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Thread: Xline placement behaving like SNAP, but SNAP off

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    Default Xline placement behaving like SNAP, but SNAP off

    Hello everyone,

    As the subject states, I'm having an issue with xlines.

    Basically, they are behaving as if the SNAP setting is on, but SNAP is off. I don't notice this at first, but upon zooming in (way in), I notice that the line will move only in certain increments. This only occurs after I type the command and before the actual placement of the line.

    I've attached an image that illustrates what I mean, with blipmode on so you can see where I clicked. I placed 6 red lines that look to be spaced equally (vertical column of blipmode points). Then I placed 3 more pink lines all snapped to the top one (horizontal blipmode points). These pink lines seem to be offset at random distances from the target line.

    The yellow line on top is a current action of a horizontal xline placement. My cursor moves smoothly like normal, but the yellow line will not follow until the next increment.

    I thought perhaps it was just that particular file, so I opened an unrelated file and same issue. I thought just my computer, but I observed again on multiple systems. I'm a bit OCD on making sure every point is accurate, so I'm always zooming in to this extent and fixing these minute differences.. but this is the first time I've ever noticed this. Happens on all xlines (hor, ver, ang).

    Can anyone educate me on what's causing this behavior? Autocad 2006.

    Thanks in advance!

    Bryan
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    Certified AUGI Addict cadtag's Avatar
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    Default Re: Xline placement behaving like SNAP, but SNAP off

    just a thought. but did you try to REGEN after you zoomed way in?

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    Default Re: Xline placement behaving like SNAP, but SNAP off

    Thanks for the reply.

    Yup, I zoom in as far as I can, regen, and repeat, until I can't zoom in any further.
    (I know it's kind of ridiculous to worry about these little differences at such a level of zoom. But I use xline quite a bit, and I don't want tons of tiny gaps scattered throughout)

    Once I place the xline, moving/copying it to the desired location still works normally. The problem only occurs when placing an xline from its command.

    Thanks.

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    Default Re: Xline placement behaving like SNAP, but SNAP off

    I did a similar test, only I left the base point the same and just added different angles. Notice that they don't appear to intersect at the same point. Also, note that the one I'm hovering over highlights at an offset. However, if I select them, the grips all show at the proper locations and properties confirms that the base point is the same for all lines. I think its just the graphics display's inability to accurately display such small tolerances.
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    Default Re: Xline placement behaving like SNAP, but SNAP off

    So in your drawing, is the xline you're hovering over actually just a single line? and the highlighted line only pops up once you hover over said line? I've seen this happen to xlines also. In this case, it wouldn't bother me so much since there is still only 1 actual snap point (please correct if I'm wrong). From what I can remember though, the xline would highlight normally when hovering, but would offset itself a small distance away once I clicked on it.

    In my drawing though, I can still snap to each of those lines. Although, like you pointed out, the properties box does show all the lines existing within one point (see attached). Which means I'm probably zoomed in WAY too much. I guess I'll just have to limit this in the future when I check for accuracy.

    Funny thing, just checked for this in another drawing and didn't notice it.

    xline6.png

    Thanks.

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    Default Re: Xline placement behaving like SNAP, but SNAP off

    Quote Originally Posted by lucid.balloon472536 View Post
    Thanks for the reply.

    Yup, I zoom in as far as I can, regen, and repeat, until I can't zoom in any further.
    (I know it's kind of ridiculous to worry about these little differences at such a level of zoom. But I use xline quite a bit, and I don't want tons of tiny gaps scattered throughout)

    Once I place the xline, moving/copying it to the desired location still works normally. The problem only occurs when placing an xline from its command.

    Thanks.

    Ok, that probably identifies the (non)problem you are seeing. That series of steps, zooming in to an infinitesimal distance, is probably exceeding the number of decimal places the computer (and hence AutoCAD) uses for its calculations. Floating point math uses a finite number of places, and if the attempted precision in your scenario exceeds that, then the results will be 'unexpected'. IIRC only 16 places are used, so that outs a hard limit on how precise the comouter can be.

    When angles are involved, the rounding becomes more 'interesting', as the computer does not actually use 90 degrees as a right angle. angles are calculated in radians, a unit which is based on the transcendetal number pi. ALL DISTANCES AND MEASUREMENTS ARE APPROXIMATIONS.

    Being concerned about nano-angstroms when the thing being designed is built in meters is not useful. Just because you can zoom in to an immeasurably small distance, doesn't mean you should.

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    Default Re: Xline placement behaving like SNAP, but SNAP off

    Quote Originally Posted by cadtag View Post
    Ok, that probably identifies the (non)problem you are seeing. That series of steps, zooming in to an infinitesimal distance, is probably exceeding the number of decimal places the computer (and hence AutoCAD) uses for its calculations. Floating point math uses a finite number of places, and if the attempted precision in your scenario exceeds that, then the results will be 'unexpected'. IIRC only 16 places are used, so that outs a hard limit on how precise the comouter can be.

    When angles are involved, the rounding becomes more 'interesting', as the computer does not actually use 90 degrees as a right angle. angles are calculated in radians, a unit which is based on the transcendetal number pi. ALL DISTANCES AND MEASUREMENTS ARE APPROXIMATIONS.

    Being concerned about nano-angstroms when the thing being designed is built in meters is not useful. Just because you can zoom in to an immeasurably small distance, doesn't mean you should.
    You're right, of course, and I'm fine with that; my intent was not to worry about measurements on a molecular scale, but to find an explanation for the observed behavior.

    The precision limit explains why the properties box shows several different lines having the same coordinates, but it still doesn't explain why the xline can only be placed in increments vs. anywhere in space (like all other objects I've tried).

    Take a look at the attached (xline8 ). I started with red line. I tried placing a yellow xline directly on top, and as you can see it was placed offset from it rather than directly on top. Yet I can draw a yellow line directly on the red xline without problem. Again, like you said, it's ridiculous to concern oneself with minute distances like these. I simply wanted to know if there was a reason for this behavior that I was unaware of.

    And just for kicks, see xline9. Not a single xline lands on top of my target. I was using nearest to snap, but the same result happens using mid/endpoints, although to a lesser degree (xline10).

    Thank you for the replies, I appreciate it.
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    Default Re: Xline placement behaving like SNAP, but SNAP off

    You may want to have a look at this thread.
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