Mars Rover Colour Vision: Generating the true colours of Mars. Dave Barnes Head of Space Robotics Aberystwyth University, Wales, UK

Transcription

1 Mars Rover Colour Vision: Generating the true colours of Mars Dave Barnes Head of Space Robotics Aberystwyth University, Wales, UK 1

2 Contents Previous work RCIPP The Mars White Point Problem Judd and the CIE Standard illuminants Creating Mars standard illuminants Application of Mars illuminants: Creating Mars natural-colour images Chromatic adaptation examples 2

3 Baseline Radiometric and Colourimetric Image Processing Pipeline (RCIPP) Developed raw image data Rover with PanCam and PCT Presented at ASTRA 2011 Surface Operations compress Mathcad Java LabVIEW ESA PSA PDS format flats/darks (temp,exp) PCT BRDF data image radiometric and colourimetric processing uncompress flat/dark correction camera response correction relative reflectance CIE XYZ CIE XYZ to srgb etc PanCam image SOC Operations camera radiometric correction pixel spectral radiance (W sr -1 m -2 nm -1 ) R* data CIE XYZ data ESA PSA PDS format reflectance ROI select PanCam spectrum 3

4 Generating Rock Spectra AMASE 2009, BOCK01 Site using calibration test target (Arctic Mars Analogue Svalbard Expedition AMASE) 4

5 440 nm 560 nm Clarach Bay 2010 Image Natural Colour Correction 470 nm 600 nm 510 nm 660 nm Narrow-band (10 nm) filtered input images Output colour corrected image in srgb format 5

6 The Mars White Point Problem Surface colour is dependent upon both surface reflectance/absorption properties AND the surface illumination. Getting the surface solar spectrum irradiance correct is vital for planetary colourimetry. Associated with a solar spectral power distribution (SPD) is its correlated colour temperature (CCT) which determines the reference white point. If the CCT is wrong, then the white point will be wrong, and hence the colours will be wrong. We need to be able to compute typical Mars solar SPDs based upon actual or desired CCT. We need Mars (standard) illuminants 6

7 The Problems of Mars Surface Solar Irradiance Spectra for Radiometric and Colourimetric Image Processing Typically past missions have applied a number of fixes to overcome this problem. For example: Use AM0 solar spectrum and attenuate to mean Mars heliocentric distance, then add attenuation factors to represent Mars atmospheric effects not a good solution! For Viking Landers a terrestrial standard illuminant C (6774K) was used no wonder Viking Landers had colour problems! MER processing has employed a constant correction using average bright regions on Mars and referenced Maki 1999 Pathfinder IMP paper. These average bright regions are not necessarily typical and no spectra details are given. Reference white point unknown. As all science target reflectance spectra and natural-colour image products are entirely dependent upon surface solar irradiance spectra, we need a better solution what can terrestrial solutions tell us? 7

8 References Infinite number of daylight illumination conditions. The use of standards revolutionised colourimetry Image courtesy W. C. Lemons Example Standard CIE illuminants Can the same be done for Mars? (CIE International Commission on Illumination) 8

9 Judd et al 1964 chromaticity of daylight compared to the Planckian Locus 622 daylight solar spectra measured at different times of day, different seasons, different locations, different weather conditions All spectra processed using PCA to find characteristic vectors Generated typical daylight solar spectra as a function of correlated colour temperature, e.g.d65 (6500K), D75 (7500K) etc. 9

10 The Correlated Colour Temperature (CCT) determines the reference white point 1. (Between 4000K and 7000K) (Above 7000K) CIE (Judd) Formulae Mean SPD (S 0 ) and two characteristic vectors S 1 and S 2 computed from all samples 4. Resultant daylight illuminant 10

11 Components of Mars Sol-light Distribution Irradiance Obtain Mars surface solar spectra by processing in situ calibration targets. Currently working on Mars Pathfinder, and Phoenix. Include MSL when available. Left Right 1. Sample target (W & LG) 2. Determine spectral irradiance from radiance and prelaunch reflectance 3. Fit cubic-spline 4. Normalise to 560nm 5. Repeat for 1 st 50 sols of data for all missions Example spectra Example 560 nm Normalised Mars Solar Spectra Data Currently only 50 spectra obtained Wavenumber Simonds Characteristics Vectors for Mars Sol-light CV1 CV2 CV3 CV4 Zero Apply PCA to derive Characteristic Vectors MER Example Wavelength 11

12 CIE y Obtained Mars surface solar spectra converted to CIE xy coordinates and plotted on chromaticity diagram ALL Mars solar spectra chromaticity versus Planckian locus and Chromaticity Loci 0.38 Green side Purple side 4200 K 4800 K 5500K 6500 K Judd Chroma Locus Planckian Locus Mars Chroma Locus IsoT-42 IsoT-48 IsoT55 IsoT-65 IsoT-75 Illuminant E Spirit C. Temp. Opp C. Temp. Phx C. Temp CIE x Currently 50 Mars solar SPDs obtained representing: Different sols, Different times, Different locations, Different atmospheric and dust conditions Note Spirit/Opportunity difference 12

13 Relative Spectral Irradiance (no units) 1. and 2. Using Mars chromaticity data, x m and y m formulae derived for specified CCT Example x m and y m results Using first 3 characteristic vectors (x m + y m + z m = 1) 4. D65 M46 M48 M50 M52 M54 E 3. M 1, M 2, and M 3 formulae derived incorporating x m, y m and z m Example M1, M2, and M3 results Wavelength (nm) Example derived Mars sol-light illuminants E m ( ) 13

14 Mars illuminant application Use CIE reflective case: Mars illuminant included Example of natural-colour Mars image: Heat Shield Rock processed with illuminant M54 14

15 Relative SPD (no units) Because we know the illuminant we can use chromatic adaptation: The scene illumination can be transformed to a different reference white point 2 Relative SPD for Illuminant Illuminant_Discrete Illuminant_Continuous Illuminant_E Wavelength (nm) Mars under illuminant M48 15

16 Relative SPD (no units) Illuminant M48 to Illuminant D65 Chromatic Adaptation 2 1 Relative SPD for Illuminant Illuminant_Discrete Illuminant_Continuous Illuminant_E Wavelength (nm) Mars under illuminant D65: Noon Daylight Mars terrain on Earth! 16

17 Relative SPD (no units) Illuminant M48 to Illuminant F2 Chromatic Adaptation 2 1 Relative SPD for Illuminant Illuminant_Discrete Illuminant_Continuous Illuminant_E Wavelength (nm) Mars under illuminant F2: Cool White Fluorescent 17

18 Relative SPD (no units) Illuminant M48 to Illuminant A Chromatic Adaptation 2 1 Relative SPD for Illuminant Illuminant_Discrete Illuminant_Continuous Illuminant_E Wavelength (nm) Mars under illuminant A: Incandescent / Tungsten 18

19 Conclusion The CIE Standard illuminants work has been applied to Mars The process and required formulae have been produced to create Mars standard illuminants Mars illuminants have been applied to: Creating Mars natural-colour images Chromatic adaptation Only 50 Mars solar SPDs have been obtained so far, but the work is ongoing to increase this number to several hundred (with MPF, PHX and MSL data when available) Full characteristic vector data and formulae will be published in the future literature: Barnes D.P. Spectral distribution of typical Mars sol-light as a function of correlated colour temperature and its application to Mars colourimetry. In preparation. 19