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I'm currently learning OpenGL at uni, and they give us questions to help us learn (these are not worth anything), however I'm stuck on this one question and would have to travel over an hour and a half to uni for an answer.

How do I do this question? Please include as many steps as you can, I want to be able to follow exactly how to do this.

Find the transformation that maps a window whose lower left corner is at (1,1) and upper right corner is at (3,5) onto:

  1. The entire device screen whose dimension is (600, 500)

  2. A viewport that has lower left corner at (100,100) and upper right corner at (400,400)

Edit: Damn sorry I should have added I am meant to find the matrix, so no code.

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1 Answer 1

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You need a combination of scale and translation matrices. You could first go to a "normalized" screen space (origin at 0,0 and scaling to 1,1) using, in pseudo-code:

MatToNormalized = Translation(-1, -1) x Scale(1/2, 1/4)

Then you can easily map to any kind of screen space. E.g. for question 1.:

MatToFullscreen = MatToNormalized x Scale(600, 500)

And for question 2.:

MatToViewport = MatToNormalized x Scale(300, 300) x Translation(100, 100)

EDIT

A Scilab dump with the actual matrices:

MatToNormalized = [1 0 0; 0 1 0; -1 -1 1] * [1/2 0 0; 0 1/4 0; 0 0 1]
    0.5    0.      0.  
    0.     0.25    0.  
  - 0.5  - 0.25    1.  

MatToFullscreen = MatToNormalized * [600 0 0; 0 500 0; 0 0 1]
    300.    0.      0.  
    0.      125.    0.  
  - 300.  - 125.    1.  

TestMinFullScreen = [1 1 1] * MatToFullscreen
    0.    0.    1.  

TestMaxFullScreen = [3 5 1] * MatToFullscreen
    600.    500.    1.  

MatToViewport = MatToNormalized * [300 0 0; 0 300 0; 0 0 1] * [1 0 0; 0 1 0; 100 100 1]
    150.    0.     0.  
    0.      75.    0.  
  - 50.     25.    1.  

TestMinViewport = [1 1 1] * MatToViewport
    100.    100.    1.  

TestMaxViewport = [3 5 1] * MatToViewport
    400.    400.    1.  

Please note that Scilab uses a column-major notation, so if you want the row-major notation, just transpose the result (' is Scilab's transpose operator):

MatToFullscreen'
    300.    0.    - 300.  
    0.      125.  - 125.  
    0.      0.      1.    

MatToNormalized'
    0.5    0.    - 0.5   
    0.     0.25  - 0.25  
    0.     0.      1.    

If you don't get why the translation and scale matrices are formatted this way, try Wikipedia.

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  • \$\begingroup\$ Accepted because I didn't explicitly imply what I wanted. But this will be useful for later on thank you. \$\endgroup\$
    – gbhall
    Commented Aug 17, 2012 at 2:43
  • \$\begingroup\$ Well you can still get the actual matrices out of this process easily, e.g. using WolframAlpha. \$\endgroup\$ Commented Aug 17, 2012 at 10:01
  • \$\begingroup\$ Any chance you could show me? I'm still stuck on this question. \$\endgroup\$
    – gbhall
    Commented Aug 27, 2012 at 16:51
  • \$\begingroup\$ @gbhall I was wrong on some bits: the scale values weren't taking viewport origins in account, and the MatToViewport multiplication was in the wrong order. Fixed + added a Scilab dump. \$\endgroup\$ Commented Aug 29, 2012 at 22:23

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