Hi, I am trying to draw a staircase in 1pt perspective: so I created a parallel vertical and horizontal ruler, a vanishing point on the horizon and an uphill vanishing point. When I would like to draw a line from the corner of the step to the uphill vanishing point, krita draws a line to the vanishing point on the horizon . The freehand tool is set to 1000 snapping and the option snap single is checked.
Please help .
Thanks in advance

Is this in a situation where the Assistant Preview lines are nearly parallel? If so, that can be tricky but I've found that if you put a definite bias toward one of the lines then it usually snaps to it.
A workaround would be to use the Line tool and use the Assitant Preview lines as guides rather than snap targets.
Can you post a link to a screenshot of the situation?

I don't know how to upload an image: there is only the option to upload an image from a website. I don't even know how to put a definite bias towards one of the lines.
Let me know how to upload an image on this forum.

For a full screen image, you need to upload it to a site like Imgur.com (registration required but it's free). It will give you a link address for the image that you uploaded then you can copy/paste that link to your topic post here.

I' ve made this post on imgur:
https://imgur.com/a/k7CLxPr
In that post there is the screenshot and the image I would like to create.

That is a tricky one. With that arrangement I have it going to either of the vanishing points or either of the parallel rulers depending how carefully I move the cursor when I start drawing a freehand line. I can make it go to the uphill vanishing point if I'm careful but it does seem more like luck than judgement.

One thing you can do is temporarily disable the parallel rulers by selecting the Painting Assistants tool and clicking on their 'eye' icon. That way, there will only be a choice of the two vanishing points and you can bias your paint stroke towards the one you want. In the case of the uphill vanishing point, that would mean starting to draw the line vertically and then it should stick to the uphill assistant line. That seems to work if I try it.

The other thing to try is using the Line Tool and position the end points by using the assistant lines to guide their placement.

Your image seems to be quite a low resolution and that might have an effect on the accuracy of attempts by krita to work out where the stroke is supposed to go. If you scale the size up by 500% that may improve matters but then you'd lose the positioning of the vanishing points. You can easily delete them and remake them and I'd suggest that you try using a higher image resolution to see if that improves the situation.

Your image seems to be quite a low resolution and that might have an effect on the accuracy of attempts by krita to work out where the stroke is supposed to go. If you scale the size up by 500% that may improve matters but then you'd lose the positioning of the vanishing points. You can easily delete them and remake them and I'd suggest that you try using a higher image resolution to see if that improves the situation.

My canvas is A3 at 300ppi. Is it a low resolution?

300 ppi is not a low resolution for printed work. I was basing my assumption on the small image you posted which has drawn lines showing obvious 'jaggies'. Are you working in a small part of the overall image? If so, my comment about accuracy of calculation would still hold.

The image I have posted is small because of the zoom level to show you where there are the painting assistants.
I don't think I am working on a small part of the image, (maybe I am wrong), you can see from the image I have posted on imgur the dimension of the first step (the rectangle with grey lines)

Based on what I can see on your image from Imgur, which is all that I can see, that step looks to be about ten pixels high. Maybe I'm wrong. I don't know because I don't have the original .kra file open in krita.
Anyway, I've done all I can do and said all I can say. I wish you luck in sorting this problem out.

It is 220 pixels high. If you tell me how to upload the .kra file you can see better the problem.

Anyhow, with the line tool I have no problems, so I will use this tool.

Indeed you don't need these two parallel rulers.
If you want do use Brush tool you can hold [V] to temporarily switch to Line Tool and then [SHIFT] to make horizontal/vertical lines easier.

You can also try change the direction of the stroke.