Jump to content
  • Advertisement
Sign in to follow this  
adzz182

OpenGL SOLVED : Help, Please : Changing the origin of Rotation

This topic is 4259 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

I currently have a setup very similar to a first person shooter - with the environment rotating around the veiwport. This is not quite the setup i wanted - i am trying to create an RPG style camera with the 'origin of rotation' being a point a point in front of the viewport (where my character would be stood). Sorry if my explanation is not very good - im quite new to openGL, havent quite got the terminology yet. The up/down/left/right strafes the viewport forwards/backwards/left/right respectively. and the L & R buttons rotate the environment around the viewport. I have tried a few methods, i believe this way should work, but it doesnt: (lol) (BEFORE DRAWING EACH FRAME) 1. Translating the environment to the position where i would like the rotation to occur - just 'in front' of the viewport. 2. Rotating the environment to the correct rotation. 3. Translating the environment again to the position of the viewport in the environment - the location on the environment. If you dont understand i will try to explain it better - or maybe it would be better if i posted my code (i am programming for the Nintendo DS) Thanks adzz182 [Edited by - adzz182 on March 26, 2007 4:53:43 AM]

Share this post


Link to post
Share on other sites
Advertisement
Think this way: Whenever a rotation is performed then the point at (0,0,0) will not change its position while every other point in space will (or at least can) change its position. This can be seen when performing the matrix-vector product of a rotation on paper. I.e. you want to have
R * p == p
where R is any rotation matrix and p a point in space. The formula says: The rotated point and the original point should be the same. This would happen if R is the identity matrix (i.e. _no_ rotation, but this isn't the case you want) or p == (0,0,0).

So you 1st have to translate the world so that the point you want not to rotate is rested at (0,0,0). Assume your point of interest (i.e. 'in front of the view') is given as (x,y,z). Then

glTranslate(x,y,z);
glRotate(<whatever>);
glTranslate(-x,-y,-z);

would do the trick. Remember that the later a transformation is given to GL, the more local it is applied to the vertices. The last transformation above is glTranslate(-x,-y,-z), so this is applied 1st to a vertex. A vertex that is exactly at that position is hence translated to (0,0,0) and the subsequent rotation doesn't change that. Then glTranslate(x,y,z) translates it back to the original position.


EDIT: Notice please that the set-up above is suitable as a view transformation in GL. If you want to rotate the camera as an object and later derive the view transformation from the camera settings, then you may want to transform your camera inverse compared to the code above.

Share this post


Link to post
Share on other sites
Thanks for the quick reply haegarr.
Im still having trouble with this bit of code though....ill try to explain a little better. Here is my current code:


while (1)
{
heading = (yrotation & 511); //used for strafing forwards/backwards
heading90 = ((yrotation+128)&511); //used for strafing left/right

if (keysHeld() & KEY_LEFT) //if LEFT is Pressed
{
xpos -= SIN[heading90] >> 4;
zpos += COS[heading90] >> 4;
}

if (keysHeld() & KEY_RIGHT) //if RIGHT is Pressed
{
xpos += SIN[heading90] >> 4;
zpos -= COS[heading90] >> 4;
}

if (keysHeld() & KEY_DOWN) //if DOWN is pressed
{
xpos -= SIN[heading] >> 4;
zpos += COS[heading] >> 4;
}

if (keysHeld() & KEY_UP) //if UP is pressed
{
xpos += SIN[heading] >> 4;
zpos -= COS[heading] >> 4;
}

if (keysHeld() & KEY_R) //if R_TRIGGER is pressed
{
yrot+=ROTSPEED;
}

if(keysHeld() & KEY_L) //if L_TRIGGER is pressed
{
yrot-=ROTSPEED;
}

int DrawGLScene()
{
glRotatef32i((512-yrot),0,(1<<12),0);
glTranslate3f32(-xpos,0.0f,-zpos);




This code does work, but not how i would like it to. Currently i have:


^
|
| -90 Degree Rotation->
|
|
o
ViewPort ViewPort o---------->


What i am looking for is:

^
|
| -90 Degree Rotation->
o ViewPort----o----->
|
|
ViewPort

o = where rotation is taking place

Share this post


Link to post
Share on other sites
I suspect that your problem may be solved by inserting a translation of (xpos, 0.0, zpos) before your rotation call, to undo the translation that you call for after that rotation, which should bring you to the situation described by haegarr.

I hope that this helps.

Share this post


Link to post
Share on other sites
Thanks alot for your replies, they did help. I have solved the problem now (i cant believe it took me so long - its sooooo simple lol). For anyone whos interested about how to do this here is the new source, which is now a sort of RPG style 'camera'.

The only difference if in the glTranslate

OLD:
glTranslate3f32(-xpos,0.0f,-zpos);
NEW:
glTranslate3f32(-xpos+(SIN[heading]<<1),0.0f,-zpos-(COS[heading]<<1));

Share this post


Link to post
Share on other sites
hey, i seem to be having the same problem as you had so if you could please post your display function code it would be much appreciated

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
×

Important Information

By using GameDev.net, you agree to our community Guidelines, Terms of Use, and Privacy Policy.

GameDev.net is your game development community. Create an account for your GameDev Portfolio and participate in the largest developer community in the games industry.

Sign me up!