# Thinking of writing a perspective tool for krita

Brendan Scott disposableemail at apps.opensourcelaw.biz
Sat Apr 25 08:42:21 UTC 2015

```[resend with attachment removed and hosted at http://www.tiikoni.com/tis/view/?id=bfd5b0a]

On 25/04/15 00:59, Wolthera wrote:
> I see.
>
> The reason the vanishing point assistant was set up the way it was is
> because of the way I understand perspective: All parallel lines share a
> single vanishing point, and making the horizon is actually a lot easier
> than you'd think: Just turn on snapping and draw a line from one vanishing
> point to the other. (Though shift+snapping is on my todo list for the
> parallel lines :) )
>
> As far as I know I wasn't eyeballing the distances: You can check these by
> drawing diagonals, and maybe I haven't drawn enough diagonals in the video
> itself. It was my third take or something, but I did take into account the
> diagonals.
>
> But, in regards to that link: let me make your life a whole lot more easy:
> A perfect circle in perspective is always an ellipse. An ellipse in
> perspective in turn becomes an egg-shaped ellipsoid. What's more, any
> perfect circle fits perfectly in a perfect square. What this means is that,
> if any given perfect circle is an ellipse, we can draw our lines around it,
> then we have a perfect square! I'm attaching an image with this in Krita.
> Sounds a lot easier than following that link's guide for a perfect square
> each and every time, doesn't it?

I'm sorry. I don't follow. If I have a length that I want to transfer from
one axis to another, I don't see how an ellipse will help.

For example, assume that you _only_ have edge G (and the two vanishing points
and vanishing lines) in the attached image.

How could you use an ellipse to create edge A or E (and therefore implicitly C/B/D/F)?
In particular, there are multiple ellipses which are tangent to both vanishing lines.
Even if I put one of the axes of the ellipse passing through both the the centre of G
and (when extended) through the vanishing point.

See: http://www.tiikoni.com/tis/view/?id=bfd5b0a
(expires in 28 days)
There is no basis for distinguishing between the ellipse on the left hand side of edge G and that on the right hand side. Is there?

This is my use case:

At the moment I'm trying to draw a person in perspective and I want to do the
perspective properly basically to try to train my eye for the right perspective
proportions.

The person I want to do has "heroic" proportions - which means they're 9 heads
tall and 2.66 heads across their shoulders. The length of the head is the yardstick.
I am doing them from waist to head, so I need to map out a prism: 4.5 heads
high, 2.66 heads wide (ie across shoulders)  and 1.4 heads deep (roughly front of chest to
back of backside).

I did this on a piece of paper then scanned it into Krita. I'd like to do it
natively within Krita.

> Similarly, you can use the perspective transform to deform any given
> imagine to a perspective plane(you can even drag around the vanishing
> points in the perspective transform for precision).(This isn't cheating,
> Krita isn't a videogame)
>
> You can use the distance measure tool to measure pixels. In the future I
> hope to get a concentric circle assistant working, which you could then use
> to draw a perfect circle around each vanishing point so you don't have to
> go and measure things.

Yeah, I think my build didn't work properly. That block of tools (crop, move,
transform, measure) doesn't work for me. My line tool is also laggy.

> I hope this e-mail doesn't sound flippant, but I want to make your life
> easier, or at the least bearable till I have time to work on the assistants
> again. :)

Not at all, but unfortunately I don't follow your explanation.

```