Visual Perception Experiments
CE16 SAPC-6 — Algorithm Evaluation
and are intended for desktop viewing on a standard sRGB monitor.
WAI Contrast Research
"Constant Background" Series, October 2019
These are tests of the contrast measurements by the SAPC algorithm(s). These test plots are for normal polarity (dark text on a light background, aka BoW).
The series on this page is "Constant Background" with the background constant for each panel, and the contrast changing in increments of 4% for each test patch.
See this page for Constant Contrast, where both the background and text change to test uniform tracking of perceptual contrast.
Jump To Test Panel:
You can also jump to or link to any particular patch by the serial number, i.e. add a hash (#) and the serial to the URL to jump to a particular test patch. For instance, to go to test patch number CE16_CC_CBNP_18, then in the location bar add #CE16_CC_CBNP_18 to the end of the URL (click to see).
NOTES, TERMS, and “What’s an SAPC Anyway?”
CONTRAST: As used on this page, mainly it means the perceived difference between two colors displayed on a computer monitor. However, it's useful to point out that “Contrast” can be used to describe the differences of other perceptions: Contrast of size, contrast of position, contrast of speed, etc.
As it happens, perception is very much a mental attribute. That is, it happens in our brain. We perceive things a certain way, but what we perceive is essentially representational and not an absolute quantity of what we're seeing, feeling, or hearing. The important understanding here, is that what we perceive is subject to our neurological system's interpretation of the physical stimuli. This is where the term “psychophysics” comes from. Partly to do with stimulus in the physical world, and partly to do with how our brain interprets it.
For example, the color "purple" is not real (purple is the perception we have when the red and blue cones in our eye are stimulated without stimulating green). Can you see the orange background on this screen being displayed on your computer monitor? Well it's not orange at all. It's separate red and green, stimulating the red and green receptors in your eye, in such a way that your eye/brain visual system interprets it—perceives it—as orange.
So this study of contrast, as it applies to colors that you might want to use a web design, is not just about the difference between two colors as some might think. It's about all the factors that we need to take into account for how we perceive the difference between two colors. And that includes things like spatial frequency, a practical application of which relates directly to the stroke width of a font.
BoW and/or NORMAL mode: means dark text on a light background (normal mode). Confusingly some researches call this negative contrast, other call it positive display. I'm pushing to move toward “NORMAL” to describe black print on white (or maybe "paper mode") and then “REVERSE” to describe light text on a dark BG, which is how people often describe that.
WoB and/or REVERSE mode: means light text on a dark background (reverse mode).
WCAG: Web Content Accessibility Guidelines. In the context used here, I refer to the math used for the 1.4.3/1.4.6 contrast assessments, which is essentially just a simple ratio of luminance between two sRGB colors with a small offset.
SAPC: The “APC” stands for “Advanced Perceptual Contrast”, the “S” is either for “sRGB” or “Somers” the inventor (eh, that would me me) ... it’s SAPC either way :) ... This is Major Iteration 6. The companion project is S-LAB, an L*a*b* type colorspace designed specifically for accessibility and emissive display uses.
Evaluations/discussions based on observations made by a trained observer using a common calibrated sRGB laptop display. More elaborate evaluations using a larger sample size of subjects and display types is forthcoming.