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I'm wondering what basic characteristics of shadows are. I'm looking from a camera image/video processing point of view. I'm considering the case of an outside scene in direct sunlight.

These are the characteristics I could quickly come up with, but I guess there are more.

  • There are strong edges on the shadow borders
  • The shadow is ‘darker’ than the surrounding area
  • The shadow usually ‘deforms’ when an object is rotated
  • Due to absence of the primary lighting in the shade, the color tends to be dominated mostly by the main diffuse lighting source (which is in most cases the sky --> blue)

I'm wondering which other characteristics I might miss.

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Shadow has very specific properties that makes it very clear way of making it distinguishable from the regular object. A lot of work in the area of background subtraction and surveillance has been using this to eliminate the shadows or to avoid them being mistaken as the actual object (or human).

As observed by Daniel Grest

To distinguish the shadows from the person in the segmented image we assume the following properties:

  • a shadow pixel is darker than the corresponding pixel in the background image,
  • the texture of the shadow is correlated with the corresponding texture of the background image.

This paper models in detail the property of color correlation and develops the Nromalized Cross-Corelation matrix method to identify the shadow pixels. See 1 for more details.

Julio Cezar uses and extends the performance by localization. It also uses it in monochromatic conditions which would be harder than color. See 2 for details.

Horprasert characterizes shadow as

Shaded background or shadow (S) if it has similar chromaticity but lower brightness than those of the same pixel in the background image. This is based on the notion of the shadow as a semi-transparent region in the image, which retains a representation of the underlying surface pattern, texture or color value

This technique is used to classify scene in background, shadow and a moving object. See 3 for more details.

In a sense, these papers are indicating the properties you mentioned - but they show the mathematical model for the same which allows computationally identifying the shadows.

  1. Daniel Grest et. al. A Color Similarity Measure for Robust Shadow Removal in Real-Time

  2. Julio Cezar et. al. Background Subtraction and Shadow Detection in Grayscale Video Sequences

  3. Thanarat Horprasert, David Harwood A Statistical Approach for Real-time Robust Background Subtraction and Shadow Detection

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More formally:

1) The saturation component of the shadow should be low.

2) Under the Lambertian assumption, an input image I arises from a product of two intrinsic images: the reflectance image and the illumination image. Invariance to illuminant colour and intensity means that such images are free of shadows, as well, to a good degree. Since shadows are mostly due to removal of some of the lighting, such an image also has shadows removed.

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The edges of the shadows are crisper where they are nearer to the shadow caster. Also, since the sun is so far away, beams of light are essentially parallel when they reach us, which means that shadows from sunlight are basically orthographic projections of the shadow casters. Also, if you knew an objects dimensions and saw its shadow on something that you knew the slope of, you could probably calculate the angle of the sun, and so maybe could calculate what time it was, and maybe something about the season or latitude??

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The characteristics of the shadow are as follows:
It is always dark regardless to the colour of the object or the coloor of the light used to make a shadow
It only shows dark outline of the object
It is formed in the opposite direction to the source of light
The size of the shadow depends upon the distance between the source of light and opaque object
The size of the shadow also varies depending on the distance between opaque object and opaque screen

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  • $\begingroup$ i hope it is very informative $\endgroup$ – amina saja Oct 14 '17 at 14:40
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A shadow has the following three characteristics

  1. It is always black, regardless of the colour of the object used to make the shadow.

  2. It only shows the shape or outline of the object not the details.

  3. The size of a shadow varies depending on the distance between the object and the source of the light, and the distance between the object and the screen.

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Shadows, despite what I have always assumed, are black or nearly so. In my studies with the LED light and the digital camera, I have discovered that the shadow, is, in fact, the color of the surface onto which is is found. Experimentally, I have, on a pure white ground that is lit with a rather weak, warm, ambient light of around 5000K, placed a subject, in this case, a feather. I then projected a very strong LED flashlight beam raking across the subject creating a shadow. This strong cool light of around 7000K will wipe out the ambient light in all areas except the shadow. The shadow then takes on the warm tone of the 5000K ambient light which is a very pleasant light orange.

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There are many characteristics of shadow,some are mentioned below - 1.Black in colour 2.Size varies 3.Shadow always forms on the opposite sides of source of light. 4.Shadows do not show details. 5.Shadow do not show exact shape, size and colour.

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