Windup window shade with warping compensation

A windup window shade (12) for regulating the entry of light through a rear window into the interior of a motor vehicle,

• having at least one windup shaft (23), which is rotatably seated,
• having at least one window shade web (35), which is fastened along one edge to the windup shaft (23), whose edge which is parallel with the one edge is connected with an associated traction rod (37), and which has lateral longitudinal edges, wherein, in the course of the extension of the window shade web (35), the traction rod (37) has a nominal position with respect to the windup shaft (23) which position is a function of the position of the window shade web (35), and in which no transverse tensions occur,
• having at least one pair of guide means (13), each of which extends laterally beside the extended window shade web (35), and each of which contains at least a guide groove (19), in which the ends of the traction rod (37) are guided,
• having a drive mechanism (45), which has a first and a second thrust member (56, 57) arranged such that an effective end of the first thrust member (56) acts together with the one end of the traction rod (37), and the effective end of the second thrust member (57) acts together with the other end of the traction rod (37), and
• having a compensating device (22, 66, 71), which is equipped to at least reduce a displacement of the traction rod (37) by the thrust members (56, 57) out of the nominal position.