This is a program designed for experimenting with constructions in classical Euclidean geometry, in particular with the restriction to compass-and-straightedge as tools. This is a document describing it from a user's perspective. If you want to build (and possibly install) it, see INSTALL. This document describes how it's supposed to work; if it doesn't describe how it actually works, it is probably a bug in the code, though it could conceivably be a mistake in this file. When you start it, you'll be faced with an empty window except for two things: (1) a crosshair which tracks the mouse, when the mosue is in the window, and (2) eight small `tool' buttons in the upper left corner, with digits on them. The program is designed for performing Euclidean geometric constructions under the classical restrictions, meaning that the only tools allowed are compass and (unmarked) straightedge. Correspondingly, the only constructions supported are - Draw a line between two points. - Extend a line segment, drawing more segments along the same line. - Draw a circle, or arcs forming parts of a circle. The first four tool buttons correspond to the above operations. Tool 1 draws a line segment between two points. Tool 2 extends a line segment, adding additional collinear segments of it. Tool 3 draws a full circle, given its centre and a point on its circumference. Tool 4 draws arcs, parts of a circle. Tool 4 is unnecessary from a theoretical point of view; it exists to allow constructions to stay relatively uncluttered by extraneous circles when only a few small arcs of them are relevant. (For example, to construct the perpendicular bisector of a line segment, the usual construction uses two small arcs of each of two circles; you could draw the full circles instead, but the result looks significantly more cluttered than necessary.) The other four tool buttons perform other functions. Tool 5 allows you to set your compass for a given distance, allowing you to (for example) have points A, B, and C, and easily draw a circle with centre C and radius equal to the distance A-B. Tool 6 zooms in on the diagram, tool 7 zooms out, and tool 8 pans around. For each of the eight tools, you can type the number on the button or you can click on it; the descriptions will speak of clicking on them for simplicity of language. Here's a brief sketch of what the various tools do. Detailed descirptions follow. Clicking on tools 1, 2, 3, or 4 simply selects that tool for use. Clicking on tool 5 lets you select two points, then locks your compass setting to the distance between them until you click tool 5 again to unlock it. This affects any use of tool 3 or 4. Clicking on tool 6 zooms in on the diagram. If you use button 1 (the left button, usually), the zoom is by a factor of about two to one (precisely, 59049 to 100000); if you use button 2, the factor is about three to four (precisely, 729 to 1000); and button 3, 9 to 10. The zoom always leaves the middle of the window unmoved. Typing 6 is equivalent to clicking with button 1. Clicking on tool 7 zooms out; it accepts the same buttons, and uses the same zoom factors, as tool 6. Clicking on tool 8 starts panning. This highlights tool 8; then, clicking in the drawing area starts dragging; clicking again stops dragging. Clicking on tool 8 again stops panning, clearing the highlight and returning to normal operation. Stopping panning with button 1 preserves the panning you've done; using any other button cancels, panning back to where you were when you clicked on tool 8. There is one feature which is important to understand. Under most circumstances, there is a snap-to feature; if the mouse is close enough (about 20 pixels) to a relevant feature of the drawing, the full-window crosshair will jump to the feature, deviating from tracking the mouse cursor. If there are multiple features within range, it snaps to the one which is closest to the mouse cursor, with priority given to points over lines and circles/arcs. If this becomes a problem, for example if you want to draw to an intermediate point on a line segment which is so short that you snap from one end to the other, simply zoom in until the feature you want to snap to is distant enough from the feature that's causing trouble. This snap-to is context-sensitive. For example, when using the extend-a-line-segment tool and selecting a line segment, only line segments and their endpoints will attract the crosshair in this way. When tool 1 is selected, click on one point to start a line segment and another point to draw the segment between them. When tool 2 is selected, click on a line or a line endpoint to extend the line. (If a point is an endpoint of multiple lines, it is effectively unpredictable which line will get extended when selecting that endpoint.) Then the extended line will be displayed dotted, with a short dotted crossing line which moves with the mouse; clicking will start drawing from the point of intersection, animating another short crossing line. After clicking a second time, the selected segment is added. (If it overlaps or touches existing segments on the same line, they are merged.) When tool 3 is selected, click to select a centre, then click to select any point on the circumference, animating a dotted circle (or, if tool 5 is active, click anywhere to start dragging a dotted circle, then click on the centre to draw the circle). When tool 4 is selected, it acts just like tool 3, except that, instead of drawing a circle, it just draws the dotted circle with a dotted line extending from the centre to the mouse cursor; clicking, moving,a nd clicking again draws an arc. To stop drawing arcs, click with any button other than button 1. To use tool 5 to lock your compass setting, click on it. This highlights its corners. Then select two points in the drawing area; after selecting the second, tool 5 will fully highlight and your compass setting will be locked. To unlock it, click on tool 5 again. If you want to cancel the setting before locking it, click on the partially-highlighted tool 5. To zoom in or out, just click tool 6 or 7. Zooming always leaves the centre of the window unmoved. To pan around, click tool 8. This will highlight it. Then click on the drawing area. The large crosshair will disappear and you can drag the drawing around; click again to drop it. When you're done panning, click tool 8 again to unhighlight it and go back to ordinary operation.