A method performed by a head mounted display, HMD, device (104; 1200) supporting rendering of a virtual reality environment, the HMD device (104; 1200) comprising: one or more sensors (1206, 1208, 1210, 1214, 1218), one or more processors, and a see-through display employing additive blending between real world and virtual world display elements, wherein the sensors (1206, 1208, 1210, 1214, 1218), processors and display are connected together, the method comprising: obtaining sensor data, from the one or more sensors (1206, 1208, 1210, 1214, 1218), describing a physical space adjoining a user of the HMD device (104; 1200); generating, by the one or more processors, a model of the physical environment by reconstructing the surface geometries of real world objects located in the physical space using the sensor data; representing, by the one or more processors, a user of the HMD device (104; 1200) in the model of the physical environment as a volumetric shape (505; 605) surrounding the user, wherein the height and location of the volumetric shape (505; 605) in the model of the physical environment is based on an estimate of the user's head height determined from the sensor data provided by the HMD device (104; 1200); determining, by the one or more processors, from the surface reconstruction, a distance between the volumetric representation of the user and the surface geometries of the real world objects using the reconstructed surface geometries; changing, by the one or more processors, a predetermined distance threshold the method being characterised in that: the threshold is changed based on the head height of the user, wherein: the predetermined distance threshold is varied from a first distance threshold to a second distance threshold greater than the first distance threshold, when the estimate of the user's head height indicates the user is seated; and the predetermined distance threshold is varied from the second distance threshold to the first distance threshold when the estimate of the user's head height indicates the user is standing; and if the distance is equal to or less than the predetermined threshold, then causing the one or more processors to operate the HMD device (104; 1200) so that the user can see the real world objects through the display without obstruction from the rendering of the virtual reality environment on the display by the HMD device (104; 1200). obtaining sensor data, from the one or more sensors (1206, 1208, 1210, 1214, 1218), describing a physical space adjoining a user of the HMD device (104; 1200); generating, by the one or more processors, a model of the physical environment by reconstructing the surface geometries of real world objects located in the physical space using the sensor data; representing, by the one or more processors, a user of the HMD device (104; 1200) in the model of the physical environment as a volumetric shape (505; 605) surrounding the user, wherein the height and location of the volumetric shape (505; 605) in the model of the physical environment is based on an estimate of the user's head height determined from the sensor data provided by the HMD device (104; 1200); determining, by the one or more processors, from the surface reconstruction, a distance between the volumetric representation of the user and the surface geometries of the real world objects using the reconstructed surface geometries; changing, by the one or more processors, a predetermined distance threshold the method being characterised in that: the threshold is changed based on the head height of the user, wherein: the predetermined distance threshold is varied from a first distance threshold to a second distance threshold greater than the first distance threshold, when the estimate of the user's head height indicates the user is seated; and the predetermined distance threshold is varied from the second distance threshold to the first distance threshold when the estimate of the user's head height indicates the user is standing; and if the distance is equal to or less than the predetermined threshold, then causing the one or more processors to operate the HMD device (104; 1200) so that the user can see the real world objects through the display without obstruction from the rendering of the virtual reality environment on the display by the HMD device (104; 1200). the threshold is changed based on the head height of the user, wherein: the predetermined distance threshold is varied from a first distance threshold to a second distance threshold greater than the first distance threshold, when the estimate of the user's head height indicates the user is seated; and the predetermined distance threshold is varied from the second distance threshold to the first distance threshold when the estimate of the user's head height indicates the user is standing; and the predetermined distance threshold is varied from a first distance threshold to a second distance threshold greater than the first distance threshold, when the estimate of the user's head height indicates the user is seated; and the predetermined distance threshold is varied from the second distance threshold to the first distance threshold when the estimate of the user's head height indicates the user is standing; and if the distance is equal to or less than the predetermined threshold, then causing the one or more processors to operate the HMD device (104; 1200) so that the user can see the real world objects through the display without obstruction from the rendering of the virtual reality environment on the display by the HMD device (104; 1200).