Comments on: Viewing output-space coordinates for a transformed image

By: Markus

Markus — Wed, 18 Feb 2009 15:46:59 +0000

Hi Steve,
thanks, you are right, I don´t need all this machinery. I solved it like this:
-shift the contour to the center (1,1) since the centroid of the contour was calculated,
-scaling
-shifting back
T-Matrix: this was just a typo :)

markus

By: Steve

Steve — Wed, 18 Feb 2009 02:22:00 +0000

Markus—Your description of what you were trying to accomplish and what you actually did is a bit vague. My only suggestion is that you apply a “sanity check” to your tform struct. Use tformfwd to transform a few points, and make sure that the points transform to the locations you expect. Also, make sure you are using the correct transform matrix for scaling - the scale factors should be on the diagonal. (I’m not sure whether your misplaced sy factor was just a typo.) Finally, do you really need all this machinery, anyway? Why don’t you just scale and shift the contour locations yourself? It might be a lot simpler.

By: Steve

Steve — Wed, 18 Feb 2009 01:56:17 +0000

Swetha—How about Paul Heckbert’s Master’s thesis, “Fundamentals of Texture Mapping and Image Warping.” It’s available his web site.

By: Markus

Markus — Mon, 16 Feb 2009 13:53:15 +0000

Hi Steve,

I am trying to apply “tformfwd” with the
following T-matrix [sx 0 0 ; sy 0 0 ; 0 0 1]
in order to shrink a 2D contour.
I understood that the images gets scaled
with respect of the image origin (1,1). So
I need to supply the origion of the contour
to the “tformfwd” function.
I calculated the origin of the contour by using “regionprops.centroid” and added the coordinates to each pixel of my contour, afterwards I applied the transformation. By it only gave reasonable results for scaling factor 1/2 and I don´t know why?
Do you have any suggestions?

Thank you
markus

By: swetha

swetha — Wed, 11 Feb 2009 10:17:51 +0000

Hello Steve,

Thankyou very much for these blogs,they have been very helpful in my projects. I understand how imtransform work,but i want to know how exactly the transformation takes place,like how we place the input image pixels after transformation in the output space. In short i want to do the transformation without using any matlab function. I just want to know which book to refer.
Thankyou
swetha

By: Mikr

Mikr — Thu, 28 Aug 2008 14:52:10 +0000

I look for answers before asking people…

“But we still can’t see the coordinates! That’s because imshow turns the axis display off. That brings us to step 3: Turn on the axis box and tick labels”

but actually how to save those coordinates I have seen using your article.
imwrite makes an image
save does not support images…
thanks

By: Steve

Steve — Thu, 28 Aug 2008 12:39:03 +0000

Mikr—You might want to take a look at the Getting Started section of the MATLAB documentation in order to become familiar with MATLAB usage and capabilities.

By: Mikr

Mikr — Thu, 28 Aug 2008 12:32:25 +0000

thanks
but is it possible to see and write to file (Excel ?) that matrix of pixel coordinates ?
instead of exporting one image format to another
is that matrix which I see after command imread without semicolon ?

By: Steve

Steve — Thu, 28 Aug 2008 10:57:53 +0000

Mikr—An image in MATLAB is simply a matrix of pixel values. It can be saved (exported) to several common image file formats using imwrite.

By: Mikr

Mikr — Thu, 28 Aug 2008 06:30:03 +0000

thanks, Steve
just started to learn matlab
and to clarify
matlab saves image files as a matrix of pixel coordinates
matrix can be seen by imread without semicolon
is this related to my previous question ? and does it make sence to export matrix from matlab ?