fig1 to 3 show an example of a kite surfboard 10 , the outer shapes of which correspond substantially to what is commercially available . the board is substantially planar and thin . however , as shown in the side view in fig2 , it has a non - negligible curvature . the lateral edges 12 of the board are curved and the board also comprises curved end transverse edges 14 . preferably , the board is slightly thicker in its center than in the vicinity of its edges 12 , 14 . the lower surface 16 of the board , which forms the hull , has , for example , a slightly concave shape , which longitudinally extends in its center , the longitudinal edges of the hull being substantially planar . the board is equipped with accessories such as , on its upper surface , bindings to tighten the user &# 39 ; s feet , and on its lower surface , four fins 18 arranged in the vicinity of the four corners of the board . on the upper surface 20 , also referred to as the deck , the board comprises bindings 22 in which the user can slide his feet in order to maneuver the board . the illustrated example of a gliding board is non - limiting , and it could have another geometry or be provided with other accessories , etc . according to the teachings of the invention , and as shown in fig9 , the board comprises a foam core 24 that is covered with an outer skin 26 , and which has zones varying in density without material discontinuity . a principle of the invention rests on the manufacture , from a preform 23 , of a foam core 24 preferably having a shape close to that of the final board , this shape being obtained prior to the core 24 being covered with the outer skin 26 , and this shape being obtained by a method having at least one thermoforming procedure with material compression . thus , one can start , for example , from a parallelepipedic initial foam blank 21 , such as shown in fig4 . this blank then has an initial length lo , a width to , and a height ho . in most cases , however , and as a function of the method for producing this initial blank , it has a relatively uniform density . possibly , if the blank 21 is , for example , made directly by molding , it can have a surface crust having a higher density , but this crust will be substantially homogeneous over the entire surface of the blank . preferably , one will use a thermoformable foam , for example , a pvc foam such as those marketed under the tradename “ airex ”. other materials can be used , especially extruded polystyrene foams , and more generally all cellular materials having a synthetic resin basis . from this rough form blank 21 , a preform 23 for the core is made . thus , in the case that is shown , a simple operation to shape the blank 21 by machining thickness - wise is provided . therefore , the preform 23 of the core schematically shown in fig5 is produced , the preform having a reduced thickness h 1 in the area of its longitudinal ends . this machining operation is particularly fast and simple for because , over a given width , the thickness of the preform is constant . this machining operation can be carried out in various ways , especially by hot - wire cutting or by planing . the preform 23 can possibly undergo other shaping operations before the procedure of thermoforming with material compression . the thermoforming procedure according to the invention involves introducing the preform 23 of the core in the mold of a press ( not shown ), preferably a hot press . the preform 23 will preferably have been heated previously and brought to a temperature close to the foam thermoforming temperature . the mold will preferably have rigid surfaces . once in the mold , the preform is subjected to a compression force , which causes a deformation of the foam by crushing , at least in certain areas , and due to the fact that this pressure is applied at a temperature that is at least close to the thermoforming temperature , this deformation becomes permanent . thus , after the cooling and opening of the mold , the foam will have definitively taken the shape of the core 24 . in the embodiment shown in fig6 , and 8 , the initial foam blank 21 can be seen to have had substantially larger longitudinal and transverse dimensions than those of the core 24 , such that it is necessary , after the thermoforming operation , to proceed with cutting the core 24 along its contour line c . alternatively , one can provide for the mold of the press to be provided with sharp edges along this contour line , the cutting operation being then carried out simultaneously with that of thermoforming . one can also provide for the preform 23 to have a contour sufficiently close to that of the core , so that no cutting would be necessary . in the example shown , the core was made in a press moveable along one direction , in this case along the direction of the thickness of the preform 23 and of the core 24 . however , a mold can be used , the lateral sides of which , corresponding , for example , to the longitudinal edges of the preform , would also be transversely moveable , so that the preform would undergo a compression in two directions . using a pvc foam having an initial density of 80 kg / m 3 , this thermoforming operation can be carried out at a temperature of 80 ° c ., at a pressure of 8 bars , for a duration of 222 seconds . in some cases , especially when certain zones are adapted to undergo a substantial densification ratio , one can provide for the thermoforming procedure with material compression to be carried out in several gradual sub - steps , these various sub - steps using , for example , different molds , for example . thus , each of the various foam core areas will have been subjected to a force and a compression amplitude that is essentially dependent upon the initial thickness of the preform in this area and upon the final thickness imposed by the corresponding surfaces of the closed mold . however , tests have shown that , under the above - mentioned conditions , it is possible to locally reduce the thickness of the foam to less than a quarter of its initial thickness , that is , by multiplying its density by at least a factor of 4 , without deterioration of the foam . conversely , the foam thus densified becomes stronger in flexion and compression . such advantages are markedly noticeable once a densification rate of about 10 to 20 percent , depending on the foam , has been reached , and are obvious at a densification rate of 100 percent , corresponding to double the initial density of the foam . as shown in fig9 , a local reinforcement of the core material can thus be obtained in the area of the edges 12 , 14 of the board 10 , which translates into a greater strength of the edges of the boards , which are exposed to direct impacts . in the example shown , in which the preform of the core has undergone a preliminary machining s to reduce the thickness of its ends to the thickness h 1 , which is less than ho , the densification rate of the core in the area of the longitudinal ends of the board will therefore be lower than the densification ratio observed near the longitudinal edges . thus , flexibility can be easily varied in various zones of the board . however , the entire peripheral area of the core has a higher density than that of the central area . one must note , however , that the thermoforming procedure according to the invention may well leave some areas of the core preform completely intact , without any deformation . likewise , it can also impart on certain areas geometrical deformations that do not lead to any noticeable material compression . certain areas can thus be only bent by thermoforming , which , involves almost no material compression , especially for elements having a low thickness . as shown in fig1 to 12 , the invention is particularly useful in locally reinforcing areas of the board other than the peripheral area , especially the areas adapted to receive accessories such as the bindings 22 or fins 18 . thus , one can provide that the preform of the core comprise excess thicknesses ( in the compression direction provided for the shaping procedure ) corresponding to the areas of the core in which one wishes to densify the foam . in the example shown , the areas of the board that are adapted to receive the bindings 20 have been chosen to be densified . indeed , these areas of the core of the board must allow the bindings to be securely anchored , on the one hand , and to directly support the pressure forces applied by the user . it is therefore particularly advantageous to reinforce them . this is achieved in a particularly simple manner due to the invention . as shown , excess thicknesses 28 are provided in the preform , in the areas to be densified . these excess thicknesses 28 are obtained directly by molding , if the initial foam blank was obtained in this manner , or they are obtained by machining , manually ( planing , sanding , etc ) or with a piece of equipment ( digitally controlled milling machine , etc ). after the material compression thermoforming procedure according to the invention , the core shown in fig1 is indeed reinforced in the predefined areas . this local reinforcement enables the positioning of inserts in the core , which enable the anchoring of the bindings . the same method of local densification of the core can be used , for example , to create “ stiffening beams ” that are directly integrated in the core , without material discontinuity . for example , if an increase in the rigidity of the board in torsion is desired , one can provide for the core to comprise diagonal beams , each connecting to two opposite corners of the board . a central longitudinal beam , which is well - known in the field of surfboard manufacturing , can thus be created in the core . finally , the material compression thermoforming technique can also be used to create simple , hollow and / or raised decorative designs , for example on the upper surface of the board . due to the invention , it is therefore possible to locally densify and reinforce the core , exactly where necessary . contrary to certain cores of the prior art , in which local reinforcements are integrated into the form of inserts made from a different material than the base material of the core , the technology according to the invention allows the continuity of the core material between the less dense areas and the denser areas to be preserved . in addition , it is very easy to create a gradual density gradient in order to avoid sudden discontinuity of the density between the less dense areas and the denser areas . this only requires that the height variation along the edge of the excess thickness provided on the preform be gradual ( as shown in fig1 for the excess thicknesses 28 ). areas of sudden variation of the core &# 39 ; s mechanical characteristics are thereby avoided , which are always areas where force constraints are concentrated , and are therefore always weakened . shaping the core by thermoforming permits , at the same time , the outer geometrical shapes that are intended for the board , especially its thickness profile , the possible hull shapes , and / or front or rear raised spatula - shaped portions to be created . however , one can also provide for the shape of the core 24 to be corrected after the thermoforming procedure by means of a complementary operation , for example , shaping by machining , therefore by material removal . the initial density of the foam core will naturally be selected as a function of the desired final characteristics for the gliding board , especially the desired stiffness and strength . of course , one will also take into account the weight / volume ratio desired for the board , especially if one wishes to have specific characteristics in terms of floatability . in this context , being able to selectively densify the foam will allow preserving a low foam density in the least biased areas . in the examples shown , the gliding board comprises only one core . however , for various reasons , the use of a plurality of cores , for example , two superimposed cores , is encompassed by the invention . within the finished board , such two cores can be separated , for example , with a reinforcement layer such as a sheet of resin - impregnated fiber fabric , a metallic strip , etc . in any case , one can then provide that only one core be shaped according to the invention . once the core is made according to the teachings of the invention , the manufacture of the board can be carried out using the usual methods . all of the usual materials can be used to make the skin adapted to cover the core . this skin can have , for example , a single thermoformed abs resin sheet , a composite sandwich complex , glass / carbon / kevlar fiber composites embedded in polyester or epoxy resins . the skin can also include localized reinforcements , such as very high - density foams , cellular materials of the honeycomb type , etc . as shown in fig9 , the board can also be provided , along its edges , with a peripheral reinforcement 30 made from plastic material , for example abs . it has been noticed with respect to making a kite surfboard that it could be advantageous to use a metallic reinforcement in the skin of the board . this metallic reinforcement can be , for example , a sheet of aluminum alloy with a thickness of several tenths of millimeters . this metallic sheet , which , for example , covers the upper surface of the board , is embedded in a ( preferably thermosetting ) resin . in order to reduce the weight thereof , one can provide that it comprises recesses , for example , holes that are regularly distributed over its entire surface . using these various materials adapted to form the outer skin can require subjecting the assembly to non - negligible temperatures and pressures . tests have shown that a core made according to the invention could withstand , without any difficulty , a procedure for manufacturing the skin requiring temperatures on the order of 120 ° c . for durations of about 10 minutes . these conditions thus allow using all the usual materials . the invention can also apply to boards for snowboarding , to skis in general ( i . e ., such as alpine skis , touring skis , cross country skis , etc ), to water skis , and to wakeboards . in the following drawing figures , various embodiments of the invention are shown in the context of other types of gliding boards . fig1 shows an embodiment of the invention used to create a reinforcement of the density of a ski core 24 in its central portion 32 ( or platform , the one adapted to receive the binding system ), as well as in stiffened areas 34 forming an x shape , which , originating from the center of the ski core , extend in the direction of the front and rear lateral ends . in an alternative embodiment , as with the kite surfboard example , one could elect to reinforce the periphery of the core . the core in fig1 is easily produced according to the invention , by machining a core preform so that it has excess thicknesses , the shape of which correspond to said zones . the final core can selectively have excess thicknesses in the area of the zones thus densified , or , conversely , a “ smooth ” upper surface . fig1 to 17 show an embodiment of the invention for manufacturing a ski core 24 having a central platform 32 that is raised and densified , as well as , at the front and rear of this platform 32 , two semi - cylindrical flanges 36 that extend in parallel to one another along a substantially longitudinal direction on the upper surface of the core . such a core , schematically and partially shown in fig1 , can be obtained from a machined preform 23 , such as shown in fig1 . this platform 23 has a very thick central portion 38 and , longitudinally on both sides , front 40 and rear 42 portions having a substantially rectangular section , the height of which corresponds substantially to the height of the flanges 36 of the final core . in this way , after the thermoforming procedure , the central platform 32 is very dense ( fig1 ) and the front 40 and rear 42 portions of the core are not dense in the area of the flanges 36 , and are more dense between the flanges 36 , as well as on both of their sides , on the lateral edges ( fig1 ). with another geometry of the machined preform , it could have also been provided that the core be conversely more dense in the area of the flanges 36 than in the adjoining areas . the invention can also be carried out for making a core having dissymmetrical properties . fig1 shows a core 24 for a snowboard having a lateral side 44 where the foam density is higher than on the opposite lateral side 46 . due to the invention , this can be easily obtained by machining a core preform 23 having the profile shown in transverse cross - section in fig1 , where one side is thicker than the other . after thermoforming , a core is obtained , having , for example , a constant thickness ( see fig2 ) which has a widthwise density gradient . in the case of a core of a snowboard , the core in the area of the zones adapted for receiving the bindings can also be densified , as seen with respect to the board for kitesurfing . the lateral dissymmetry described here can , of course , be provided as a longitudinal dissymmetry , or the like . generally speaking , the various alternative embodiments , which have just been mentioned can be provided and used regardless of the type of gliding board envisioned , and can generally be combined together .