CapTrack — swept path

All five catalogue vehicles can be run: VP, CO, O, SR and RE. See The vehicle catalogue.

What it does

Typing CAPCAD_CAPTRACK opens the options window. The window is modeless and single per session: typing the command again just brings it back.

The window's Run and Run with the mouse buttons run the same command, which reads the options from the window and then:

1. Asks for the path: a CAD curve (line, arc, circle, polyline, spline, ellipse) or, in Civil 3D with CapCad's Civil 3D module loaded, an alignment. The run starts at the end of the path nearest to where you clicked: to drive the other way, click near the other end. With Reversing ticked, the path is that of the last unit's rear axle. Any other entity is refused with a message and the prompt repeats.
2. Samples the path into points: the spacing is a quarter of the shortest tracking length in the chain, clamped to between 0.10 m and 0.50 m. Arcs and curves are turned into straight segments at this stage.
3. Runs the simulation. The command point advances step by step along the sampled path; at each step every unit is pulled by the tractrix and its body is recorded. A progress window shows the steps and then the building of the envelope; Cancel stops the run and nothing is drawn.
4. Builds the output: the track of each wheel point, the union of all footprints inflated by the clearance, the parts of the obstacles inside the envelope (when obstacles are selected) and the swept width sections (when the Options page has an interval).
5. Bakes the result into the drawing (Run) or hands over the preview that follows the mouse (Run with the mouse). If the same vehicle already ran on the same path, from the same end and the same way (forward or reversing), the previous result is erased and replaced; any other combination adds to the drawing.

The command line reports the outcome: vehicle name, number of steps, how many wheel tracks, how many rings the envelope has and the clearance applied; how many entities of the previous run were replaced; the obstacle check; the largest swept width, with the table of sections; and, when reversing, a warning when the vehicle folds past 90° (jackknife).

The tractrix, in one line

For a rigid unit with command point F (the steering axle of the tractor, or the trailer's hitch) and rear axle R, separated by the tracking length L, when F moves to a new position:

R_new = F_new + L × normalise(R_old − F_new)

The unit's heading is the direction from R_new to F_new. The coupling point sits at a fixed distance from the rear axle, measured along the heading, and it is what pulls the next unit. Repeated along the chain and along the path, that single step is the simulation.

At the start, the whole chain is laid out straight, aligned with the path's initial tangent, each unit set back by its L from the one before. That is why the first step already comes out coherent instead of shaking itself straight over the first few metres.

Workflow

1. Draw (or choose) the polyline or alignment that represents the path travelled by the vehicle's steering axle (when reversing, by the last unit's rear axle), not the edge of the carriageway nor the road centre line, if they do not coincide.
2. Run CAPCAD_CAPTRACK. The window opens floating.
3. Choose the vehicle in Design vehicle (1). The preview (5) draws the chain in plan, with the wheel points; an incomplete vehicle shows there the list of what is missing, and Run (6) is disabled.
4. Enter the Envelope clearance (m) (2) if you want the margin added to the envelope. Zero is the default.
5. To check kerbs, edges, poles or islands, click Select next to Obstacles (3) and select the curves. They apply to every following run, until Clear.
6. Click Run (6) to draw the result straight away, or Run with the mouse (4) to move the vehicle with the mouse. The CapTrack window steps aside and the CAD asks for the path: click near the end it starts from. The chosen path stays loaded (the Path line on the Vehicle page): the following runs use the same path, re-read from the drawing each time (an edit to the polyline counts), the same way round, without asking again. The X beside it frees the path, and the next Run asks for another; a path that was erased or belongs to another drawing is forgotten too. Drive with the mouse and the Learn sample load the path they draw. Whenever CapTrack asks for something in the drawing (path, obstacles, points, the mouse preview), the window steps aside and comes back at the end.
7. With Run, the result is already in the drawing. With Run with the mouse, move the cursor along the path to see the vehicle at each station and click to bake, or Esc to discard.

The form

No screenshot. The window has the CapCad side panel with icons on the left and eight pages: Vehicle (controls 1, 3 and 5 and the loaded path), Options (2, 8, 10 to 13 and 15), Layers (14 and the vehicle code in the layer names), Speed (see Speed), Animation (see Animation), My vehicles (see Your own vehicles), Signage (see Signage: unused pavement) and Learn (videos, samples and the Learning Zone). On Learn, the sample Path with bends and a U-turn draws a path with S-bends and a U-turn (courtesy of George Arakaki) on the driven-path layer and zooms to it; a drawing with no coordinate system yet gets the sample's, SIRGAS 2000 / UTM 23S, so the Animation page's satellite image lines up. Then choose the vehicle, click Run and select the path. Reset erases the sample path. Drive with the mouse (9), Run with the mouse (4), Run (6) and Close (7) stay at the bottom on every page, with an orange Reset link beside them on every CAD: it erases what the last run drew and brings back what it replaced. One Ctrl+Z undoes a whole run. Between sessions the window remembers the vehicle, the type filter, the left and right clearance, the swept width, the vehicle code in the layer names and reversing.

What is drawn

The layers are created through CapCad's layer catalogue (the CAPTRACK family), which gives the colours in the table, or the active style's when it redefines them, and the style's prefix (CAPCAD_CAPTRACK-ENVELOPE with the CapCad style); layers that already exist are reused as they are. With Vehicle code in the layer names ticked on the Options page (the default), the vehicle's code is added at the end, CAPCAD_CAPTRACK-ENVELOPE_SR and CAPCAD_CAPTRACK-WHEELPATH_SR, so each vehicle's paths sit on their own layers and can be frozen, compared or erased apart. Characters a layer name cannot hold are replaced: a space by _, a decimal comma by ., a slash by - (AASHTO P/T gives _AASHTO_P-T).

Each wheel track carries, in the curve's properties, the unit (tractor, semitrailer, fulltrailer, rigid…) and the axle number, so they can be identified afterwards.

Obstacles

Obstacles are curves in the drawing (line, arc, circle, polyline): kerbs, edges, poles drawn as circles, islands. Each run clips every obstacle by the envelope (with the clearance) and draws the parts inside it on the CAPTRACK-CONFLICT layer. The command line says how many obstacles were hit and how many metres of them are inside, or that none touches the envelope. Obstacles selected in another drawing are not checked.

Swept width

With an interval on the Options page, CapTrack draws, every so many metres from the start of the path, a line square to the path from the right edge to the left edge of the area the vehicle swept, without the clearance, and writes the width beside it (text height: a fifth of the vehicle width, at least 0.25 m). The command line gives the largest width, where it occurs and how much it exceeds the vehicle width (the widening), and the table of sections with the distance, the width and the parts left and right of the path.

With Mark the largest swept width, CapTrack measures the width every 0.5 m along the whole path, whatever the interval, and draws the widest section with the text MAX 6.41 m (+3.81 m): the width and how much it exceeds the vehicle width. The command line gives that width and the distance from the start where it occurs. Sections every so many metres can miss the widest point of a bend: for an SR on a 90° bend of 15 m radius, sections every 20 m read at most 6.23 m, and the largest width is 6.41 m, 4 m earlier. On a polyline corner with no arc, the square section would be the corner's bisector and read the width slanted, wider than it is; those points are left out.

The swept width is the body's only, without the clearance. To check that a lane is wide enough for the turn, compare the swept width plus the clearance on each side against the lane width.

Reversing

With Reversing, the path is that of the last unit's rear axle, driving backwards, facing away from the direction of travel. Each unit is solved from the one behind it: the last unit lies tangent to the path, a unit with its fifth wheel over the axle follows the direction its coupling travels, and a unit with the coupling off the axle follows the tractrix at that distance. Nothing limits the steering: a path that asks more than the vehicle can do shows as a large angle between units, and the command line warns when it passes 90° (jackknife).

Drive with the mouse

Drive with the mouse asks for no path. Click the start (when reversing, where the last unit's rear axle is) and the direction the vehicle leaves in; then, as you move the mouse, CapTrack shows the path to the cursor, the wheel paths and the vehicle where it would arrive. Each click fixes a point and the path carries on from there; Undo drops the last point, Enter finishes and Esc cancels. Between the points the path is straight, and each corner becomes an arc of the tightest radius the vehicle makes, worked out from the outer front wheel's minimum radius (when reversing, the last unit's rear axle's). With a Design speed and a Steering lock to lock time on the Speed page, the driver does not turn the wheel at once: each arc gets a spiral (clothoid) in and out, as long as the distance the vehicle covers while the wheel reaches the bend's angle. The faster, the longer the spiral and the more room a corner needs. A point that asks for a tighter turn, or leaves no room for the arc, shows in red and the click is refused with a message. A vehicle with no minimum radius joins the points with corners. When you finish, the path stays in the drawing as a polyline on the CAPTRACK-PATH layer, to edit and run again with the other buttons, and the run is drawn as Run draws it, with obstacles and swept width. Reset erases the path too.

Speed

The Speed page checks the path for a speed. Enter the Design speed (km/h) (0, the default, does not check), the Maximum superelevation e (%) and the Maximum side friction f of the project's norm; CapTrack assumes none of these values, and with a speed an empty friction turns red and the run buttons wait. In Brazil (the Brazil country profile), the page cites where each rule comes from: the smallest radius from DNIT IPR-706, Manual de Projeto Geométrico de Rodovias Rurais, item 5.4.3, and f from Quadro 5.4.3.1, page 71. The command line gives the same source with the smallest radius, and Open in the manual under the table opens DNIT's manual at that page. f from offers Tabela do DNIT: f comes from Quadro 5.4.3.1 of DNIT's Manual de Projeto Geométrico de Rodovias Rurais for the design speed, and the field is locked. Between two of the table's speeds the next one up is used (65 km/h takes 70 km/h's f, the safe side); outside 30 to 120 km/h the table has no value, so the field goes back to typed and stays red until filled. The table shows at the bottom of the page, for reference, with the column in use highlighted. In other countries f is always typed. The page shows the smallest radius, R = V² / 127 (e + f). Each run reads the path's radius every 0.5 m, through the circle of three points 3 m apart, and draws on the CAPTRACK-SPEED layer the stretches tighter than the smallest radius; the command line says how many there are, where each runs (in metres from the start), its tightest radius and the side friction that radius needs, f = V² / 127 R − e, next to the f entered (for example radius 180.0 m, needs f = 0.22, the norm allows 0.15), or that no bend is too tight. On a path of lines with no arcs, each corner counts as a tight bend, because that is how it is driven.

Steering lock to lock (s) is for Drive with the mouse: the time the driver takes to turn the wheel from one lock to the other (0 = instantaneous). With it, each corner gets its spirals in and out; it is off with no speed.

Superlargura (widening)

In Brazil, the Speed page also works out the widening (superlargura) of each curve of the path, by item 5.4.4 of DNIT IPR-706 (two-lane road): S = LT − LB, with LT = 2 (GC + GL) + GBD + FD. Enter the Two-lane road width on the straight LB, the whole road, for example 7.20 m (0, the default, does not work it out), with a design speed. A width that cannot take two vehicles as wide as the chosen one turns red and the run buttons wait. The Lateral clearance GL comes from the GL table on page 76 by the width (6.00 to 6.40 m: 0.60; 6.60 to 6.80 m: 0.75; 7.00 to 7.20 m: 0.90; between two rows the next one up) and is locked; outside 6.00 to 7.20 m it is typed and stays red until filled. The dynamic clearance FD = V / (10 √R) is how speed enters the widening: an extra margin, not a change of trajectory.

Envelope clearance = GL/2 on each side applies Figura 5.4.4.1 (page 75) to the vehicle alone: each vehicle's lane is GC + GL, with half of GL on each side of the body. Ticked, both clearances on the Options page become GL/2 (0.45 m on a 7.20 m road), are locked with the note DNIT GL/2 (Speed page), and the envelope shows the space DNIT requires for the vehicle. Unticked, the typed clearances come back. The clearance applies along the whole path, not only on the curves.

Each curve (radius under 1000 m) gets two values:

• by DNIT's formula, with the chosen vehicle's dimensions: GC = LV + E²/2R (for an articulated vehicle E² = E1² + E2², the note of item 5.4.4, adding the pivot-to-pivot lengths when there are more units) and GBD = √(R² + BD(2E + BD)) − R;
• by the simulation, the same formula with GC and GBD measured in the run: GC = LV + the largest distance of the last unit's rear axle from the path, and GBD = the largest distance of a front corner from the path less half the width, over the curve and the vehicle's length after it.

Both are rounded as in DNIT's tables (checked against Quadro 5.4.4.1): to the centimetre, not needed below 0.40 m and, above that, to the nearest multiple of 0.20 m. The command line gives, for each curve, where it runs, its radius, both values with their GC and GBD, and FD, citing DNIT's item; on the swept-width layer, each curve gets a short tick across the path where it starts and another where it ends and, at its middle, a section square to the path, as long as the total width the curve needs (LT, by the simulation), centred on the vehicle's swept strip, with the text Superlargura 1,00 m (fórmula 1,00 m) and, below it, the breakdown: LT 8,14 m = LB 7,20 + S 0,94; GL 0,90, GC 2,93, GBD 0,03, FD 0,45. The section shows the width and where it applies; the road edges are not drawn, because CapTrack knows the vehicle's path, not the road's centreline or edges. The section texts (swept width, the largest width and the widening) always read the right way up. When reversing the widening is not worked out. On a path of lines with no arcs, each corner reads a radius that is far too small: use a path with arcs.

Signage: unused pavement

The Signage page finds the pavement no design vehicle goes over, to zebra-mark: the closed areas between the road edges and the envelopes in the drawing.

1. First run every vehicle that uses the place (each movement: turning right, left, straight on). The envelopes count all together, every vehicle's, read from the drawing's envelope layers; a run drawn as a block does not count.
2. Under Road edges, click Select and select the kerbs and edges (lines, arcs, polylines) and the lines that close the studied area, stop lines for example, or any line across the road. Without them, the gap between the edge and the envelope runs on along the straights to where the edge ends and closes no area. Clear forgets the selection.
3. Click Find the areas. Each area comes out as a closed polyline on the CAPTRACK-ISLAND layer; a new search replaces the previous one, and Reset erases it. The command line gives how many areas and the total in m².
4. To zebra-mark, click Zebra with the assistant and select the area: the zebra assistant picks the marking (area, two or three flows).

Smallest area (m²) drops smaller areas (default 1.00). Smallest width (m) trims off the parts of an area narrower than it (default 1.00): the narrow gap between the kerb and the envelope along the straights, which nobody paints, goes, and the corner area stays. Use the envelope with the clearance the marking should keep, DNIT's GL/2 for example.

Where an envelope runs over an edge (a conflict), the part beyond the edge can come out as an area; fix the conflict first.

Animation

The Animation page shows the last run in CapCad's viewer (drag to pan, wheel to zoom, left-drag for a zoom window): the path dashed in grey, the lines drawn in each output's colour and the vehicle, in magenta, at the current position, with the distance travelled at the bottom. Play drives the vehicle along the path and becomes Pause; the slider moves the vehicle by hand, forward and back. Speed goes from 0.5× to 8×, 1× being 10 km/h. With In the drawing too ticked (the default), the vehicle moves in the drawing as well, as temporary graphics that are not saved; it leaves the drawing when the window closes or on a new run. Satellite image puts the imagery underneath, as in the road-width previews; it needs the project's coordinate system (CAPCAD_GEO_CRS). Fit shows the whole run again.

Record video... records the whole run to a 1280×720 MP4 at 25 frames a second, at the chosen speed, framed as the viewer shows it and, when ticked, with the satellite image. The video is made by the encoder that comes with Windows, no extra program, and is limited to three minutes. A progress window shows the frames; Cancel deletes the file it started.

Start and end of a run

With Whole vehicle on the path at the start and at the end ticked (the default), the run starts once the whole vehicle is on the path: driving forward, with the last unit's rear axle at the start of the path; reversing, with the first unit's front axle there. It ends with the axle that follows the path at its end, the vehicle still on the path. So the start and the end look alike. Unticked, the vehicle starts straight, with the axle that follows the path at its start and the rest of the vehicle behind it, off the path. A path shorter than the vehicle runs whole and the command line says so. Remember too that a run starts at the end of the path nearest to where you clicked: clicking near the other end reverses it.

The vehicle, as CapTrack describes it

CapTrack does not know what a truck is: it knows a chain of rigid units, each with four pieces of data.

When the vehicle comes from an engineering catalogue, those four things are derived from the manual's dimensions:

Every unit that pulls another gives its coupling in couplingOffsetFromRearAxle (a tractor may use the older name, kingpinOffsetFromRearAxle); without it the vehicle is (incomplete) and the preview names the missing field. So a B-double, a bi-articulated bus, a tractor + semitrailer + trailer or a road train are just more units in the list. A drawbar trailer is described as a dolly (the drawbar and front axle) followed by a semitrailer (the body, turning on the dolly).

Half the wheel track is half the width minus 0.15 m, because the source gives the body width and not the axle track.

The vehicle catalogue

The default catalogue ships with the product. A catalogue placed in a CapTrack folder in the CapCad installation folder takes precedence over it.

It carries five design vehicles, with the overall width and length from the geometric-design tables of the norm it was built from:

All five have their per-axle geometry (wheelbase, overhangs, kingpin) from the dimensioned vehicle drawings of the norm. On SR, the kingpin position and the front of the semitrailer are not dimensioned in the norm and were measured at the drawing's scale; the dimension printed as 7.90 was taken as 8.00 so that the chain adds up to the printed 16.80 m. RE follows its drawing's chain (0.90 / 3.35 / 6.05 / 2.85 / 6.05 / 0.60 = 19.80): a tandem tractor, a semitrailer and a drawbar trailer with its own front axle. What the drawing does not dimension was measured at its scale: the kingpin over the tractor's rear tandem, the semitrailer front 1.55 m ahead of the kingpin, the drawbar eye 0.65 m behind the semitrailer axle (a 2.20 m drawbar) and the trailer front 0.60 m ahead of its front axle. Simulated at the norm's radius, RE gives an inside radius of 6.97 m, against the norm's 6.90 m. Each vehicle also carries the norm's minimum radius of the outer front wheel (7.30 m for VP, 12.80 m for CO and O, 13.70 m for SR and RE), which draws its turning template.

An AASHTO catalogue follows, with 19 design vehicles of the 2004 Green Book (metric): P, SU, BUS-12, BUS-14, CITY-BUS, S-BUS11, S-BUS12, A-BUS, WB-12, WB-15, WB-19, WB-20, the double and triple combinations WB-20D, WB-30T and WB-33D, MH, P/T, P/B and MH/B, each listed as AASHTO <symbol>, with the Green Book's dimensions and minimum design turning radius. Tandem axles are taken at their centre. In the doubles and the triple, each rear semitrailer rides on a converter dolly hitched to the rear of the semitrailer ahead. Simulated through a 180° turn at that radius, every one of them passes within 0.25 m of the Green Book's minimum inside radius, except WB-33D, which gives 4.21 m against 4.5 m. On WB-20D the printed overall length (22.4 m) does not match the sum of the figure's own dimensions (22.03 m); the geometry uses the dimensions.

A further catalogue carries specific manufacturer vehicles, listed after those: in this version the Mercedes-Benz OF 1726L/59 bus at 11 m (body dimensions from a bus builder's turning template, wheelbase 5.95 m from the manufacturer's data sheet) and at 13.2 m (the same wheelbase, the extra length split between the overhangs in the same proportion; a first estimate that can change), both with a 9.80 m minimum radius (half the data sheet's 19.6 m kerb-to-kerb turning circle).

There are also two samples, with provisional dimensions:

Sample: VP matches the overall dimensions of the catalogue's passenger car (2.10 × 5.80 m); Sample: SR does not match the catalogue's articulated vehicle. Both are provisional.

Your own vehicles

The My vehicles page adds vehicles that are not in the catalogue, for example a specific bus or truck from the manufacturer's drawing. New clears the fields; Copy from... lists the catalogue vehicles and starts a new vehicle with the chosen one's dimensions, named "copy of ...", to change and save (every vehicle in the list; the dolly of a drawbar trailer, as in RE and the AASHTO doubles and triple, becomes the drawbar of the semitrailer behind it; a vehicle whose units differ in width comes with the widest). Type a Name, choose the Type (Rigid or Tractor + semitrailers) and enter the width, the front overhang, the wheelbase and the rear overhang. For a tractor with semitrailers, also the kingpin position (ahead of the tractor's rear axle, negative when behind it) and the Number of semitrailers, 1 to 12, with no B-double limit: a sugar-cane road train with as many semitrailers as the tractor can pull. In Semitrailer no. choose which semitrailer the fields below show: the drawbar ahead of it (a drawbar trailer on a dolly; 0 for a semitrailer on a kingpin), front to kingpin, kingpin to rear axle, rear overhang and where the next one's hitch is, behind the axle (negative when ahead of it; off on the last one). Raising the number copies the last semitrailer. On a bi-articulated bus, each joint is a "kingpin" and each car a semitrailer. These fields stay off for a rigid vehicle. The outer front wheel minimum radius (half the kerb-to-kerb turning circle of the data sheet; 0 when unknown) draws the turning template; a radius too small for the wheelbase turns red. Mirrors beyond the width is how far the mirror's tip sticks out past the side of the body (0 = no mirrors) and Mirrors: distance from the front where it sits behind the vehicle's front; the latter is off with no mirrors. Enter the dimensions of the finished vehicle, body included, not of the bare chassis: the envelope is drawn from the body. When the fields do not fit the window's height, the list scrolls.

The page shows the overall length and draws the vehicle as you type. A missing name, a name already in the list, or a wheelbase of zero turn the field red and leave Save off, with the reason under the fields. Saved vehicles appear in the Design vehicle list after the catalogue, on every CAD, for the Windows user who saved them (they are kept in %LocalAppData%\CapCad\CapTrack\vehicles.user.json). Delete removes the selected one after a confirmation.

A catalogue of your own can also be written in JSON, either in the engineering format (manual dimensions, derived by CapCad) or in the direct format (tracking length, outline and wheel points already worked out). Both schemas ship in CapTrack/data/ in the product, and the direct format is the quickest way to add the design vehicle of your own norm: enter its tracking length, coupling offset, body outline and wheel points.

How the options combine

The simulation step is not the path sampling spacing. They are two different quantities, both derived from the shortest tracking length in the chain:

For Sample: VP (L = 3.40 m), the path is sampled every 0.50 m and simulated every 0.17 m. In other words: the simulation is fine, but it walks over a path that has already been turned into half-metre chords. On a short, tight curve it is the sampling that limits fidelity, not the step.

The discrete tractrix cuts corners slightly compared with the continuous solution — the larger the step, the larger the cut. Since neither the step nor the sampling is configurable in the window, the result is what it is: a representative envelope, fit for checking manoeuvring space, and not a metrological instrument.

The clearance (2) inflates the envelope only. The wheel tracks always come out in their real position.

The interactive preview

With Run with the mouse, the whole simulation has already been computed when the preview starts. Moving the cursor recomputes nothing: the command projects the cursor onto the path, converts the station into a step number and redraws the vehicle outline at that step. That is why it stays fluid even for a combination.

• The wheel tracks appear in cyan and stay still.
• The vehicle outline appears in yellow (ACI 2) and follows the cursor.
• All of this is transient graphics: it is not in the drawing, it does not join a selection, it is not plotted.
• The click bakes — and it bakes the whole result, the same one Run would produce. Where you stopped the cursor changes nothing about what is drawn.
• Esc discards, and the command line reports the cancellation.

Remembered between sessions

The window writes %LocalAppData%\CapCad\CapTrack\settings.json on each run and when it closes: the vehicle, the type filter, the left and right clearance, the swept width, the vehicle code in the layer names, reversing, the whole vehicle on the path, the continuous envelope, the hatch, the block, the largest-width mark, the Draw lines, the Speed page's fields and the layer names you changed, which come back the next time it opens. The obstacles are not remembered.

The vehicle catalogue is re-read every time the window opens.

Notes

• Replacing applies to the same run. The same vehicle, on the same path, from the same end and the same way, erases the previous result; another vehicle, the other end or reversing adds to the drawing. Each vehicle's layers stay apart when the vehicle code is in the names.
• The path is the route of the steering axle, not the edge of the carriageway nor the road centre line. A path taken from the centre line makes the whole vehicle run offset.
• It is a low-speed model. There is no tyre slip and no speed-based widening: the wheels' paths are the same at any speed. The Speed page checks the path's radius and, in Drive with the mouse, the steering, but does not change the envelope. For checking manoeuvres at junctions, roundabouts, yards and accesses it is the right model. At speed, DNIT's norm treats the effect as a margin (the widening's dynamic clearance), and that is how CapTrack takes it; the rear's drift at high speed, which would depend on each vehicle's axle loads and tyres, is not worked out.
• The simulation is planar. The path is read in X and Y only — a 3D polyline is flattened.
• Every command-line message is in English, in any product language: the path prompt, the incomplete-vehicle warning, the final summary. The window's own labels are translated.
• The window opens centred on the parent window and may appear over the drawing area; drag it aside before picking the path.