative black to white widths are the same across all image resolutions, for example the left of the image below is from the 2K test pattern and the right is from the 4K test pattern.

  • The points included might be used to evaluate star quality are progressively 1 pixel wide, 2 pixels wide, 3 pixels wide and 4 pixels wide.

  • The solid inset colour swatches are 50% and 100% saturation. The grey scale inset swatches are 25%, 50%, 75% and 100% saturation.

  • The colour aspects of this test pattern can be more difficult to evaluate. One aspect they are intended to highlight is colour banding due to a reduced colour range in the projector or in the compression codec. The ramp from black to primary or secondary colour and then to white in the top left and bottom right occurs in 255 steps, as much as 8bit colour can support.

  • The test image as it stands is suited to estimating the quality of realtime computer generated graphics. If it is to be used to estimate movie playback quality then it needs to be encoded into a movie. Only then will some compression artifacts be observed. The movie should consist of this image slowely rotating about the north pole, that is, this image rotated about the center as each frame of the movie. Note that performing this rotation "live" (if supported) is NOT necessarily the same as actually encoding it into a movie. The rotation should be slow in order to pick up aliasing and blending errors. Shimmering of lines is the classic indication of aliasing that occurs when a high resolution image is downscaled to a lower resolution one.

  • The images have been created with a gamma of 2.0 and a colour profile: sRGB IEC61966-2.1. Any excessive amount of black or white in the greyscale (or colour) ramps would indicate the gamma is not being preserved during the image/movie encoding or playback process.

  • While not the main purpose of this document, the test pattern is an excellent way of identifying any non-smooth playback. The author recommends a 360 rotation in 1 minute, 1800 frames for a 30fps playback system and 3600 frames for a 60fps playback system.


Notes

  • The use of "K" to represent 1024 rather than 1000 is due to historical reasons within the digital fulldome movie industry. These stem from the fact that for computer playback frames that are a power of 2 in dimension can more readily take advantages of GPU hardware. Although this is increasingly becoming less of a consideration.

  • Overlooked in this discussion is dynamic range and colour depth. These are more difficult to quantify through observation although the colour ramps are an attempt to do that in a simplistic way. They are also confounded by many lossy compression codecs many/most of which reduce colour depth as well as spatial resolution.

  • The author can supply the slow rotating movie frames referred to above at a range of requested resolutions but this would involve a fee for a USB drive and postage. Noting that for a correct test each frame should be rendered from the infinite version of this test pattern rather than simply rotating the representative image provided. Please contact the author by email for more information.


Other backgrounds

Note these have constant real-world width of elements, unlike the test pattern above which has pixel scale elements.

2048 x 2048

4096 x 4096

8192 x 8192

2048 x 2048

4096 x 4096

8192 x 8192

2048 x 2048

4096 x 4096

8192 x 8192

2048 x 2048

4096 x 4096

8192 x 8192