OpenGL sprites are trembling slightly at rendering - iphone

Question

I made a class which manage sprites in an iphone application by using opengl es.
rendering works well but when the sprite sheet is large (more than 512x512px) and the sprite has a part which doesn't move, this part seems trembling slightly (like if antialiasing was computed differently for each frame).
Is it normal or is it a bug in my code ?
Here is the code :
//set bounds of the cell
int t_x1 = position.x;
int t_y1 = MAX_SPRITE_HEIGHT - position.y;
int t_x2 = t_x1 + width;
int t_y2 = t_y1 - height;
//set vertices position
GLshort verticesBuffer[4*3];
verticesBuffer[0] = t_x1;
verticesBuffer[1] = t_y1;
verticesBuffer[2] = depth;
verticesBuffer[3] = t_x1;
verticesBuffer[4] = t_y2;
verticesBuffer[5] = depth;
verticesBuffer[6] = t_x2;
verticesBuffer[7] = t_y1;
verticesBuffer[8] = depth;
verticesBuffer[9] = t_x2;
verticesBuffer[10] = t_y2;
verticesBuffer[11] = depth;
//set texture coordinates
GLfloat texCoordBuffer[2*4]
texCoordBuffer[0] = a_cellLayer.origin.x / CGImageGetWidth(texture.textureImage);
texCoordBuffer[1] = a_cellLayer.origin.y / CGImageGetHeight(texture.textureImage);
texCoordBuffer[2] = texCoordBuffer[0];
texCoordBuffer[3] = texCoordBuffer[1] + (float)(t_y1-t_y2) / CGImageGetHeight(texture.textureImage);
texCoordBuffer[4] = texCoordBuffer[6];
texCoordBuffer[5] = texCoordBuffer[1];
texCoordBuffer[6] = texCoordBuffer[0] + (float)(t_x2-t_x1) / CGImageGetWidth(texture.textureImage);
texCoordBuffer[7] = texCoordBuffer[3];
//set texture
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, [texture getTexture]);
//set vertices
glVertexPointer(3, GL_SHORT, 0, verticesBuffer);
glEnableClientState(GL_VERTEX_ARRAY);
//apply texture
glTexCoordPointer(2, GL_FLOAT, 0, texCoordBuffer);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
//render
glColor4f(1.0, 1.0, 1.0, 1.0); // R V B Alpha
glNormal3f(0.0f, 0.0f, 1.0f);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4*nbOfXSudivisions*nbOfYSudivisions);
I tried to change the type of my texture coordinates with GL_FIXED but it doesn't change anything
Does anybody have an idea ?

Answer 1

I don't use depth buffer and I've set the projection matrix like that :
const GLfloat zNear = 100, zFar = 1000, fieldOfView = 22.5;
glMatrixMode(GL_PROJECTION);
GLfloat size = zNear * tanf(DEGREES_TO_RADIANS(fieldOfView) / 2.0);
CGRect rect = oGLView.bounds;
glFrustumf(-size, size, -size / (rect.size.width / rect.size.height), size /
(rect.size.width / rect.size.height), zNear, zFar);
glViewport(0, 0, rect.size.width, rect.size.height);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
//set perspective
gluLookAt(PAGE_WIDTH/2, PAGE_HEIGHT/2, PAGE_WIDTH/2.0f/(tanf(DEGREES_TO_RADIANS(fieldOfView/2.0f))),
PAGE_WIDTH/2, PAGE_HEIGHT/2, 0,
0, 1, 0);