opengl - Strange behaviour with shadow mapping using GLSL shaders

Question

I've integrated basic shadow mapping. However I noticed a strange behaviour with the rendering. To summarize, the shadow mapping technics requires 2 step. The first to render the scene from the light point of view (filling of a depth texture) and the second from the camera point of view. Here's the C++ code foir the first step :

void Basic::GLSLRenderSystem::renderSceneFromLight(void)
{
    SceneNodeList::iterator nodeListIt = this->m_pScene->getRootNode()->begin();

    for (; nodeListIt != this->m_pScene->getRootNode()->end(); ++nodeListIt)
    {
        EntityList::const_iterator  Ite = (*nodeListIt)->getListEntity().begin();

        (*nodeListIt)->beginTransformations();
        (*nodeListIt)->applyTransformations();

        for (; Ite != (*nodeListIt)->getListEntity().end(); ++Ite)
        {
            //Send matrix

            glm::mat4 modelMatrix = (*nodeListIt)->getModelMatrix();
            glm::mat4 modelViewMatrix = this->m_ViewMatrix * modelMatrix;
            glm::mat4 MVPMatrix = this->m_ProjectionMatrix * modelViewMatrix;

            this->m_pShadowProgram->setUniform("MVP", MVPMatrix);

            (*Ite)->getMesh()->getVertices()->lock();

            //Send vertice positions

            glEnableVertexAttribArray(0);
            glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), OFFSET_BUFFER(0));

            (*Ite)->getMesh()->getVertices()->unlock();

            //Draw primitive

            if ((*Ite)->getMesh()->getVertices()->getCount() == 2)
                glDrawArrays(GL_LINES, 0, (*Ite)->getMesh()->getVertices()->getSize());
            else if ((*Ite)->getMesh()->getVertices()->getCount() == 3)
                glDrawArrays(GL_TRIANGLES, 0, (*Ite)->getMesh()->getVertices()->getSize());
            else if ((*Ite)->getMesh()->getVertices()->getCount() == 4)
                glDrawArrays(GL_QUADS, 0, (*Ite)->getMesh()->getVertices()->getSize());
        }
        (*nodeListIt)->endTransformations();
    }
}

Here's the output :

There's acnee on the floor. But now if I erase the code below the part corresponding the sending of the vertices to the program shader :

void Basic::GLSLRenderSystem::renderSceneFromLight(void)
{
    SceneNodeList::iterator nodeListIt = this->m_pScene->getRootNode()->begin();

    for (; nodeListIt != this->m_pScene->getRootNode()->end(); ++nodeListIt)
    {
        EntityList::const_iterator  Ite = (*nodeListIt)->getListEntity().begin();

        (*nodeListIt)->beginTransformations();
        (*nodeListIt)->applyTransformations();

        for (; Ite != (*nodeListIt)->getListEntity().end(); ++Ite)
        {
            //Send matrix

            glm::mat4 modelMatrix = (*nodeListIt)->getModelMatrix();
            glm::mat4 modelViewMatrix = this->m_ViewMatrix * modelMatrix;
            glm::mat4 MVPMatrix = this->m_ProjectionMatrix * modelViewMatrix;

            this->m_pShadowProgram->setUniform("MVP", MVPMatrix);

            //Draw primitive

            if ((*Ite)->getMesh()->getVertices()->getCount() == 2)
                glDrawArrays(GL_LINES, 0, (*Ite)->getMesh()->getVertices()->getSize());
            else if ((*Ite)->getMesh()->getVertices()->getCount() == 3)
                glDrawArrays(GL_TRIANGLES, 0, (*Ite)->getMesh()->getVertices()->getSize());
            else if ((*Ite)->getMesh()->getVertices()->getCount() == 4)
                glDrawArrays(GL_QUADS, 0, (*Ite)->getMesh()->getVertices()->getSize());
        }
        (*nodeListIt)->endTransformations();
    }
}

I have the following output (with no acnee) :

Here's is the vertex shader :

#version 400

layout (location = 0) in vec3 VertexPosition;

uniform mat4 MVP;

void main(void)
{
    gl_Position = MVP * vec4(VertexPosition, 1.0f);
}

As you can see I transform my vertices in light (view) position. So in my C++ code if I don't send (in the second case) the vertices I wonder how it works ! (I just send the light MVP matrix !) And the most amazing thing is there is no more acnee! How can you explain this behaviour ? Do I send the vertices to the shader like in the first case (and find a solution to erase the acnee on the floor) or not ?