CODEWITHSUNDEEP
c++copenglgraphicstransformation
UnreachableCode
UnreachableCode

Reputation: 1801

How to reverse(or cancel) OpenGL SRT Transformation with another Transformation?

I'm studying for an exam, and a previous exam had this question:

1. void drawGLScene(){ 
2. glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); 

3. glLoadIdentity(); 
4. glTranslatef(0, 1, 0); 
5. glRotatef(-90.0f, 0.0f, 0.0f, 1.0f); 
6. glScalef(.5, 2, 1); 
7. drawChevron();  
8. glFlush(); 
9. }

"Write three lines of OpenGL that when inserted in between lines 6 and 7 would reverse (cancel) the effect of the transformations implemented by lines 4 to 6. Do not use glLoadIdentity()."

I'm pretty sure it has to be another Translate, Rotate, Scale. But I know it isn't as simple as just using what I think are the reverse parameters. I'm having a lot of trouble understanding which way the x and y axes are facing, and whether scale effects the translation.

Would anyone be able to explain to me how to correctly do this?

Upvotes: 0

Views: 1815

Answers (1)

Roman Byshko
Roman Byshko

Reputation: 9062

The three lines would be:

glScalef(1.0/.5, 1.0/2, 1.0/1);
glRotatef(90.0f, 0.0f, 0.0f, 1.0f);
glTranslatef(0, -1, 0);

Basically you need to perform inverse transformations in reverse order. You start with scaling. Inverse for scaling would be to scale back but this time with 1/factor. Then you rotate it back by 90 degrees and translate back 1 unit on y axis.

Upvotes: 3

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