Hi Friends,
This may actually be better suited for the General forum, I'm not sure. But mods, please move it if necessary.

I've been trying to recreate a black and white line drawing, so that it's perfectly symmetrical side-to-side (but not top to bottom). So I set a Guide line exactly in the center horizontally (a vertical Guide). Well, with no zoom, it's hard to hit exactly the middle So I zoom way in, and then position the Guide right exactly in the middle. And I write down it's position on the ruler, because I need it to make sure I'm positioning the nodes on one side the exact same distance from the center Guide as the other side.

At 1st, I noticed that once I zoomed back out, the Guide no longer looks like it's in the middle -- it looks like it's shifted by a pixel or 2. Nothing dramatic, and I thought it was some sort of optical illusion, or just some display glitch association with zooming and unzooming. But then later, I realized it really HAS moved. So for example, when I'm zoomed in as far as it goes (25,600%), the Guide placed in the middle of the drawing is at 534.28 pixels. Then I unzoom, back to 100%, check the position of the Guide, and now it's at 533.00 px. And if I unzoom out as far as possible (1%), the Guide is at 500.00!

Once I started experimenting and gathering info to post here, I noticed that if I mouse over the Guide, to get its exact position, with my mouse outside the image or page borders, it's exactly one(1) px different from the position revealed by mousing over inside the border. So in the above example, it's 534.00. This is at 100% zoom, and the inside page/image border position vs outside differs by varying amounts, depending on the zoom level....although I haven't figured out exactly the pattern of the inside vs outside border differences. At the highest and lowest possible zooms (25,600% and 1% respectively) there's no difference in the Guide's position taken outside the page/image border. And finally, this 1 px difference I've sometimes found outside the page border, and other times outside the image selection border (marching ants), but not the page border.

So, I'm baffled! The short version of my question is "What gives?" The long version is "Why is this happening, and how can I work around this, to keep my image exactly balanced side to side?" And of course, as always, many sincere THANKS for your kind help, comments and/or advice

Are you really sure you're measuring the position of the guide in a good way? Double click on the guide and youll get a small popup where you can position it accurately.

I also get a little mismatch at some zoom levels but I think thats just a display issue (size and resolution). If I have a guide at 460px sometimes its completely lined up with the 460px mark on the ruler and sometimes its just a little bit to the left, but no matter what zoom level Im in if I double click the guide it always shows 460px.

btw in your example making a symmetric drawing - in addition to the central guide also add 2 more guides that are equally distant from the center one. That way the nodes will snap automatically (don't forget to turn snapping to guides On). For example if your drawing is 600px in width, the central guide can go on 300px and the 2 side guides on 100px and 500px.

Hhhm, interesting....
No, I had never double-clicked, or even single-clicked the Guide, once I had it in place. Didn't even know about it. But, in my few attempts since I just read your reply, it does look like double-clicking reveals the same position no matter how it's zoomed. And that really helps me with the image I'm working on. So thanks for that, prkos

So what about the X and Y coordinates which are displayed in the bottom right corner, beside the Z zoom control? That's what I was using before -- I was putting the mouse pointer on the Guide, so that it turned red, and then looked down at the coordinates. What is their use, if they're not so accurate

...(don't forget to turn snapping to guides On).

OMG, I only wish I could get the snapping to turn OFF. I mean, I understand what you mean about snapping the nodes. I don't find the snapping very helpful though, because it doesn't look like they snap exactly to the guide. Maybe that's because I've been using those same coordinates in the bottom, right corner, though. But I'll find myself trying to position a selection, and it's just impossible to position a selection near a Guide, but not snapped to the Guide, if snapping is turned on. So I've made a habit of turning off snapping whenever I begin a new image. But somehow, it ALWAYS gets turned back on I'm guessing there's some default action that engages the snapping when some particular tool is used. But so far, I haven't figured out how to really disable snapping. So no worries, I couldn't forget to turn on snapping, if I wanted to!

....but if those coordinates in the bottom, right corner are not so reliable, how can one determine the exact coordinates of any given point in an image??

brynn wrote:....but if those coordinates in the bottom, right corner are not so reliable, how can one determine the exact coordinates of any given point in an image??

The problem is that any given point under your mouse does not have an exact set of coordinates. It could if you just had a 1:1 pixel grid, but that would be kind of annoying and limiting. When you zoom in, a "pixel" in the document takes up more than one pixel on screen, so you get fractional coordinates. Likewise, when you zoom out, one screen pixel represents more than one "document pixel".

One way to deal with the problem is just don't be so particular. (This is the method I'm learning to take.) If it looks good, why bother with making sure you're exactly exact. Another method is to use guides, and position them exactly by double-clicking. Or you can set the grid lines to be one pixel apart on each axis, and turning on snapping to the grid.

brynn wrote:So what about the X and Y coordinates which are displayed in the bottom right corner, beside the Z zoom control? That's what I was using before -- I was putting the mouse pointer on the Guide, so that it turned red, and then looked down at the coordinates. What is their use, if they're not so accurate

Just as kelan said above, our monitor screens aren't nearly as powerful in displaying precision coordinates as vector editing software is in computing them. The coordinates you see in the bottom right corner aren't (to the best of my knowledge) meant to be used as a high precision tool, they are just helping in general orientation.

If you need precision you can (and should) use guides and grid (you'll see its not so difficult to learn), use various snapping options, use the width, height, horizontal and vertical position options (you can see them on top toolbar when the main Selector tool is active).
Snapping options can be a bit confusing because you can choose if you want to snap object nodes or the bounding box of an object, you've probably noticed by now that those don't "sit" on the same lines if you have stroke on an object. That happens because Inkscape applies stroke equally on the inside and on the outside of an object, if your stroke is 6px, half of that will be on the outside of the object, and the other half (3px) will cover the object (or should I say its fill). Heres an example where I've applied a gradient to the stroke that gets more transparent as you go vertically downwards, so you can see that the stroke "sits in the middle of the road"


Thats probably the thing that was confusing you when you said it doesn't seem to snap exactly to the guide. You probably wanted the bounding box to snap but instead node snapping was turned on, so half of the stroke ended up on the other (wrong) side of the guide. When you feel a snap you can be quite sure that something really snapped lol, if the result wasn't what you expected it means you need to adjust your (snapping) settings to get the snap type you need. If you can't tell when a snap happens your snap sensitivity is too high (some people would say low) so try choosing the Always snap option until you get the hang of it.

When you want to position an object near a guide but it keeps snapping on it, theres an easy way to do it: just hold SHIFT while moving the object, it disables snapping temporarily. You could also reduce the snapping distance or disable snapping to guides altogether then turn it back on after positioning but Shift is the easiest way.

Every time you create a new document Inkscape uses the same template for it. You can make your own template though and set Inkscape to use that file as your default. I'm not sure if it remembers snapping options though, Ill go test it and let you know. But all in all you should be happy to have a feature like snapping available cos it makes life sooooo much easier (once you learn how to use it )
The place where you tweak snapping is File > Document properties > Snapping

If you really care about the snapping, then please use a recent development build (if you're not already using one). Snapping has improved a lot since v0.45.1. BTW, you cannot turn snapping to grids or guides off, you'll simply have to make them invisible. This is easily done using the "#"-key (i.e. shift-3) and the "|" keys.

Diederik

Oh wow, thank you all for so much helpful info!
I think you're probably right about the snapping to guide vs grid, node vs bounding box, stroked or not...

There is just so very much to learn, especially as a non-graphic artist. In every case I've posted and asked for help, it's turned out that what I thought was something broken or just not working very well, is actually another great feature of Inkscape, that I just need to understand, and learn how to use effectively. Yes, the snapping has been annoying, but after reading all these responses, I realize it's a helpful feature, and I just need to understand how to use it. I will definitely try to make a habit of leaving the snapping turned on, and use Shift to temporarily disable it.

So anyway, thanks again for everyone's help.
I'm moving on to my next lesson!