Gaps in Color Constancy

1. Gaps in Color Constancy Poster B 39 at the VSS 2004 Sarasota meeting Adam Reeves Northeastern University, Boston MA Kinjiro Amano, David Foster UMIST, Manchester, UK RKFvss04.ppt

2. Color Constancy (CC) in brief displays In viewing natural scenes, where objects are known, and one is adapted to the prevailing illuminant, CC is fairly good. When surface colors are re-arranged to form abstract patterns, as in Mondrians, and presented too briefly for light adaptation, • Observers can accurately distinguish the cause of a spectral change,. i.e. a change in material or in illumination (Foster & Nasciamento, P. R. Soc. B, 1994), and do so rapidly, effortlessly, and in parallel (Foster et al, PNAS). • But, CC is poor (40%) when matching material ('paper match'), and • CC is nearly absent (16%) with direct (hue/sat) matches (Arend at al, JOSA 1991). Note: Good CC matches (84%) occur with >10 secs light adaptation (Brainard, JOSA, ‘98) To study this task effect further, observers RATED the quality of simulated illumination and material changes. • Ratings were used to extend the binary choice (material vs. illum.) of Foster et al ‘94). Mondrians were shown with gaps or without gaps: • No-gap: the 49 Mondrian papers abutted; the Mondrians followed on with no delay • Gap: black (5’ arc) lines separated the 49 papers; delay was 0.2 sec. • The spatial gaps break the papers up into local apertures, somewhat like a stained glass window. The temporal gap further separates the two sets of papers.

3. Method Pairs of Mondrians subtending 5.5 deg were shown in succession, each for 1 sec, in darkness, on a calibrated monitor. We simulated 49 different Munsell papers, chosen at random, on each trial. The ‘local’ illuminant (that on the central patch) was offset at random by one of 9 steps along the daylight locus. Global Illuminants were either 1600 K (1st) and 6700 K (2nd), or 4000 K (1st Mondrian), 6700 K (2nd). First (16 or 4K) Second (6.7K)

4. * Examples of illumination and (a subtle) material change • Original illumination illumination+material Observers and Training 14 Observers with normal color vision (Ishihara, RG and YB anomoloscopy). • All 14 were trained to distinguish illumination and reflectance using verbal instruction and example Mondrians. • 11 were also shown demonstrations of material and illumination changes in Portuguese outdoor scenes *. • 7 also saw A and C illuminations of Farnsworth paint-chip box #3. • All explained back what they had learnt. All but 2 understood well. • The level of training was categorized as 1 (low), 2 (med), or 3 (high).

5. Ratings of Quality Observers rated the quality of the simulation of the central patch, from 0 to 100%, either in hue-saturation (as in a ‘direct’ match), or in material (as in a ‘paper’ match). They were told the simulation might be good or poor -- their job was just to rate how good it seemed. They also rated the quality of the illuminant shift between the first and second Mondrians.

6. Observer differences: HSD, LSD Some observers distinguished hue-saturation from material changes, but others seemed unable to do so. To capture this individual difference, observers were split into two equal groups using the standard deviation (SD) of the differences between their hue-saturation and material ratings. High SD observers (HSD) were better than average in distinguishing hue-saturation from material; LSD observers were worse than average. This split is independent of the observers’ color constancy.

7. Results: 16K, LSD

8. Results: 16K: HSD

9. Results: 4K, LSD

10. Results: 4K, HSD

11. color constancy index CCI = 1 - (b-c) / (a-b) • b is the u’ of the central patch in the second Mondrian, • a is the u’ of the central patch in the first Mondrian. • c is the u’ at the peak of the rating function, and • CCI=1 for perfect color constancy and CC=0 for none (Arend et al, 1991). MATERIAL HUE-SAT Low SD Observers (poor task discrimination) High SD Observers

12. Conclusions • Color constancy is task-dependent, even when observers cannot adjust to match (as in Arend & Reeves) but must rate the stimuli. • Training may matter; the 7 HSD observers’ mean training level was 2.5, that of the 7 LSD observers, 1.8. • LSD: mean CC was 0.50 for both tasks • HSD: mean CC was 0.83 in material, 0.33 in hue-sat. • CC is higher in rating (here) than in matching (Arend et al, JOSA 1991) • 66% rating vs. 40% matching, in the material task; • 42% rating vs. 16% matching, in the hue-sat task. • CCI was not affected by the gaps (mean 54% with and without gaps), even though the spatial gaps appeared to break up the display. • So if the signals which support rapid color constancy are changes in local edge-ratios at the cone level (Foster & Nasciamento, Proc. R. Soc. B, 1994), these ratios depend on encodings which can span across clearly visible gaps.