Calculating matrix for linear transformation of orthogonal projection onto plane.











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0
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The question goes like this:



"Calculate the matrix P for the linear transformation of an orthogonal projection of vectors onto the plane



$$ 2x_1+2x_2+x_3^{}= 0" $$



So I am thinking that projection is the way to go. What I basically will do is use the normal of the plane. Which is:



$$ left[
begin{array}{cc|c}
2\
2\
1
end{array}
right] $$



That would be my perpendicular part. And the vectors that I will project onto the plane will naturally be the basis vectors $$
|e_1| = left[
begin{array}{cc|c}
1\
0\
0
end{array}
right], |e_2| =left[
begin{array}{cc|c}
0\
1\
0
end{array}
right], |e_3| =left[
begin{array}{cc|c}
0\
0\
1
end{array}
right]$$



Basically, what I will do is set up an equation



$$ Proj V_n + |n| = left[
begin{array}{cc|c}
1\
0\
0
end{array}
right]$$



So I solve for the projection and that would be my first column of my matrix P. But I keep getting the wrong answer. Where is my thinking going wrong?



Thanks in advance.










share|cite|improve this question
























  • Do you mean orthogonal projection?
    – José Carlos Santos
    Nov 21 at 15:30












  • Hmm, do you mean the question? The question literally is saying : " Calculate the matrix for the linear transformation of projections of vectors onto the given plane"
    – Synchrowave
    Nov 21 at 15:32












  • There are infinitely many such projections, but only one orthogonal one. So, yes, I do mean the question.
    – José Carlos Santos
    Nov 21 at 15:35










  • I would say that they look for the projection that is parallell to the plane.
    – Synchrowave
    Nov 21 at 15:39










  • So yes! It is correct. Orthogonal projection. I just looked it up and edited the question. Thanks for the heads up!
    – Synchrowave
    Nov 21 at 15:59















up vote
0
down vote

favorite












The question goes like this:



"Calculate the matrix P for the linear transformation of an orthogonal projection of vectors onto the plane



$$ 2x_1+2x_2+x_3^{}= 0" $$



So I am thinking that projection is the way to go. What I basically will do is use the normal of the plane. Which is:



$$ left[
begin{array}{cc|c}
2\
2\
1
end{array}
right] $$



That would be my perpendicular part. And the vectors that I will project onto the plane will naturally be the basis vectors $$
|e_1| = left[
begin{array}{cc|c}
1\
0\
0
end{array}
right], |e_2| =left[
begin{array}{cc|c}
0\
1\
0
end{array}
right], |e_3| =left[
begin{array}{cc|c}
0\
0\
1
end{array}
right]$$



Basically, what I will do is set up an equation



$$ Proj V_n + |n| = left[
begin{array}{cc|c}
1\
0\
0
end{array}
right]$$



So I solve for the projection and that would be my first column of my matrix P. But I keep getting the wrong answer. Where is my thinking going wrong?



Thanks in advance.










share|cite|improve this question
























  • Do you mean orthogonal projection?
    – José Carlos Santos
    Nov 21 at 15:30












  • Hmm, do you mean the question? The question literally is saying : " Calculate the matrix for the linear transformation of projections of vectors onto the given plane"
    – Synchrowave
    Nov 21 at 15:32












  • There are infinitely many such projections, but only one orthogonal one. So, yes, I do mean the question.
    – José Carlos Santos
    Nov 21 at 15:35










  • I would say that they look for the projection that is parallell to the plane.
    – Synchrowave
    Nov 21 at 15:39










  • So yes! It is correct. Orthogonal projection. I just looked it up and edited the question. Thanks for the heads up!
    – Synchrowave
    Nov 21 at 15:59













up vote
0
down vote

favorite









up vote
0
down vote

favorite











The question goes like this:



"Calculate the matrix P for the linear transformation of an orthogonal projection of vectors onto the plane



$$ 2x_1+2x_2+x_3^{}= 0" $$



So I am thinking that projection is the way to go. What I basically will do is use the normal of the plane. Which is:



$$ left[
begin{array}{cc|c}
2\
2\
1
end{array}
right] $$



That would be my perpendicular part. And the vectors that I will project onto the plane will naturally be the basis vectors $$
|e_1| = left[
begin{array}{cc|c}
1\
0\
0
end{array}
right], |e_2| =left[
begin{array}{cc|c}
0\
1\
0
end{array}
right], |e_3| =left[
begin{array}{cc|c}
0\
0\
1
end{array}
right]$$



Basically, what I will do is set up an equation



$$ Proj V_n + |n| = left[
begin{array}{cc|c}
1\
0\
0
end{array}
right]$$



So I solve for the projection and that would be my first column of my matrix P. But I keep getting the wrong answer. Where is my thinking going wrong?



Thanks in advance.










share|cite|improve this question















The question goes like this:



"Calculate the matrix P for the linear transformation of an orthogonal projection of vectors onto the plane



$$ 2x_1+2x_2+x_3^{}= 0" $$



So I am thinking that projection is the way to go. What I basically will do is use the normal of the plane. Which is:



$$ left[
begin{array}{cc|c}
2\
2\
1
end{array}
right] $$



That would be my perpendicular part. And the vectors that I will project onto the plane will naturally be the basis vectors $$
|e_1| = left[
begin{array}{cc|c}
1\
0\
0
end{array}
right], |e_2| =left[
begin{array}{cc|c}
0\
1\
0
end{array}
right], |e_3| =left[
begin{array}{cc|c}
0\
0\
1
end{array}
right]$$



Basically, what I will do is set up an equation



$$ Proj V_n + |n| = left[
begin{array}{cc|c}
1\
0\
0
end{array}
right]$$



So I solve for the projection and that would be my first column of my matrix P. But I keep getting the wrong answer. Where is my thinking going wrong?



Thanks in advance.







linear-algebra linear-transformations






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share|cite|improve this question













share|cite|improve this question




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edited Nov 21 at 16:09

























asked Nov 21 at 15:27









Synchrowave

375




375












  • Do you mean orthogonal projection?
    – José Carlos Santos
    Nov 21 at 15:30












  • Hmm, do you mean the question? The question literally is saying : " Calculate the matrix for the linear transformation of projections of vectors onto the given plane"
    – Synchrowave
    Nov 21 at 15:32












  • There are infinitely many such projections, but only one orthogonal one. So, yes, I do mean the question.
    – José Carlos Santos
    Nov 21 at 15:35










  • I would say that they look for the projection that is parallell to the plane.
    – Synchrowave
    Nov 21 at 15:39










  • So yes! It is correct. Orthogonal projection. I just looked it up and edited the question. Thanks for the heads up!
    – Synchrowave
    Nov 21 at 15:59


















  • Do you mean orthogonal projection?
    – José Carlos Santos
    Nov 21 at 15:30












  • Hmm, do you mean the question? The question literally is saying : " Calculate the matrix for the linear transformation of projections of vectors onto the given plane"
    – Synchrowave
    Nov 21 at 15:32












  • There are infinitely many such projections, but only one orthogonal one. So, yes, I do mean the question.
    – José Carlos Santos
    Nov 21 at 15:35










  • I would say that they look for the projection that is parallell to the plane.
    – Synchrowave
    Nov 21 at 15:39










  • So yes! It is correct. Orthogonal projection. I just looked it up and edited the question. Thanks for the heads up!
    – Synchrowave
    Nov 21 at 15:59
















Do you mean orthogonal projection?
– José Carlos Santos
Nov 21 at 15:30






Do you mean orthogonal projection?
– José Carlos Santos
Nov 21 at 15:30














Hmm, do you mean the question? The question literally is saying : " Calculate the matrix for the linear transformation of projections of vectors onto the given plane"
– Synchrowave
Nov 21 at 15:32






Hmm, do you mean the question? The question literally is saying : " Calculate the matrix for the linear transformation of projections of vectors onto the given plane"
– Synchrowave
Nov 21 at 15:32














There are infinitely many such projections, but only one orthogonal one. So, yes, I do mean the question.
– José Carlos Santos
Nov 21 at 15:35




There are infinitely many such projections, but only one orthogonal one. So, yes, I do mean the question.
– José Carlos Santos
Nov 21 at 15:35












I would say that they look for the projection that is parallell to the plane.
– Synchrowave
Nov 21 at 15:39




I would say that they look for the projection that is parallell to the plane.
– Synchrowave
Nov 21 at 15:39












So yes! It is correct. Orthogonal projection. I just looked it up and edited the question. Thanks for the heads up!
– Synchrowave
Nov 21 at 15:59




So yes! It is correct. Orthogonal projection. I just looked it up and edited the question. Thanks for the heads up!
– Synchrowave
Nov 21 at 15:59










2 Answers
2






active

oldest

votes

















up vote
1
down vote













Your notation is a bit hard to decipher, but it looks like you’re trying to decompose $mathbf e_1$ into its projection onto and rejection from the plane. That’s a reasonable idea, but the equation that you’ve written down says that the projection of $mathbf e_1$ is equal to $mathbf e_1-mathbf n = (-1,-2,-1)^T$. Unfortunately, this doesn’t even lie on the plane: $2(-1)+2(-2)+1(-1)=-7$.



The problem is that you’ve set the rejection of $mathbf e_1$ from the plane to be equal to $mathbf n$, when it’s actually some scalar multiple of it. I.e., the orthogonal projection $Pmathbf e_1$ of $mathbf e_1$ onto the plane is $mathbf e_1-kmathbf n$ for some as-yet-undetermined scalar $k$. However, $kmathbf n$ here is simply the orthogonal projection of $mathbf e_1$ onto $mathbf n$, which I suspect that you know how to compute.






share|cite|improve this answer





















  • I overcomplicated things. My TA helped me through using the same concepts you use when computing reflection about a plane. Basically. You subtract the vector in question, in my case I started out with $$ e_1 $$ the projection of that vector on the normal of the plane. The result will be a vector lying on the plane.
    – Synchrowave
    Nov 22 at 15:28












  • @Synchrowave Yep. For reflections, decomposing into components perpendicular and parallel to the reflector and then reassembling is usually a much easier way to go.
    – amd
    Nov 22 at 19:25


















up vote
0
down vote













Since $(2,2,1)$ is orthogonal to the plane, you wnat that its projection is the null vector. Now, take two linearly independent vectors from your plane. For instance, take the vectors $(1,0,-2)$ and $(0,1,-2)$. You want the each one is projected into itself.



So, take the only linear map $Pcolonmathbb{R}^3longrightarrowmathbb{R}^3$ such that





  1. $P(2,2,1)=(0,0,0)$;


  2. $P(1,0,-2)=(1,0,-2)$;


  3. $P(0,1,-2)=(0,1,-2)$.


A simple computation shows that the matrix of $P$ with respect to the canonical basis is$$frac19begin{bmatrix}5 & -4 & -2 \ -4 & 5 & -2 \ -2 & -2 & 8end{bmatrix}.$$






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    2 Answers
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    up vote
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    down vote













    Your notation is a bit hard to decipher, but it looks like you’re trying to decompose $mathbf e_1$ into its projection onto and rejection from the plane. That’s a reasonable idea, but the equation that you’ve written down says that the projection of $mathbf e_1$ is equal to $mathbf e_1-mathbf n = (-1,-2,-1)^T$. Unfortunately, this doesn’t even lie on the plane: $2(-1)+2(-2)+1(-1)=-7$.



    The problem is that you’ve set the rejection of $mathbf e_1$ from the plane to be equal to $mathbf n$, when it’s actually some scalar multiple of it. I.e., the orthogonal projection $Pmathbf e_1$ of $mathbf e_1$ onto the plane is $mathbf e_1-kmathbf n$ for some as-yet-undetermined scalar $k$. However, $kmathbf n$ here is simply the orthogonal projection of $mathbf e_1$ onto $mathbf n$, which I suspect that you know how to compute.






    share|cite|improve this answer





















    • I overcomplicated things. My TA helped me through using the same concepts you use when computing reflection about a plane. Basically. You subtract the vector in question, in my case I started out with $$ e_1 $$ the projection of that vector on the normal of the plane. The result will be a vector lying on the plane.
      – Synchrowave
      Nov 22 at 15:28












    • @Synchrowave Yep. For reflections, decomposing into components perpendicular and parallel to the reflector and then reassembling is usually a much easier way to go.
      – amd
      Nov 22 at 19:25















    up vote
    1
    down vote













    Your notation is a bit hard to decipher, but it looks like you’re trying to decompose $mathbf e_1$ into its projection onto and rejection from the plane. That’s a reasonable idea, but the equation that you’ve written down says that the projection of $mathbf e_1$ is equal to $mathbf e_1-mathbf n = (-1,-2,-1)^T$. Unfortunately, this doesn’t even lie on the plane: $2(-1)+2(-2)+1(-1)=-7$.



    The problem is that you’ve set the rejection of $mathbf e_1$ from the plane to be equal to $mathbf n$, when it’s actually some scalar multiple of it. I.e., the orthogonal projection $Pmathbf e_1$ of $mathbf e_1$ onto the plane is $mathbf e_1-kmathbf n$ for some as-yet-undetermined scalar $k$. However, $kmathbf n$ here is simply the orthogonal projection of $mathbf e_1$ onto $mathbf n$, which I suspect that you know how to compute.






    share|cite|improve this answer





















    • I overcomplicated things. My TA helped me through using the same concepts you use when computing reflection about a plane. Basically. You subtract the vector in question, in my case I started out with $$ e_1 $$ the projection of that vector on the normal of the plane. The result will be a vector lying on the plane.
      – Synchrowave
      Nov 22 at 15:28












    • @Synchrowave Yep. For reflections, decomposing into components perpendicular and parallel to the reflector and then reassembling is usually a much easier way to go.
      – amd
      Nov 22 at 19:25













    up vote
    1
    down vote










    up vote
    1
    down vote









    Your notation is a bit hard to decipher, but it looks like you’re trying to decompose $mathbf e_1$ into its projection onto and rejection from the plane. That’s a reasonable idea, but the equation that you’ve written down says that the projection of $mathbf e_1$ is equal to $mathbf e_1-mathbf n = (-1,-2,-1)^T$. Unfortunately, this doesn’t even lie on the plane: $2(-1)+2(-2)+1(-1)=-7$.



    The problem is that you’ve set the rejection of $mathbf e_1$ from the plane to be equal to $mathbf n$, when it’s actually some scalar multiple of it. I.e., the orthogonal projection $Pmathbf e_1$ of $mathbf e_1$ onto the plane is $mathbf e_1-kmathbf n$ for some as-yet-undetermined scalar $k$. However, $kmathbf n$ here is simply the orthogonal projection of $mathbf e_1$ onto $mathbf n$, which I suspect that you know how to compute.






    share|cite|improve this answer












    Your notation is a bit hard to decipher, but it looks like you’re trying to decompose $mathbf e_1$ into its projection onto and rejection from the plane. That’s a reasonable idea, but the equation that you’ve written down says that the projection of $mathbf e_1$ is equal to $mathbf e_1-mathbf n = (-1,-2,-1)^T$. Unfortunately, this doesn’t even lie on the plane: $2(-1)+2(-2)+1(-1)=-7$.



    The problem is that you’ve set the rejection of $mathbf e_1$ from the plane to be equal to $mathbf n$, when it’s actually some scalar multiple of it. I.e., the orthogonal projection $Pmathbf e_1$ of $mathbf e_1$ onto the plane is $mathbf e_1-kmathbf n$ for some as-yet-undetermined scalar $k$. However, $kmathbf n$ here is simply the orthogonal projection of $mathbf e_1$ onto $mathbf n$, which I suspect that you know how to compute.







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered Nov 21 at 21:26









    amd

    29k21050




    29k21050












    • I overcomplicated things. My TA helped me through using the same concepts you use when computing reflection about a plane. Basically. You subtract the vector in question, in my case I started out with $$ e_1 $$ the projection of that vector on the normal of the plane. The result will be a vector lying on the plane.
      – Synchrowave
      Nov 22 at 15:28












    • @Synchrowave Yep. For reflections, decomposing into components perpendicular and parallel to the reflector and then reassembling is usually a much easier way to go.
      – amd
      Nov 22 at 19:25


















    • I overcomplicated things. My TA helped me through using the same concepts you use when computing reflection about a plane. Basically. You subtract the vector in question, in my case I started out with $$ e_1 $$ the projection of that vector on the normal of the plane. The result will be a vector lying on the plane.
      – Synchrowave
      Nov 22 at 15:28












    • @Synchrowave Yep. For reflections, decomposing into components perpendicular and parallel to the reflector and then reassembling is usually a much easier way to go.
      – amd
      Nov 22 at 19:25
















    I overcomplicated things. My TA helped me through using the same concepts you use when computing reflection about a plane. Basically. You subtract the vector in question, in my case I started out with $$ e_1 $$ the projection of that vector on the normal of the plane. The result will be a vector lying on the plane.
    – Synchrowave
    Nov 22 at 15:28






    I overcomplicated things. My TA helped me through using the same concepts you use when computing reflection about a plane. Basically. You subtract the vector in question, in my case I started out with $$ e_1 $$ the projection of that vector on the normal of the plane. The result will be a vector lying on the plane.
    – Synchrowave
    Nov 22 at 15:28














    @Synchrowave Yep. For reflections, decomposing into components perpendicular and parallel to the reflector and then reassembling is usually a much easier way to go.
    – amd
    Nov 22 at 19:25




    @Synchrowave Yep. For reflections, decomposing into components perpendicular and parallel to the reflector and then reassembling is usually a much easier way to go.
    – amd
    Nov 22 at 19:25










    up vote
    0
    down vote













    Since $(2,2,1)$ is orthogonal to the plane, you wnat that its projection is the null vector. Now, take two linearly independent vectors from your plane. For instance, take the vectors $(1,0,-2)$ and $(0,1,-2)$. You want the each one is projected into itself.



    So, take the only linear map $Pcolonmathbb{R}^3longrightarrowmathbb{R}^3$ such that





    1. $P(2,2,1)=(0,0,0)$;


    2. $P(1,0,-2)=(1,0,-2)$;


    3. $P(0,1,-2)=(0,1,-2)$.


    A simple computation shows that the matrix of $P$ with respect to the canonical basis is$$frac19begin{bmatrix}5 & -4 & -2 \ -4 & 5 & -2 \ -2 & -2 & 8end{bmatrix}.$$






    share|cite|improve this answer



























      up vote
      0
      down vote













      Since $(2,2,1)$ is orthogonal to the plane, you wnat that its projection is the null vector. Now, take two linearly independent vectors from your plane. For instance, take the vectors $(1,0,-2)$ and $(0,1,-2)$. You want the each one is projected into itself.



      So, take the only linear map $Pcolonmathbb{R}^3longrightarrowmathbb{R}^3$ such that





      1. $P(2,2,1)=(0,0,0)$;


      2. $P(1,0,-2)=(1,0,-2)$;


      3. $P(0,1,-2)=(0,1,-2)$.


      A simple computation shows that the matrix of $P$ with respect to the canonical basis is$$frac19begin{bmatrix}5 & -4 & -2 \ -4 & 5 & -2 \ -2 & -2 & 8end{bmatrix}.$$






      share|cite|improve this answer

























        up vote
        0
        down vote










        up vote
        0
        down vote









        Since $(2,2,1)$ is orthogonal to the plane, you wnat that its projection is the null vector. Now, take two linearly independent vectors from your plane. For instance, take the vectors $(1,0,-2)$ and $(0,1,-2)$. You want the each one is projected into itself.



        So, take the only linear map $Pcolonmathbb{R}^3longrightarrowmathbb{R}^3$ such that





        1. $P(2,2,1)=(0,0,0)$;


        2. $P(1,0,-2)=(1,0,-2)$;


        3. $P(0,1,-2)=(0,1,-2)$.


        A simple computation shows that the matrix of $P$ with respect to the canonical basis is$$frac19begin{bmatrix}5 & -4 & -2 \ -4 & 5 & -2 \ -2 & -2 & 8end{bmatrix}.$$






        share|cite|improve this answer














        Since $(2,2,1)$ is orthogonal to the plane, you wnat that its projection is the null vector. Now, take two linearly independent vectors from your plane. For instance, take the vectors $(1,0,-2)$ and $(0,1,-2)$. You want the each one is projected into itself.



        So, take the only linear map $Pcolonmathbb{R}^3longrightarrowmathbb{R}^3$ such that





        1. $P(2,2,1)=(0,0,0)$;


        2. $P(1,0,-2)=(1,0,-2)$;


        3. $P(0,1,-2)=(0,1,-2)$.


        A simple computation shows that the matrix of $P$ with respect to the canonical basis is$$frac19begin{bmatrix}5 & -4 & -2 \ -4 & 5 & -2 \ -2 & -2 & 8end{bmatrix}.$$







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Nov 21 at 16:24

























        answered Nov 21 at 16:11









        José Carlos Santos

        147k22117218




        147k22117218






























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