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| The presence of layers is one of the
basic features of any true CADD program. Generic CADD's implementation of 256
layers which can be turned on and off, and it's ability to edit on the single
current layer or all layers visible on the screen is simple, elegant, and
powerful. Unfortunately, there is often a tendency for the use of layers to be limited to the capabilities of their hand-drawing counterpart, physical overlays. This month's column introduces another way of thinking about layers, as a simple means of manipulating sets of entities. Once you leave behind the idea that a layer is like a sheet of paper and think of it more as a method for tagging entities with a specific code, defining groups rather than levels, the usefulness of layers multiplies. The Change commands offer ample opportunity to take advantage of this way of thinking about layers. Some specific examples: Suppose that you have just filleted all of the corners of a complicated shape with arcs of the wrong radius. Instead of Object Erasing each arc individually, use the Window Change command to banish all arcs on the current layer, of the proper color and line type to Layer 99 and then erase Layer 99 with a Layer Erase command. You want to copy or move several objects by the same distance, perhaps rotating them when they get to the new location, when both their current and new locations are crowded with other entities. First use the Object Change and/or Window Change command to put the desired entities onto their own layer. make that the current layer, and use Window Move, Copy, Rotate, etc., on them with All Layers turned off. As a layer, they can be picked out from a crowd and manipulated however you like. You can also use any of the Layer editing commands on them, of course. The two examples above should give you some ideas of your own about using layer numbers as tagging devices. Once you begin doing this, you may want to make use of another often overlooked layer characteristic. You know that you can edit either All Layers or only the Current Layer. But what if you want to edit all BUT one or two layers, leaving selected entities intact. You can hide these layers, of course, but you might want to still see them for visual reference, and you have to wait for the Redraw anyway...or do you? No. Although nothing may appear to happen when you Hide a layer until you Zoom or Redraw, in fact something very significant happens. Generic CADD's editing commands ignore the hidden layer, even though it is still visible on the screen. Voila! Is Layer 7 in the way? YH7, perform the edit, then YD7. No mess, no fuss, NO REDRAW. As a reminder, now that you've got layers working for you more effectively, don't forget about the other Layer commands that you may not be using. Layer Save is great for moving selected information out of one drawing and into another via Drawing Load or Layer Load. Layer Load is nice for loading a drawing temporarily for reference (I like Layer 99 for this purpose), then erasing it with a Layer Erase once you're done with it. |
