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From: Parker on 19 Feb 2010 18:13
after reading an image to a variable A, I met an error of "Indexing
cannot yield multiple results" when I want to assign the r, g, b
components to 3 varaiables like:

[r,g,b] = A(1,1,:)
==================
due it's a long loop, so I want to save the r,g,b components to speed
up the function.
Is there any way for me to do that?

"r = A(1,1,:); g=A(1,1,2);b=A(1,1,3);" is not the answer I want,
because it has to fetch values from the image array 3 times.

Thanks in advance.

From: James Tursa on 19 Feb 2010 18:21
Parker <xenoszh(a)gmail.com> wrote in message <764ee50d-dcdb-452e-aa43-a3b06481669d(a)g19g2000yqe.googlegroups.com>...
> after reading an image to a variable A, I met an error of "Indexing
> cannot yield multiple results" when I want to assign the r, g, b
> components to 3 varaiables like:
>
> [r,g,b] = A(1,1,:)
> ==================
> due it's a long loop, so I want to save the r,g,b components to speed
> up the function.
> Is there any way for me to do that?
>
> "r = A(1,1,:); g=A(1,1,2);b=A(1,1,3);" is not the answer I want,
> because it has to fetch values from the image array 3 times.
>
> Thanks in advance.

How is the r-g-b info actually stored? Your example code doesn't make sense to me. Are the r,g,b values interleaved in memory? If so, then a simple c-mex routine can generate the separate variables by accessing the original array only once. Let me know.

James Tursa
From: Walter Roberson on 19 Feb 2010 18:18
Parker wrote:
> after reading an image to a variable A, I met an error of "Indexing
> cannot yield multiple results" when I want to assign the r, g, b
> components to 3 varaiables like:
>
> [r,g,b] = A(1,1,:)
> ==================
> due it's a long loop, so I want to save the r,g,b components to speed
> up the function.
> Is there any way for me to do that?
>
> "r = A(1,1,:); g=A(1,1,2);b=A(1,1,3);" is not the answer I want,
> because it has to fetch values from the image array 3 times.

I could provide code that could do it, but it would be less efficient than the
individual assignments.

r = A(:,:,1); b = A(:,:,2); g = A(:,:,3);

When the first two dimensions are allowed to vary and the third is fixed, then
the data extracted comes from a sequential block of memory and would be
written in a sequential block in exactly the same order -- the most efficient
memory copying operation that there is.

The alternatives that we could provide could do the assignments to [r,g,b] in
a single line, but for syntactic reasons a temporary variable would be
required, and it would be necessary to copy the original data into the
temporary variable.
From: Parker on 20 Feb 2010 03:53
On Feb 19, 11:21 pm, "James Tursa"
<aclassyguy_with_a_k_not_...(a)hotmail.com> wrote:
> Parker <xeno...(a)gmail.com> wrote in message <764ee50d-dcdb-452e-aa43-a3b064816...(a)g19g2000yqe.googlegroups.com>...
> > after reading an image to a variable A, I met anerrorof "Indexing
> > cannot yield multiple results" when I want to assign the r, g, b
> > components to 3 varaiables like:
>
> > [r,g,b] = A(1,1,:)
> > ==================
> > due it's a long loop, so I want to save the r,g,b components to speed
> > up the function.
> > Is there any way for me to do that?
>
> > "r = A(1,1,:); g=A(1,1,2);b=A(1,1,3);" is not the answer I want,
> > because it has to fetch values from the image array 3 times.
>
> > Thanks in advance.
>
> How is the r-g-b info actually stored? Your example code doesn't make sense to me. Are the r,g,b values interleaved in memory? If so, then a simple c-mex routine can generate the separate variables by accessing the original array only once. Let me know.
>
> James Tursa

the image was read as A = imread('test.jpg'), the A(:,:,1), A(:,:,2)
and A(:,:,3) was the 3 components of R, G and B. due to I need to
fetch the R, G, B values at a specified position within a rather long
loop, I want to get them at the same time like [r,g,b]=A(i,j,:),
unfortunately it didn't work.

I think I need to try other ways to modify the algorithm to avoid the
long loop.
From: Parker on 20 Feb 2010 04:07
On Feb 19, 11:18 pm, Walter Roberson <rober...(a)hushmail.com> wrote:
> Parker wrote:
> > after reading an image to a variable A, I met anerrorof "Indexing
> > cannot yield multiple results" when I want to assign the r, g, b
> > components to 3 varaiables like:
>
> > [r,g,b] = A(1,1,:)
> > ==================
> > due it's a long loop, so I want to save the r,g,b components to speed
> > up the function.
> > Is there any way for me to do that?
>
> > "r = A(1,1,:); g=A(1,1,2);b=A(1,1,3);" is not the answer I want,
> > because it has to fetch values from the image array 3 times.
>
> I could provide code that could do it, but it would be less efficient than the
> individual assignments.
>
> r = A(:,:,1); b = A(:,:,2); g = A(:,:,3);
>
> When the first two dimensions are allowed to vary and the third is fixed, then
> the data extracted comes from a sequential block of memory and would be
> written in a sequential block in exactly the same order -- the most efficient
> memory copying operation that there is.
>
> The alternatives that we could provide could do the assignments to [r,g,b] in
> a single line, but for syntactic reasons a temporary variable would be
> required, and it would be necessary to copy the original data into the
> temporary variable.

thx, for optimizing purpose, it's better to do the assignments in
separate line, if there's no way to avoid getting values for 3 times.
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