OpenGL OES Iphone glCopyTexImage2D - iphone

Question

I am new to openGL OES on iPhone and have a memory issue with glCopyTexImage2D. So far i understood, this function should copy the current framebuffer to the binded texture. But for some reason it always allocates new memory, which i can see in instruments checking the allocations.
My goal is to read texture images and draw on it, after drawing i want to save the new texture , so i can scroll through the painting. So here is may code:
1) init opengl and framebuffer:
context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES1];
if (!context || ![EAGLContext setCurrentContext:context]) {
[self release];
return nil;
}
glEnable(GL_TEXTURE_2D);
glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_SRC_COLOR);
// Setup OpenGL states
glMatrixMode(GL_PROJECTION);
CGRect frame = self.bounds;
CGFloat scale = self.contentScaleFactor;
// Setup the view port in Pixels
glOrthof(0, frame.size.width * scale, 0, frame.size.height * scale, -1, 1);
glViewport(0, 0, frame.size.width, frame.size.height * scale);
glDisable(GL_DEPTH_TEST);
glDisable(GL_DITHER);
glMatrixMode(GL_MODELVIEW);
glEnableClientState(GL_VERTEX_ARRAY);
// Set a blending function appropriate for premultiplied alpha pixel data
glEnable(GL_POINT_SPRITE_OES);
glTexEnvf(GL_POINT_SPRITE_OES, GL_COORD_REPLACE_OES, GL_TRUE);
glPointSize(64 / kBrushScale);
2) now i load the saved images into the framebuffer:
if ([[NSFileManager defaultManager] fileExistsAtPath:path]) {
// load texture
NSData* data = [[NSData alloc] initWithContentsOfFile:path];
glGenTextures(1, &drawBoardTextures[i]);
glBindTexture(GL_TEXTURE_2D, drawBoardTextures[i]);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 1024, 1024, 0, GL_RGBA, GL_UNSIGNED_BYTE, [data bytes]);
// free memory
[data release];
}
3) and finally render the texture:
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
int width = 1024;
GLfloat quad[] = {0.0,1024.0,1024.0,1024.0,0.0,0.0,1024.0,0.0};
GLfloat quadTex[] = {0.0,1.0,1.0,1.0,0.0,0.0,1.0,0.0};
for (int i=0; i<10; i++) {
quad[0] = width * i;
quad[2] = quad[0] + width;
quad[4] = quad[0];
quad[6] = quad[2];
glBindTexture(GL_TEXTURE_2D, drawBoardTextures[i]);
glVertexPointer(2, GL_FLOAT, 0, quad);
glTexCoordPointer(2, GL_FLOAT, 0, quadTex);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
glBindTexture(GL_TEXTURE_2D, 0);
}
4) for now everything works fine, with gltranslatef i can scroll through the textures and also there is no allocation yet observed in instruments. so now i draw on the current window and want to save the result like followed:
int texIndex = offset.x/1024;
float diff = offset.x - (1024*texIndex);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glBindTexture(GL_TEXTURE_2D, drawBoardTextures[texIndex]);
glBindTexture(GL_TEXTURE_2D, drawBoardTextures[texIndex]);
glCopyTexSubImage2D(GL_TEXTURE_2D, 0, diff, 0, 0, 0, 1024-diff, 1024);
glBindTexture(GL_TEXTURE_2D, drawBoardTextures[texIndex + 1]);
glCopyTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 1024-diff, 0, diff, 1024);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glFlush();
No the problems starts. instead of writing it directly into the generated textures, it writes it into client memory. for every copied texture it uses ~4 MB of Ram, but every recopy doesn't need any memory. i really don't know what i did wrong.
Does anyone know what the problem is? Thanks alot for your help.
cheers
chris