Is it possible to center the original part of the canvas when enlarging the image without knowing the previous size?

[tettsu]

Thanks for a good module.
I'm thinking of using ngx_small_light.

What I want to do is not to make the original part bigger when making the image bigger, but to add a margin around it so that the original is centered.
I want to do this without knowing the original size of the image.
Can it be achieved?

Below is what I tried.

• original
  http://myinstance/static/sample3.jpg
  

• I thought it could be achieved with this request, but the original part is not centered
  http://myinstance/static/sample3.jpg?dw=1500&dh=1300&cw=1500&ch=1300&cc=ffffff
  

• This request can be fulfilled, but we need to know the size of the image before resizing. That is not my desire.
  http://myinstance/static/sample3.jpg?dw=795&dh=1060&cw=1500&ch=1300&cc=ffffff
  

Thanks.