What are two reasons an artist uses chiaroscuro or shading in an artwork?

1. Contrast and realism

2. Three dimensionality

Explanation:

    Chiaroscuro is an oil painting technique developed during the Renaissance that uses strong tonal contrasts between the three-dimensional forms of light and dark for the model, often for dramatic effects. The underlying principle is that the solidity of the form is best achieved by the light falling against it. Artists known for the development of the technique include Leonardo da Vinci, Caravaggio and Rembrandt. It is a key element of black and white and low photography.

    The claroscuro, in a two-dimensional image, such as a wall, a panel, a screen, a paper photographic print, gives the illusion of relief, volumes mimicking the effects that light produces on these volumes in real space. Modeling, this representation of volumes, can be done by means other than by claroscuro, as was the case in the Middle Ages.

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i believe the answer is  the similarity is that both are sacrifices which have the same purpose and sacrifice - jesus christ, but the difference is in offering of that sacrifice.

the similarity is that both are sacrifices which have the same purpose (repenting our sins) and sacrifice - jesus christ.  the difference is in the way of the offering that sacrifice. the sacrifice of jesus on the calvary is sacrifice of his actual living body and blood, but the sacrifice of the mass is the sacrifice of the jesus through bread (body) and blood (wine). in the sacrifice on calvary jesus was offered for us; while at mass he offers himself through us.