I would suggest reading the chapter on viewing from the Red Book. Here
is a version online.
http://www.glprogramming.com/red/
The problem is that your translations are out of order. Try this order.
gl.glTranslatef( 10.0f, 10.5f, -0.0f ); //back to previous position
gl.glRotatef( 45.0f, 0.0f, 0.0f, -1.0f ); //rotate
gl.glTranslatef( -10.0f, -10.5f, 0.0f ); //to the origin
Really this is an OpenGL question and not an Android question.
Leigh
On 6/18/2010 1:42 PM, Paolo wrote:
I have to traslate into the z-axis a bit, otherwise I wouldn't see
anything on the screen.
Anyway, I'm doing this step by step as you said to me ;)
At first i traslate the quad to the origin and in fact i see it there.
Then I rotate of 45 degree around the z-axis and I see it rotated
right.
At last I traslate it again to the previous position, but the quad
goes to a different position from that which I expected.
Instead, if i do after the rotation:
glTraslete(1.0f, 0f,0f);
the quad moves of an unit along x-axis as if also this was rotated of
45 degrees, nord-east direction (in order to intend us) and not east-
direction as I expect. I hope you undestrand me now :(
Paolo
p.s. if you prefer I can post my code
On 18 Giu, 18:45, Leigh McRae<[email protected]> wrote:
I am not exactly sure what you're saying actually. At any rate, I
noticed that you're translating along the z-axis. If you're rotating
around the z-axis you don't need to translate along z. Also it could be
that you're translating the quad offscreen since you haven't mirrored
the z translation.
Anyway, what are you seeing? Did you change the order? I would do it
step by step. Start with getting the translation to the origin right.
Once that looks right then add the rotation. Finally add the translation
back.
Leigh
On 6/18/2010 12:35 PM, Paolo wrote:
Thanks for your answer.
my matrix is GL_MODELVIEW.
I have noticed that once I have traslated and rotated the quad, also
the aces x,y,z seem to be rotated of 45 degree. If it is right, it's
normal that when i applied the second traslalte the quad goes in
another position.
is it normal? is there something wrong?
On 18 Giu, 18:21, Leigh McRae<[email protected]> wrote:
OpenGL applies it's transforms in reverse so maybe swap the two
translates. Sounds lame but this is the sort of thing you get right
once and don't look at again for sometime. Also make sure your matrix
mode is GL_MODELVIEW.
Leigh
On 6/18/2010 11:58 AM, Paolo wrote:
Hi leigh!
I've tried, but it doesn't work... the quad goes in a wrong position :
(
My quad has these coordinates
//vertex bottom left
vertex[0] = 9.5f;
vertex[1] = 10f;
vertex[2] = 0.0f;
//top left
vertex[3] = 9.5f;
vertex[4] = 11.0f;
vertex[5] = 0.0f;
//bottom right
vertex[6] = 10.5f;
vertex[7] = 10f;
vertex[8] = 0.0f;
//top right
vertex[9] = 10.5;
vertex[10] = 11f;
vertex[11] = 0.0f;
and this is my draw function...
public void draw(GL10 gl){
gl.glPushMatrix();
gl.glTranslatef(-10, -10.5f, 0); //to the origin
gl.glRotatef(45, 0, 0, -1); //rotate
gl.glTranslatef(10, 10.5f, -2.0f); //back to previous position
// Enabled the vertices buffer for writing and to be used during
// rendering.
gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
// Specifies the location and data format of an array of vertex
// coordinates to use when rendering.
gl.glVertexPointer(3, GL10.GL_FLOAT, 0, markerBuffer);
gl.glDrawArrays(GL10.GL_TRIANGLE_STRIP, 0, NUMERO_VERTICI);
// Disable the vertices buffer.
gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);
gl.glPopMatrix();
}
On 18 Giu, 17:31, Leigh McRae<[email protected]> wrote:
Translate the quad to the origin, rotate, translate back.
Leigh
On 6/18/2010 11:12 AM, Paolo wrote:
Hi guys!
I'm a newbie in OpenGL. :D
I'm trying to rotate a quad around its center.
This is my scenario: I've a quad, drawn with TRIANGLE_STRIP, at
specified coordinates. Now I want to rotate of 45 degrees around its
center, without move it from its position, not around the origin of
the aces.
I hope you are able to understand my english... :)
How can I do that?
Thanks
Paolo
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Leigh McRae
www.lonedwarfgames.com
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