(artist unknown, entrance to Galla Placidia) (c.425 to 426). Good Shepherd [ceiling mosaic].

I believe the shepherd figure is not Christ but one of his apostles. He is caring for his flock, a handful of innocent, doe-eyed sheep who, based on their gestures, live through him. He is peacefully yet vigilantly surveying the area near at-hand, where the off-canvas portion of his flock grazes. Figures and landforms are located in an evenly-spaced manner on the picture plane. Value contrasts define shapes; many are framed by dense plant forms and respectable modeling. |

This is a work of semi-abstract realism, due to the medium. The arrangement of figures, lines of force and formatting choices, such as color and value, suggest deliberate design. I believe this work is meant to illustrate the internal character of the central figure through his humble, watchful and protective activity. He is dressed in the style that would be used by the imperial royalty of the time. 
Shape, value and color are roughly equal in their support of this image. Value defines shapes more than color. I cannot identify a color scheme. Greens tend to be desnse, blues reside at the value terminals and gold-oranges at the high portion of the key. It is difficult for me to say if the tonality pattern supports the careful arrangement of shapes, or the other way around. 
This work has some of the most developed depth extant in pre-Renaissance two-dimensional art. The designed arrangement of foreground shapes overlapping mid- and background shapes, in addition to foreshortening and slight modelling, create a sense of depth that is not quite shallow. The properties of color tend to support depth as well. Light, warm colors move forward, midtoned dirty establish the midground, and dense, cool colors “sink” to the background. 
As far as attention goes, the captive audience of the flock directs. Their lines-of-sight, to include the ones that are relaxing, all converge on the central figure. The seamless balance of this work has a radial flow, supported by the uncommon shape of the canvas. This and the vertical orientation of the central figure help to place focus on him. Small, rounded shapes are the most numerous, resulting in the larger, elongated form attracting still more interest.

The entire work is formatted with solid, natural forms. The use of the value is roughly equivalent across the range. The artist expertly takes advantage of the textural quality native to mosaic-as-a-medium. The location of this mosaic in relation to the observer leaves her or him with the impression that they are a member of the flock, drawing them in to the protection offered by the shepherd, who it turns out is Christ.