In my 2D Game Engine I'm struggling with correctly rendering sprites if those objects are in a parent-child relationship. The picture explains the problem. I use a scenegraph for rendering and use the visitor pattern for traversal. I want the rotation of the parent to only rotate the child in place.
//spriterenderer.cpp
// sprites are positioned & rotated around the center
GLfloat vertices[] = {
// Pos // Tex
-0.5f, 0.5f, 0.0f, 1.0f,
0.5f, -0.5f, 1.0f, 0.0f,
-0.5f, -0.5f, 0.0f, 0.0f,
-0.5f, 0.5f, 0.0f, 1.0f,
0.5f, 0.5f, 1.0f, 1.0f,
0.5f, -0.5f, 1.0f, 0.0f
};
// this gets called if a GameObject has children
bool SpriteRenderer::Enter(GameObject & node)
{
...
RenderSprite(...);
// save the current modelMatrix on the stack
m_matrixStack.push_back(m_modelMatrix);
// apply transformation. I assume this is where the mistake is made
m_modelMatrix = glm::translate(m_modelMatrix, glm::vec3(node.GetLocalPosition(), 0.0f));
m_modelMatrix = glm::rotate(m_modelMatrix, node.GetLocalRotation(), glm::vec3(0.0f, 0.0f, 1.0f));
m_modelMatrix = glm::scale(m_modelMatrix, glm::vec3(node.GetLocalScale(), 1.0f));
return true;
}
// after drawing all children of a node restore the previous model matrix
bool SpriteRenderer::Leave(GameObject & node)
{
m_modelMatrix = m_matrixStack.back();
m_matrixStack.pop_back();
return true;
}
// if a node doesn't have children
bool SpriteRenderer::Visit(GameObject & node)
{
RenderSprite(...);
}
void SpriteRenderer::RenderSprite(...)
{
// save the current transformation
m_matrixStack.push_back(m_modelMatrix);
// apply model transform
m_modelMatrix = glm::translate(m_modelMatrix, glm::vec3(gameObject.GetLocalPosition(), 0.0f));
m_modelMatrix = glm::rotate(m_modelMatrix, gameObject.GetLocalRotation(), glm::vec3(0.0f, 0.0f, 1.0f));
m_modelMatrix = glm::scale(m_modelMatrix, glm::vec3(textureSize, 1.0f));
....
//restore previous transform
m_modelMatrix = m_matrixStack.back();
m_matrixStack.pop_back();
}
I've found a working solution by keeping track of the rotation in an additive way.
So, instead of
// in SpriteRenderer::Enter
m_modelMatrix = glm::rotate(m_modelMatrix, node.GetLocalRotation(), glm::vec3(0.0f, 0.0f, 1.0f));
I use m_additiveRotation += node.GetLocalRotation();
and in SpriteRenderer::Leave I substract the amount again.
Finally, in SpriteRenderer::RenderSprite it changes to
m_modelMatrix = glm::translate(m_modelMatrix, glm::vec3(gameObject.GetLocalPosition(), 0.0f));
m_modelMatrix = glm::rotate(m_modelMatrix, m_additiveRotation, glm::vec3(0.0f, 0.0f, 1.0f));
m_modelMatrix = glm::rotate(m_modelMatrix, gameObject.GetLocalRotation(), glm::vec3(0.0f, 0.0f, 1.0f));
m_modelMatrix = glm::scale(m_modelMatrix, glm::vec3(textureSize, 1.0f));