This is a book about the visual perception of all the things we encounter in our daily lives. To that end, it is a book about everything you already know first hand, such as the beautiful glow of a baby's skin, the lush green of a well-watered lawn or the shimmering glean of your favourite drink in a frosty glass. In casting light on these things you already know, it’s also a book about everything you’ve likely never even thought about: namely, how does the brain make sense of all the nuanced variations in the patterns of light reaching our eyes in a way that allows us to even speak of these everyday things. It is thus a book about how our brains reduce the immeasurable complexity of these light patterns down into the consumable chunks of knowledge that we call everyday things. In short, this is a book about the miracle of visual perception.
Another source of light reaching our eyes is literally sources of light. Light sources are said to emit light, and depending on the nature of the source, this light can be emitted in all directions or in specific directions. The sun emits light in all directions, for example, all though our eye can only sample light reaching it along direct lines of sight. A torch, by comparison, tends to emit light along a focused path that we call the beam. The problem of how our visual system distinguishes between light reflected from surfaces and emitted light will also become a key consideration in this book.
The final major source of light variation that we will consider is light that passes through objects, either entirely or partially. We say that objects are transparent if they transmit light through their surfaces, and we say objects are translucent if light only partially penetrates the object before being emitted at some other point of the surface. This latter effect is called sub-surface scattering and it is a physical phenomenon that lies somewhere between light reflection and emission (Figure 3).
Thus, we arrive at the four major sources of variation in sensed light that inform us of the visual world: diffuse/specular light reflection and light emission/scattering.
One of the most important & highly cited papers in all of computer graphics, "Poisson image editing" describes a gradient-based integral system for performing all sorts of image manipulations, including seamless cloning, where target objects are copy/pasted into a scene. This method underlies the cloning tool familiar to Photoshop users.
I conjecture that a similar gradient-based integral system underlies many computational properties of human vision, including color filling defined over segmented, isolated domains.
Studies have shown that displacing specular highlights from their natural locations in images reduces perceived surface gloss. Here, we assessed the extent to which perceived gloss depends on congruence in the position and orientation of specular highlights relative to surface shape and the diffuse shading from which surface shape is recovered. The position and orientation congruence of specular highlights with diffuse shading was altered while preserving their compatibility with physical surface shape (Experiment 1). We found that perceived gloss diminished as the position of highlights became incompatible wit h the surface's global diffuse shading maxima. These results suggest the visual system assesses both position and orientation congruence between specular highlights and diffuse shading to estimate surface gloss.
@TonyVladusich yeah, it changes from gloss to a bright light shining on it... like a rectangular hole in the roof shining sunlight down onto the object.
Translucency perception is another fascinating topic with applications in computer graphics. Here's a very nice recent review of the topic:
Translucency is an optical and a perceptual phenomenon that characterizes subsurface light transport through objects and materials. Translucency as an optical property of a material relates to the radiative transfer inside and through this medium, and translucency as a perceptual phenomenon describes the visual sensation experienced by humans when observing a given material under given conditions. The knowledge about the visual mechanisms of the translucency perception remains limited. Accurate prediction of the appearance of the translucent objects can have a significant commercial impact in the fields such as three-dimensional printing. However, little is known how the optical properties of a material relate to a perception evoked in humans. This article overviews the knowledge status about the visual perception of translucency and highlights the applications of the translucency perception research. Furthermore, this review summarizes current knowledge gaps, fundamental challenges and existing ambiguities with a goal to facilitate translucency perception research in the future.
I just remembered a dream I had last night about @huwr. You rocked up to my house in a van and I hopped in (innocent that I am). I then poked you on the arm and asked “Are you real?”, and you replied “Of course”. I then promptly woke up and said to myself “liar”.
@TonyVladusich that wasn’t a dream. We went to get ice cream and you got sprinkles but you dropped your ice cream and cried so we had to get more. Great day
Me: I’m cooking pizza
Wife: can u pls put vegetables on it, like mushrooms?
Me: what you don’t think I know what a vegetable is?
Wife: …
Me: fair point
Intrinsic image decomposition. The "inverse shading" approach is similar to the blackness representation in gamut relativity and the Colors app, being a log inverted & anchored map from the luminance domain. Very interesting. cc @troy_s https://dl.acm.org/doi/10.1145/3630750