GB2212765A - Aids for perspective drawing - Google Patents

Abstract

Perspective drawing aids are described, including (a) A parallel linkage arrangement (Fig. 2) having a pointer pivoted to a fixed point on one arm of the linkage, and to a fitting selectably positionable along the mutually parallel arm (exact indication of "vanishing point"). (b) A pair of equal length arms pivoted to respective ends of a link, the other ends of the arms being pivoted at mutually adjustable positions to a support, and the pointer being attached to the link perpendicular thereto (Fig. 3). (c) A pair of intersecting linear tracks mutually pivotable about the point of intersection, and a link having its ends slidable along respective tracks, the pointer being attached to the link perpendicular thereto (Fig. 4) ((c) and (d) give only an approximate indication of "vanishing point"). Various "3-axis" devices are described which are based upon the above, notably devices derived from (a) above in which the parallelism of the arms is maintained by (i) an elongate inextensible flexible element (Figs. 5-8) (ii) an arrangement of links. <IMAGE>

Description

Title: Perspective Drawing Aid DESCRIPTION This invention relates to drawing aids, particularly to assisting achievement by a draughtsman of drawings with accurate perspectives.

Perspective drawings require non-parallel lines that converge to vanishing points normally known as the left, right and vertical vanishing points (LVP, RVP and VVP) whose actual positions, whether on or off the surface of any particular drawing board, can be different according to what particular perspective is required. Hitherto, the most common resource available to draughtsmen has been in the form of prepared grids of converging lines relative to particular positions of vanishing points, i.e. requiring a different prepared grid for each set of vanishing points. Those grids are emplaced on the drawing board and visible through normal tracing paper or transparent film. As well as limiting draughtsmen to perspectives for which grids are available, problems can also arise from having to interpolate between spaced grid lines to the same vanishing point.

It is an object of this invention to'provide an adjustable mechanical system for assuring that a drawing board affords to the user a means for setting up accurate perspective lines at varying positions on the board.

According to this invention there is provided associated with or for association with a drawing board apparatus comprising a part that is movable relative to the board which movement is relative to a direction representing an axis through an intended vanishing point, means operative during said movement to maintain an attitude of the part substantially constant relative to said intended vanishing point, and means for adjustably setting the part as to its attitude in accordance with different intended vanishing points along said direction.

Such movable part can serve as or in conjunction with line defining means so as to define a line at any angle to the intended vanishing point that is within the range of movement of the part for which its attitude is maintained as aforesaid.

Apparatus hereof is capable of implementation as a mechanism. One investigated approach to a mechanism involves a said part that tilts between spaced support positions as it and its support positions move towards and away from said direction during its said movement.

For a said part supported at spaced positions, such support can be in divergent guides that force the required tilt as the part moves along the guides, and which guides may be adjustable as to their angle of divergence corresponding to different vanishing points and/or themselves pivotal in a controlled manner. An additional variable could be the spacing of the support positions for the part. Alternatively, a said part supported at spaced positions may be pivotted at those positions to respective arms extending from spaced pivot positions, which arms may be adjustable as to their angle of divergence, say by adjustment of the spacings of the pivot positions and/or the part positions. Both alternatives will be described later in terms of a said part that extends between its support positions in an attitude nominally normal to a line from the intended vanishing point.For simple mechanisms of this approach, i.e. with only one adjustment, say the divergence of the guides or the spacings of the part positions for the two examples quoted, respectively, reasonable accuracy is obtainable for relatively long or distant vanishing point settings, but accuracy becomes less as vanishing point settings become relatively short or close.

Another investigated approach to a mechanism involves a said part that is effectively pivotal about one support position as it and its support position move towards and away from said direction during its said movement. A particularly neat and simple implementation has its said part in or on a pivot at one end of one of a pair of parallel arms whose other ends are pivoted for movement together relative to positions spaced along the said direction through possible vanishing points, and the said part pivots as the parallel arms pivot in said movement of the part under control of a restraint, tie or guide means that is then at a fixed position along the length of said other arm but permits the distance between the part's pivot and the fixed position to vary. The fixed position along the length of the other arm is adjustable according to vanishing point distance.Such mechanisms are capable of sustained high accuracy throughout permitted ranges of movement of said part, i.e. for relatively long or distant and for relatively short or close vanishing points along said direction.

Accordingly, investigation has concentrated upon these mechanisms.

In preferred embodiments, provision is made for setting up vanishing points in more than one direction, preferably for all of the above-mentioned left, right and vertical vanishing points. Then, the fixed pivot position for said one or other arm can be an origin point on the drawing board, i.e. where the line through the left and right vanishing points intersects the line through vertical vanishing points. The terms "left", "right" and "vertical" need not be taken too literally as it is a further preference hereof that the drawing board itself be on a pivot with an axis through its nominal origin point so that the effective horizon can also be changed.

For preferred embodiments, said restraint, tie or guide means can conveniently comprise cooperation between an elongate straight formation fixed relative to the corresponding and pivotted part and cooperating with an associated mount to the corresponding said other arm which mount engages the elongate straight formation so as to permit lengthwise movement of the latter and twisting movement relative to said other arm. Further, for preferred embodiments, their said parts can constitute releasable holders for a ruler then extending over the front of the drawing board always pointing towards the intended vanishing point, conveniently following the line of the associated said elongate straight formation.

In one preferred embodiment, three pairs of parallel arms are provided each for a different direction of selectable vanishing points and each having associated pivotted said parts and restraint, tie or guide means. Advantageously, the three said other arms can be branches or legs of a single T-piece pivotted at its junction, the three said one arms will be end-pivotted at positions spaced along perpendiculars crossing at the T-junction, two positions being to opposite sides of the same perpendicular (for left and right vanishing points) and the third being on the other perpendicular (for vertical vanishing points). Typically, the base of the T-piece passes between the two said other arms with only one of their pivot positions between the base and the stem of the T-piece, and the third said other arm is between the base and the stem of the T-piece to the other side of the stem. A particularly interesting means for holding the pairs of parallel arms in the required relation, and facilitating rotation of the drawing board to change the horizon, is by way of a chain or toothed belt, preferably the latter, and toothed arcuate segments on such said other arms, idlers, and a toothed wheel on the board's rotation axis.

In another embodiment having three pairs of parallel arms each for a different direction of selectable vanishing points and each having associated pivotted said parts and restraint, tie or guide means, the three said other arms can be branches of a T-piece, or more preferably a cruciform shape, whose fourth branch carries a counter-balance weight. The piece is pivotted at the junction of the three said other arms, and the three said one arms will be end pivotted at positions spaced along perpendiculars crossing at the T-junction, two positions usually being to opposite sides of the same perpendicular and the third usually being on the other perpendicular, with pivot positions preferred at substantially equilateral spacing and with said junction medially of a line interconnecting two of said pivots.

A particularly convenient way of holding the pairs of arms in the required relation and facilitating rotation thereof, is by way of pivotal links serving to interconnect each arm with its adjacent arm. More particularly, said parts are provided on a carrier which is itself pivotally movable on an axis coincident with said junction and arranged for movement relative to said other arms and preferably also relative to the board which is also conveniently pivotally mounted on said junction. For that purpose an arm provides for locking adjustment of said carrier relative to the board. Such a simplified construction provides required range of vanishing points for the majority of practical applications.

In another preferred embodiment, a mechanism having a single pair of pivotted parallel arms with associated said part and restraint, tie or guide means is provided but locatable and adjustable to serve for more than one direction of selectable vanishing points.

That is particularly readily done for a substantially circular drawing board where the mechanism is movable bodily about an axis at the centre of the board, or at least the abovementioned origin point, to get three positions of location, i.e. for left, right and vertical vanishing points, preferably with its said other arm pivotted to a mechanism carrier itself relocatable by swinging about said centre or origin, and with parallelism of the pair of arms secured by a link pivotted between the arms as a parallelogram link relative to end pivots for those arms, actually preferably connected between extensions of the arms.

It will be appreciated that arms of mechanisms hereof will normally be behind the drawing board but with the said part accessible at the front of the drawing board, conveniently with said one arm(s) extending beyond the periphery of the board for arcuate movement in relation to a set vanishing point or points. A hinged drawing board readily permits access to the rear for adjustment of vanishing points.

However, at least for mechanisms where one pair of arms serves more than one direction of vanishing points, and thus requiring setting and resetting for drawing lines relative thereto (but normally to the same intended vanishing point for particular directions), it is advantageous to provide for adjustment of vanishing point distances without having to swing the drawing board about a hinge to gain access to its rear.

One suitable such provision is by way of a pivotted linkage normally having at least two links, one of which is pivotted to the setting for the restraint, tie or guide so at to move the latter relative to said one arm of the associated pair of arms and also pivotted to the other link, which may be cranked for medial pivotting at the pivot for said other arm and have a free end going beyond the edge of the drawing board. Fixing of such free end for any particular vanishing point setting can then usefully be by way of a readily releasable clamp at such free link end and operative relative to a sector member rigid with said other arm and preferably graduated to indicate vanishing point dstance.

Practical implementation of this invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is used in explaining a principle involved in preferred embodiments; Figure 2 further assists in indicating practical embodiments of the Figure 1 principle; Figures 3 and 4 show principles for alternative embodiments; Figure 5 shows application of Figures 1 and 2 to a mechanism for separately setting left, right and vertical vanishing points; Figures 6, 7, and 8 are rear perspective, rear plan and side views, respectively of a mechanism for Figure 4 application; Figures 9 and 10 are rear plan and side views, respectively, of a mechanism for alternatively setting more than one direction of vanishing points; Figures 11 and 12 are broken plan and side views of an alternative to Figures 9 and 10;; Figures 13 and 14 are plan and side views of an adjuster head; Figure 15 shows a preferred aid to using embodiments of this invention; arid Figures 16 and 17 are front and rear views of an alternate embodiment of mechanism.

In the drawings, referring first to Figures 1 and 2, the line VD represents a direction for setting distance of an intended vanishing point P. Two arms 10 and 20 are shown in parallel relation and both pivotted, see 12 and 22, at fixed spaced positions along the line VD. One of the arms, 10, is shown with a pivot connection 14 spaced from fixed pivot 12 along the length of that arm so. The other of the arms, 20, is shown with a pivot connection 24 that is adjustable (see arrow 26) along its length towards and away from its fixed pivot 22 the same side of the line VD as the pivot connection 14.

For immediate purposes, the pivot connections 14 and 24 are conveniently considered only as representing positions along the arms 10 and 20, respectively. A line connecting such positions and extended (see L) to intersect the line VD, shown (Figure 2) at the intended vanishing pzat P maintains such intersection as the arm 10 is pivotted about its fixed pivot 12, see dashed arc 14X, so long as the arm 20 is also allowed to pivot about its fixed pivot 22, see dashed arc 24X, parallel relation with the arm 10, see dashed and chain-dashed lines 10A, 20A, LA and 10B, 20B, LB. It will be evident that the position of the point P as an intersection of the line L with the line VD can be changed by adjusting the position represented by the pivot connection 24 along the arm 20. Specific embodiments to be described make use of that geometrical property.However, it is to be equally appreciated that changing of intended vanishing point p along the line VD is achievable by holding the position represented by the pivot connection 24 fixed along the length of the arm 20 and having the pivot connection 14 adjustable along the length of the arm 10. Indeed, both such pivot positions could be adjustable along the lengths of the arms 10 and 20.

It will be appreciated that the range distance of intended vanishing point P along the line VD depends on the range of angular adjustment of the line L.

Preferably, that distance range is from infinity, where the line L is parallel with the line VD (see limit LM and full line L in Figure 2), and a minimum appropriate to requirements including practical mechanical implementation (see limit Lm).

Figure 2 represents a more specific embodiment of the type just mentioned. The arm 20 is shown with a slot 26 along part of its length and along which the pivot connection 24 is adjustable, for which purposes the pivot connection 24 can be of trunnion type that freely slidingly and pivotally accommodates a restraint tie or guide 30 generally following the line L and held for only pivotal movement in the pivot connection 14.

Such a mechanism will be adjustable for different vanishing points P along the line 24 by adjusting the position of the trunnion 24 along the arm 20, and will have its restraint tie or guide member 30 movable to point along any line to any set vanishing point P by moving the arm 10 about its pivot 12. A parallelogram link 32 is indicated as pivotally connected between the arms 10 and 20 parallel to the line VD to assure parallelism of the arms 10 and 20 throughout such angular movement of the arm 10. It will be evident that the arm 10 and pivot connection 14 could alternatively or additionally be made relatively adjustable and permitting of free sliding of the member 30.

A part, see dashed at 40, fixed at the pivot connection 14 to move with the member 30 will maintain a constant attitude to the intended vanishing point P.

The same is, of course, true of any part fixed relative to the member 30 which may be of any convenient length.

Such attitude-maintained part can conveniently be a holder for a ruler, usually for holding a ruler in a readily releasable manner.

Before going on to further describe specific preferred embodiments of this invention based on Figure 2, reference is made to alternative types of mechanism that are viable at least for relatively distant intended vanishing points P along line VD.

Thus, in Figure 3, the direction of intended vanishing points VD bisects a spacing between fixed pivots 50, 51 for arms 52, 53 extending away to each side of the direction VD and interconnected by a link part 54 pivoted thereto at positions 55, 56 equally spaced along the arms 52, 53, and of adjustable length, see dashed at 57. With the arms 52, 53 parallel (Figure 3A), a pointer 58 extending normal to the link part 54 will maintain parallelism with the direction VD as the arms 52, 53 are rotated about their fixed pivots 50, 51. When the length of the link part 54 is increased, rotation of the arms 52, 53 about their fixed pivots 50, 51 produces a tilting of the link part 54 that increases with arms rotation.With the arms 52, 53 slightly divergent from their pivots 50, 51, the pointer 58 will give a reasonable approximation for lines to a distant vanishing point in the direction VD.

However (see Figure 3B), with the arms 52, 53 more divergent, as required for a closer vanishing point along the direction VD, that approximation is less accurate, see P1 and P2.

In Figure 4, guides 60 and 61 are provided for ends 62 and 63, respectively, of a part 64 shown carrying a pointer 65 extending normal thereto. The pointer 65 extends parallel with the directions VD when the guides 60 and 61 are superposed and normal to that direction, see Figure 4A, which relation persists throughout movement of the part 64 along the guides 60 and 61. When the guides 60 and 61 are displaced relatively angularly and oppositely out of their superposed relation, movement of the part 64 therealong causes it to tilt increasingly with its movement.

Slight divergence of the guides 60 and 61 gives a reasonable approximation by the pointer 65 with lines directed towards a distant vanishing point along the diretion VD. However (see Figure 4B), with the guides more divergent, as required for a closer vanishing point along the line VD, that approximation is less accurate, see P3, P4.

Accuracy of mechanisms based on Figures 3 and 4 could, of course, be improved by additional control means operative relative to the lengths of the link part 54 or either or both of the arms 52 and 53 or even the positions of the fixed pivots 50 and 51 of Figure 3, or relative to the length of the part 64 or the angle of divergence of either or both of the guides 60, 61 of Figure 4. Work on developing a suitable control means, especially one of an entirely mechanical nature (for example based on employing a fusee) has not been taken to a practical conclusion in view of the inherent accuracy of mechanisms based on Figure 2.

One such mechanism is shown in principle in Figure 5 for individually servicing the normal three, vanishing points, i.e. left, right and vertical, effectively using three parallel-arm systems of Figure 2, see subscripts L, R and V in Figure 5. The three arms 26 are, in fact, different limbs of a single T-piece 80 and share a common fixed pivot 22C, which will be at the origin point for the intersecting directions VDL, VDR and VDV and is further preferably a rotation axis for the drawing board (not shown).

Individual adjustment of the trunnions 24L, R, V permits setting of the vanishing points PL, PR, PV in any way desired within the limits of the slots 26L, R, V. The fixed pivots 12 are shown with associated segments 70R, 70L and 70V by which they can be turned together by a belt 72 shown passing over idlers 74 and also about a central wheel 76 rotatable on the same axis as pivot 22C. For a rotatable drawing board, the system allows drawing relative to a tilted horizon.

Further detail of a practical embodiment of a Figure 5 mechanism is given in Figure 6, 7 and 8 in which the same references advanced by 100 are used as are used in Figures 2 and 5 for the corresponding parts. It will be noted that restraints, ties or guides 130 are shown as straight rods, also that the slotted parts of the arms 120 are marked with a scale of adjustment of the trunnions 124. Also, arcs AL, AR and AV are shown in Figure 6 for practical ranges of movement of the arms 110 outside a circular drawing board 182. The latter has a first bracket 184 hinged to a second bracket 186 affixable to a table or shelf by clamp screws 188. An adjusting wheel 190 is also shown at the hinging for setting tilt of the drawing board 182.

Use of toothed segments 170, wheel 176 and a corresponding toothed belt 172 can assure positive and accurate movement of the mechanism via any of the arms 110 once vanishing points PL, PR and PV are set. A ruler is preferably detachable associated with any one of holders rigid with the rods 130to, 130R, 130V.

Turning to Figures 9 and 10, a mechanism is shown using only one pair of parallel arms 210, 220 and associated restraint tie or guide rod 230 end-pivotted at 214 and going through trunnion 224 in slot 226. In this embodiment, fixed pivot 212 for arm 210 is at the centre of the drawing board 282 and pivot 222 for arm 220 is spaced therefrom on a carrier2g, see limb 291A thereof. Another limb 291B of that carrier extends beyond the edge of the board 282 to serve in moving the mechanism to serve vanishing points in different directions, see clamp 292 for location purposes relative to the board edge, say relative to marks on that edge corresponding to particular vanishing points, see PRM, PLM and PVM. Parallelism of the arms is shown maintained by a link 293 between cranked limbs 210A and 220A of the arms 210 and 220.

Moreover, there is a provision for adjusting the vanishing point distance from a side of the board by way of a pivotted linkage comprised of a cranked link 294 pivotted at pivot 222 for the arm 220 and a link 295 pivotted between link 294 and trunnion 224. An adjustment scale is shown marked on a sectoral extension 296 of the arm 220 with which a clamp 297 on the link 294 cooperates. The clamp 247 may be of spring-loaded squeeze-release type as shown. In addition, Figures 9 and 10 show a holder 240 for a ruler 242.

Figures 11 and 12 show an alternative to Figures 9 and 10 aimed at reducing overall thickness and simplifying generally. Equivalent parts are given like references advanced by one hundred. Drive linkage between master arm 310 and slave arm 320 is now by way of pulleys 401, 402 at their respective journals 312 and 322 and a circumscribing belt or cord 403 shown with two associated idler belt/cord guide pulleys 404, 405 of which the latter is conveniently adjustable for tensioning purposes. Also, there are upper turntable and support plate 406, 407 and intermediate plate 408 and mount 409 involved in the turntable as such.

Adjuster head 340 is also shown in a particularly convenient self-locking or clamping construction.

Referring to Figures 16 and 17, there is illustrated a yet further embodiment based on the mechanism of Figure 2 and constituting a variation of the embodiment of Figure 5 (and applications of same as illustrated in Figures 6 to 8) and in which the same references advanced by 400 are used as are used in Figure 2 for the corresponding parts. The embodiment again serves for individually servicing the normal three vanishing points i.e. left, right and vertical, using three arms 420L, R and V which are, in fact, different limbs of a single cruciform piece 480, having a fourth limb 480a carrying a balance weight 480b, and share a common fixed pivot 422c at the origin point which is further preferably a rotation axis for the drawing board 482. Individual adjustment of trunions 424L, R, V permit setting of the vanishing points as before.

Fixed pivots 412 for respective arms 410 are on a carrier plate 500 and spaced along perpendiculars crossing at the junction of the arms 420, i.e. pivot axis 422c about which carrier 500 is pivotable. An arm 502 depending from the carrier to extend beyond the periphery of the drawing board 482 provides for rotational movement of the carrier and with it the parallel arms 410, 420 and has means for engagement and location with the board, conveniently for adjustment in steps, such as 5 degree intervals. This serves for setting the basic average angle of working in preference for rotating the drawing board. Movement of an arm 502 over arc 504 controls all left VP's whilst movement over arc 505 controls all right VP's. All vertical lines use the arm 502 set on 0 degrees.

Adjacent arms 410 are linked together by way of an interposed link 506 pivotally connected at each end to an adjacent arm 410 as at 508. Guides or ties 430L, R, V are provided as before pivotally connected at 414 to arms 410L, R, V and slidably received in the respective trunion 424L, R, V marked for adjustment an arm 420 in slots 426 as previously discussed for setting vanishing point distances. Respective arms or the like ruler parts 442 are carried from parts 414 and arranged for positioning when in use in parallel relation to ties 430 (as at 442R) by a suitable keying arrangement, and allowing for repositioning clear of the board as at 442L, V, easily achieved by a lift and turn action to key with the pivot part fixed to each tie 430.

As shown in Figures 16 and 17 the board need only be part circular actually truncated to top and bottom in normal orientation. Rotation of the board is advantageous for lining in at a convenient angle and it is preferred to have the board mounted from its central pivot conveniently by a frame work which allows for angular adjustment of the plane of the board as operators prefer to view perpendicular. A stand having a deck chair-like adjustment provision can readily serve for that purpose.

Lastly, reference is made to Figure 15 which is a printed sheet 300 or other surface for association with the top of the drawing (or for display separately if desired) as an aid and guide to usuing mechanisms at least as described for Figures 6 to 8 or 9, 10. The sheet 300 contains material subject to copyrights, and shows a table 310 giving columns (A, B, C) of left, vertical and right vanishing points settings (according to calibrations for trunnion position) to realise the basic cubes drawn out at 320. Those basic cubes also bear, at appropriate faces, standard ellipses available from stencils. Also, there is a set at 330 of cubes showing typical variations from isometric (all vanishing points set to infinity) through table values and table values divided by 2, 3 and 6. In addition, there is an exemplary cube/vanishing points indicator at 340, a line division aid at 50, and an ellipse family at 360.

Claims (20)

1. A perspective drawing aid associated with or for association with a drawing board, comprising a part that is movable relative to the board which movement is relative to a direction representing an axis through an intended vanishing point, means operative during said movement to maintain an attitude of the part substantially constant relative to said intended vanishing Point, and means for adjustably setting the part as to its attitude in accordance with different intended vanishing points along said direction.

2. A perspective drawing aid as claimed in claim 1 in which said part serves as or in conjunction with line defining means.

3. A perspective drawing aid as claimed in claim 1 or 2 in which said part tilts between spaced support positions as it and its support positions move towards and away from said direction during its movement.

4. A perspective drawing aid as claimed in any one of claims 1, 2 or 3 in which said part is pivotal about one support position as it and its support position move towards and away from said direction during its movement.

5. A perspective drawing aid as claimed in any one of the preceeding claims in which said part is in or on a pivot at one end of one of a pair of parallel arms whose other ends are pivoted for movement together relative to positions spaced along the said direction through possible vanishing points, and the said part pivots as the parallel arms pivot in said movement of the part under control of a restraint, tie or guide means that is then at a fixed position along the length of said other arm but permits the distance between the parts pivot and the fixed position to vary.

6. A perspective drawing aid as claimed in claim 5 in which the fixed position along the length of the other arm is adjustable according to the vanishing point distance.

7. A perspective drawing aid as claimed in claim 5 or 6 in which the fixed pivot position for said one or other arm is at an origin point on the drawing board.

8. A perspective drawing aid as claimed in any one of claims 5, 6 or 7 in which said restraint, tie or guide means comprises cooperation between an elongate straight formation fixed relative to the corresponding and pivotted part and coperating with an associated mount to the corresponding said other arm which mount engages the elongate straight formation so as to permit lengthwise movement of the latter and twisting movement relative to said other arm.

9. A perspective drawing aid as claimed in any preceeding claim in which said part constitutes a holder for a ruler in a position to extend over the front of the drawing board pointing towards the intended vanishing point.

10. A perspective drawing aid as claimed in claim 5 or any one of claims 6 to 9 when appendant to claim 5 in which three pairs of parallel arms are provided each for a different direction of selectable vanishing points and each having associated pivoted said parts and restraint, tie or guide means.

11. A perspective drawing aid as claimed in claim 10 in which the three said other arms are branches or legs of a single piece pivoted at its junction.

12. A perspective drawing aid as claimed in claim 11 in which the three said one arms are end pivotted at positions spaced along perpendiculars crossing at the junction.

13. A perspective drawing aid as claimed in claim 12 in which said end pivoting of the three said one arms is to a carrier pivoted at the junction of said other arms.

14. A perspective drawing aid as claimed in any one of claims 5 to 13 in which the pairs of parallel arms are held in the required relation and movably interconnected by way of a flexible drive band cooperating with arcuate segments on such said other arms, rollers, and a wheel on the boards rotation axis.

15. A perspective drawing aid as claimed in any one of claims 5 to 13 in which the pairs of parallel arms are held in the required relation and movably interconnected by way of links interconnecting adjacent said one arms.

16. A perspective drawing aid as claimed in any one of claims 5 to 9 having a single pair of pivoted parallel arms with associated said part and restraint, tie or guide means which is locatable and adjustable to serve for more than one direction of selectable vanishing points.

17. A perspective drawing aid as claimed in claim 16 in which the mechanism is movable bodily about an axis at a notional centre of the board or origin point with its said other arm pivoted to a mechanism carrier itself relocatable by swinging about said centre or origin, and with the parallelism of the pair of arms secured by a link pivoted between the arms as a parallelogram link relative to end pivots for those arms.

18. A perspective drawing aid as claimed in claim 16 or 17 in which means is provided for adjustment of the vanishing point distances by way of a pivoted linkage normally having at least two links, at least one of which is pivoted to the setting for the restraint, tie or guide means so as to move the latter relative to said one arm of the associated pair of arms and also pivoted to the other link.

19. A perspective drawing aid as claimed in claim 18 in which the other link pivoted at the pivot for said other arm and has a free end for fixing for any particular vanishing point setting by way of a readily releasable clamp at such free link end and operative relative to a member rigid with said other arm.

20. A perspective drawing aid constructed and arranged and adapted to operate substantially as hereinbefore described with reference to Figures 1 or 2, or 3 or 4, or 5 to 8, or 9 and 10, or 11 and 12 or 15 and 16.