The combination of the diamond-anvil cell, synchrotron x-ray diffraction in radial geometry and simultaneous Rietveld refinement with texture analysis allows us to quantitatively investigate the plastic deformation behaviour of materials at very high pressures. Our study of copper to 30 GPa shows in ideal experimental geometry a [110] fibre texture component, as is typical for axial compression of soft face centred cubic metals. Locally a plane strain texture develops which is energetically favoured (curling). A transition from compressional to plane strain/pure shear texture can be monitored by analysing individual images taken at different positions in the diamond cell.