I've got a scan of "Maciewoski Bible" form the Internet. There is a good view of greaves on Goliaph. I'm sure, you know it. But there is another picture in the Bible (see attachment). Some my friends are sure that prisoner (on the left) wears greaves and poleyns. I'm not so sure about it. What do you think?

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The way the edge of the defense is verically straight and not shown as a series of slope sided triangles as was usually
how maille chaussets of the variety laced around the leg were depicted lends itself to the possibility that greavesare being depicted here . Additionally there is a distinct difference in the method used to represnt maille here and the
way the surface of these item is depicted . They may be representing something else but they quite possibly may be
greaves .

It could merely be a certain shadowing technique; if you look at the maille leggings on the front man, especially his forward leg, you'll note that his legs also show a straight line, with the same bluish hue to the right of it.

Checking my "Old Testament Miniatures", where the Maciejowski Bible is shown in all its glory, it is clearly evident that it isn't some shadowing technique, but something that appears to be a greave.

The legs of the fellow in front of him have the same lines used to represent maille as are found on his hauberk as
well as on all other maille representations in the portion of the panel shown in the picture while the legs of the fellow in question are devoid of such lines so I don't believe it to be a shading technique but as Bjorn said meant to represent
plates or greaves .

umm whats a greave?

Greaves are plate armor (or hardened leather) for the front of the lower leg, or shin. I think they were sometimes called "schynbalds." Some cover the shin from the ankle to the bottom of the knee, and some extend to the top of the knee. They're strapped around the back of the calf.