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fooman_69

OpenGL Degrees on rotation

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Hiya, Is it just me or is the rotation circle a bit backwards in openGL?? Because when I start my quad at 0 degrees (pointing right), it is fine. But when I start it at 270 (supposed to be pointing down), the pic on the quad points up. And the same goes for 90 degrees, it is opposite. Is this how it is, or am I setting things up improperly somewhere??

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OpenGL uses a right-handed coordinate system and (AFAIK) right-handed rotations. This means that if you hold up your right hand, stick out your thumb, and wrap your other fingers around, the curl of your fingers is the direction of positive rotation about an axis pointing in the direction of your thumb. If you are for some reason expecting positive rotation to go the other way, then that would explain why things seem wrong to you. Otherwise, I'm not sure.

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Well usually the right is 0 degrees, and the top is 90 degrees, left side of ur hand is 180, and bottom is 270 right??

But for me, Im getting the 90 and 270 opposite.

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Is this 2d? If you're looking at the xy plane, with the +z axis pointing towards you, what you describe should be correct. You might try drawing the +x and +y axes onscreen to make sure you're oriented correctly. Also, are you using glRotate(), or your own matrix library? Using your own library, it's easy to get things transposed, resulting in the rotations going the wrong way.

Anyway, I'd recommend giving as much information about the problem as you can; then it should be easy to solve.

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Im not sure what other info I can give you to be completely honest.
I am using 2d.
I have an image of an arrow pointing right initially. I always rotate the image I am having issues with by x amount of degrees. I start at 90 degrees, and it points down. When I am at 270 degrees that arrow points up. Then I allow it to rotate in the quadrants of 0-90, and 360-270 (which is half a circle).

The only problem is, the circle is upside down... as I have described. Everything works, but dammit I wanna know why this is happening.

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Alright, well to be honest there isn't much code that is relevant... haha. It's one of those things you gotta see because everything seems to work except just not the way you expected.
Relevant code in Entity.cpp
void Entity::IncreaseAngle()
{
if(ammoStr.degree < 0)
ammoStr.degree += 360;

if((ammoStr.degree > 269 || ammoStr.degree < 91) || !incrDeg)
ammoStr.degree--;

incrDeg = TRUE;
Show();
return;
}

void Entity::DecreaseAngle()
{
if(ammoStr.degree > 360)
ammoStr.degree -= 360;

if((ammoStr.degree > 269 || ammoStr.degree < 91) || incrDeg)
ammoStr.degree++;

incrDeg = FALSE;
Show();
return;
}

Relevant code in Video.cpp
//Draw arm of player who is throwing
float p1_x = ent1->GetMapX();
float p1_y = ent1->GetMapY();
float p2_x = ent2->GetMapX();
float p2_y = ent2->GetMapY();

glBindTexture(GL_TEXTURE_2D, g_textureID[ent1->GetTextureID()]);
glLoadIdentity();
switch(turn)
{
case PLAYER1_TURN:
if(ent1->GetIndex() == AIMING)
{
glTranslatef(p1_x * TILE_SIZE + TILE_SIZE/2, p1_y * TILE_SIZE - TILE_SIZE/2, 0.0f);
glRotatef((float)ent1->GetAngle(), 0.0f, 0.0f, 1.0f);
glBegin(GL_QUADS);
glTexCoord2f(SPRITE_SIZE * 2, SPRITE_SIZE * 6); glVertex2f((float)(-TILE_SIZE/2), (float)(-TILE_SIZE/2)); // Bottom Left Of the Texture and Quad
glTexCoord2f(SPRITE_SIZE * 3, SPRITE_SIZE * 6); glVertex2f((float)(TILE_SIZE/2), (float)(-TILE_SIZE/2)); // Bottom Right Of the Texture and Quad
glTexCoord2f(SPRITE_SIZE * 3, SPRITE_SIZE * 7); glVertex2f((float)(TILE_SIZE/2), (float)(TILE_SIZE/2)); // Top Right Of the Texture and Quad
glTexCoord2f(SPRITE_SIZE * 2, SPRITE_SIZE * 7); glVertex2f((float)(-TILE_SIZE/2), (float)(TILE_SIZE/2)); // Top Left of the Texture and Quad
glEnd();
}
break;
case PLAYER2_TURN:
if(ent2->GetIndex() == AIMING)
{
glRotatef((float)ent2->GetAngle(), 0.0f, 0.0f, 1.0f);
glBegin(GL_QUADS);
glTexCoord2f(SPRITE_SIZE * 2, SPRITE_SIZE * 6); glVertex2f((float)(p1_x * TILE_SIZE), (float)(p1_y * TILE_SIZE)); // Bottom Left Of The Texture and Quad
glTexCoord2f(SPRITE_SIZE * 3, SPRITE_SIZE * 6); glVertex2f((float)(p1_x * TILE_SIZE + TILE_SIZE), (float)(p1_y * TILE_SIZE)); // Bottom Right Of The Texture and Quad
glTexCoord2f(SPRITE_SIZE * 3, SPRITE_SIZE * 7); glVertex2f((float)(p1_x * TILE_SIZE+ TILE_SIZE), (float)(p1_y * TILE_SIZE - TILE_SIZE)); // Top Right Of The Texture and Quad
glTexCoord2f(SPRITE_SIZE * 2, SPRITE_SIZE * 7); glVertex2f((float)(p1_x * TILE_SIZE), (float)(p1_y * TILE_SIZE - TILE_SIZE)); // Top left of the texture and quad
glEnd();
}
break;
}

So yea, I start off with my Entity having a degree of 90, which makes the arrow point down (rather than up). And in my Video.cpp file I render the arrow if the player is aiming. Not much to it.
Worse comes to worse, I'll post the project. I'm still very curious why this happens.

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I'm not seeing any problem...

270

180 o 000

090
Why do you think clockwise rotation is abnormal?
Are you expecting to be rotating the cameraview matrix or the modelview?

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modelview
I think it's odd because I usually see circular diagrams with the 90 and 270 opposite.
But whatever, it's not a big deal I guess.

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