Matlab 3-layered Image Matrix

Question

I'm currently learning Matlab and having troubles understand the image section. I have this powerpoint slide about matlab image:

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****Image Matrices*

3-layered image matrices -- Read 'rainbow.jpg' into im

Subset im to im2 – eg( 155:184 , 145:164, : )

*1 layer of an image – Get the red layer in im2****

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I would like to ask what does(155:184, 145:164, :) represent? What does each value in parenthesis represent? Also, what does the semi-colon represent?

Thank you!

Answer 1

Say you have a 3 dimensional matrix A and you are indexing your matrix. I will use the example above:

A(155:184,145:164,:)

155:184 in the first entry means take rows 155 to 184 145:164 in the second entry means take columns 145 to 164 and the semi-colon in the last entry means take EVERY element along the 3rd dimension. So if you have 200x200x3 , the semi-colon will take the 3 arrays along that 3rd dimension.