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Thread Subject:
Image processing

Subject: Image processing

From: Harsha Dissanayake

Date: 22 Apr, 2010 16:32:25

Message: 1 of 4

I have a problem in image processing. I want to know how do I calculate a height by using a pixel value in a color image. In here my approach is to measure height of a teak tree. I am using color images of a teak trees.

Subject: Image processing

From: Kadi

Date: 22 Apr, 2010 16:54:22

Message: 2 of 4

In order to convert pixel to inches you need to know how many pixels are there in an inch (pixels per inch-ppi) and this again depends on the height and width of your image.

Kadambari

"Harsha Dissanayake" <lantrapop@gmail.com> wrote in message <hqptmp$4j5$1@fred.mathworks.com>...
> I have a problem in image processing. I want to know how do I calculate a height by using a pixel value in a color image. In here my approach is to measure height of a teak tree. I am using color images of a teak trees.

Subject: Image processing

From: Sean

Date: 22 Apr, 2010 16:55:21

Message: 3 of 4

"Harsha Dissanayake" <lantrapop@gmail.com> wrote in message <hqptmp$4j5$1@fred.mathworks.com>...
> I have a problem in image processing. I want to know how do I calculate a height by using a pixel value in a color image. In here my approach is to measure height of a teak tree. I am using color images of a teak trees.

Can you post your image on a website somewhere so we can see it?

Subject: Image processing

From: Walter Roberson

Date: 22 Apr, 2010 17:13:30

Message: 4 of 4

Harsha Dissanayake wrote:
> I have a problem in image processing. I want to know how do I calculate
> a height by using a pixel value in a color image. In here my approach is
> to measure height of a teak tree. I am using color images of a teak
> trees.

You need a reference height to calibrate against, preferably as close to
the target section as possible (to prevent parallax errors) but not so
far away that the reference height markings are distorted.

Reference heights can be either explicit (e.g., an object of known
height placed in view) or implicit (e.g., the size of human eyes varies
little as they do not grow over time, though the distance between human
eyes may vary.)

As well as a reference height you need a reference distance to the
target section, or at least something by which you can calculate the
distance triganometrically. Taking perspective into account is
important, and depending on the quality of the camera and the distance
to the target section, it may also be important to correct for lens
distortions of various kinds -- e.g., a "wide-angle lens" does not
provide constant distances across the image.

And with something like a tree, there can be difficulty in deciding what
exactly the "height" of the tree is. Do you include the leaves? When the
leaves might in fact be quite big? When the leaves might change angle
with different times of day? For something like a Banana "tree", do you
include the pseudo-stems that the fruit (technically berries) grow upon?

Sometimes tree heights are defined in terms of the highest horizontal
branches -- which can end up discarding the effect of the soft "crown"
on something like a pine tree (which does tend to grow horizontal
branches). Sometimes tree heights are defined in terms of the highest
"hardened" (second year) growth, knowing that the tree may well send out
extensive "suckers" that will mostly die off and fall off (e.g., a
willow tree.)

So... you have two important tasks to start here: come up with a
calibration system, and come up with a robust (and deducible from the
image) definition of what it means for the trees to be particular heights.

You have an additional difficulty that teak often grows in or adjacent
to swamps, and the edge of swamps are often of fairly variable
geography; if a teak tree grows on a hump in the middle of a swamp, then
to be fair, you have to somehow subtract off the height of the part of
the hump the tree is growing on in order to accurately calculate the
height of the tree... though if the tree is growing _in_ the swamp then
you have to know how deep the swamp is there to calculate the tree
height... which in turn gets you into arguments about what depth in the
much or grasses in a swamp you should consider to be base point for
height calculation purposes.


There simply isn't an easy answer to your question, especially for teak
(but mangrove would be probably even worse, since you would then have to
deal with questions about where one mangrove tree ends and another
begins.) You have some decisions to make.

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