First, let’s think about what images are.

As photographers and artists we are image makers. We can make flat, two dimensional images that represent three dimensional scenes in the real world.

Another way of representing a three dimensional scene is to sculpt it; and in fact life-size realistic sculpture was one of the first expressions of thoroughgoing representational realism in Western art.

Forget colour, for now

Colour images have fundamental underlying achromatic tonal skeletons that embody all their shape and form.

We’re all at least passingly familiar with Classical Greek sculpture. We’ve strolled amongst the life-size figures on pedestals in galleries and museums. These are monochromatic now — marble-white or grey bronze — but originally they would have been painted to look more like living human figures.

Paint adds colour, nothing more. Without their paint these sculptures are not more difficult to see, it isn’t any harder to apprehend their form and volume — just the opposite in fact. And that’s a clue to what light is for in images.

Separating light values from colour

Let’s think of images as being made up of points that reflect light back to our eyes. (We’re saying images are made up of pixels.) Our vision discriminates only two fundamental properties of the light from any point: the intensity of it, and the wavelengths of which it is composed.

Our sense of the intensity of the light reflected from any point is derived from the rate we detect photons to be coming from it: how many photons per time period. We perceive this as the brightness of the point.

We perceive the wavelengths of light from the point as the colour of the point.

In colour theory, and in colour models based on human perception of colour, the property of intensity or brightness is also termed luminance. This property is a component of all colour. The measure of this property of a colour is its value. That’s to say, when we put a number on it, or some other measure, that’s a brightness or luminance value.

The term tone is very closely related to brightness. The values of areas of an image, and how they gradate, are its tones. It’s predominantly with tone, and tonal contrast, that the shapes of objects in images are delineated; and it is with tone, and tonal gradation, that their form and texture are modelled.

Colour images have fundamental underlying achromatic tonal skeletons that embody all their shape and form. When we remove colour from an image, what we are left with is its underlying tone, a panchromatic black and white version of the image. This is what a black and white image is: all the information about the brightness of light reflected from a scene, with no information about its wavelengths.

Form is modelled by how much light it reflects

It’s light that shows us the Classical Greek sculptures. It’s with light that we can see their shape and form, and get a sense of their voume in space.

Their surfaces reflect light to our eyes. Some parts may reflect most of the light falling on them directly back at us — perhaps the flat middle of a forehead, say. Others gradually reflect less light to us, and more in other directions, as they curve away from us — think of hips and thighs. There’s a gradation in the light they reflect.

Perceiving three-dimensionality

Most of us have heard that having two eyes gives us stereoscopic vision, that this is a hunter-adaptation, and that we depend on having both eyes for a good sense of distance and three dimensional vision. In fact most of us have a perfectlly good sense of the three dimensionality of the world around us, and the things in it, even when seen through only one eye.

Our instinctive understanding of perspective, and the way things should be, gives us a perfectly workable sense of relative distances and whether one thing is farther away than another.

And our instinctive understanding of gradation in the light that things reflect back at us enables us to get a near-perfect sense of their form and volume.

(Steroscopic vision is just basic range-finding on a single focus point, and adds little to our perception of the world. It might help you catch a ball, but it won’t improve your sense of how round it is.)

This is one of the reasons why we can appreciate the illusion of flat, two dimensional images as a satisfyingly realistic representation of the real world.

Coming back to colour

Classical greek sculpture is a high-point of realistic representation of the human form. When time removed the statues’ paint it allowed the underlying art of the sculptur to shine through — in a heightened, slightly hyper-real way — and it showed that aspect of it in a degree of isolation from the final statue considered as a whole. It’s not unlike what happens when a photographer makes a panchromatic black and white image of our, in-reality coloured, world.

Does the superficial surface matter? If we re-apply the paint, does it change the apparent form and volume of the statue? No. It might change the texture of the surface, and we do have a visual sense of texture. But it won’t alter how the underlying form models the volume of the figure in space. In a sense, this shines through the paint. If the statue were less perfectly modelled, and an enhanced sense of its volume had to be built up using effects of coloured paint, it would be far less satisfyingly realistic.

It is the angle of points on the surface relative to the observer that determines what proportion of the incident light they reflect to him. It’s this, not the paint on them, that models form. The form underlies the colour. Gradation in the brightness of any particular colour of paint we see on a statue is caused by the underlying forms beneath it, not by some property of the paint itself. The underlying form modulates the brightnesses we perceive, and this models the volume of the statue for us.

Still images

Classical Greek figures have something further in common with photographic images (and paintings): they are still. It’s interesting that their stillness often captures a moment of the figure in motion. The sculptor hasn’t just frozen a moment; he has carefully chosen to freeze the exact moment that most wholly represents and expresses the entire movement.

Becoming flat

In Classical times, and for long after, sculpture was the pre-eminent visual art. Painting images on flat surfaces would not even begin to rival it for many centuries. But there were flatter forms of the sculpted image. Figures sculpted or modelled in relief appeared on friezes and other features, like bodies half-emerging from the clay or stone.

We can’t walk around them, they are tableaux with a single definite (rather than implied) picture plane; but their shape and form are still modelled in three real-world dimensions, even if the third one lacks depth.

If we can flatten these reliefs completely we’ll have arrived at realisticly two-dimensional images of our three dimensional world. But we need to represent or simulate the way light is reflected differentially from forms, or gradates on them. We need to shade them in.