How to fill the deph buffer in OpenGL 1 with a depth texture?

Question

I separate moveable 3d object and non moveable 3d. I can generate 2 textures for non moveables : background color texture and background depth texture . I have no problem. The values are ok for both. I check the value with a gray gradient texture for the depth.

And I would like to write / fill the color buffer and the depth buffer in opengl with those texture but the depth doesn't work (the color buffer is fine on the screen).

My goal is to never recompute the non movable 3d objects when the camera doesn't move.

(rem : The depth buffer bits size is always in 16 bits)

This is my code for generate the depth texture :

U16 m_backgroundDepthBuffer[4096 * 4096]; // it's a member of my class

glReadPixels(0,0, width(), height(), GL_DEPTH_COMPONENT, GL_UNSIGNED_SHORT, (void*)m_backgroundDepthBuffer);

// Allocate GPU-memory for the depth-texture.
glDeleteTextures(1, &m_backgroundDepthTextureId);
glGenTextures(1, &m_backgroundDepthTextureId);
glBindTexture(GL_TEXTURE_2D, m_backgroundDepthTextureId);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT16,
            width(), height(), 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_SHORT, m_backgroundDepthBuffer);

This is my code for filling the color buffer :

void Application::Render()
{
    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LESS);

    glClear(GL_DEPTH_BUFFER_BIT|GL_COLOR_BUFFER_BIT);


    // render background
    Render::RenderImage(m_backgroundTextureId, 0,0, width(), height());

    // for debug
    // Render::RenderImage(m_backgroundDepthGrayTextureId, 0,0, width(), height());


    // This is how I fill the depth buffer : it's work but slow
    glDrawPixels(width(), height(), GL_DEPTH_COMPONENT, GL_UNSIGNED_SHORT, (GLvoid*) m_backgroundDepthBuffer); // 

    // doesn't work
    // Render::RenderDepth(m_backgroundDepthTextureId, 0,0, width(), height());

}

This is my Render::RenderImage() : it works

void Render::RenderImage(U32 tex, int x, int y, int w, int h, float anchorX, float anchorY)
{
    glClear(GL_DEPTH_BUFFER_BIT);

    GLboolean depth = glIsEnabled(GL_DEPTH_TEST);
    if (depth)
        glDisable(GL_DEPTH_TEST);

    glMatrixMode(GL_PROJECTION);
    glPushMatrix();
    glLoadIdentity();
    gluOrtho2D(0, s_width, s_height, 0);

    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();

    glColor3ub(255, 255, 255);

    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, tex);

    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glDisableClientState(GL_COLOR_ARRAY);

    x -= (int)(anchorX * w);
    y -= (int)(anchorY * h);

    gVertices[0] = VECTOR3F(x, y, 0);
    gVertices[1] = VECTOR3F(x + w - 1, y, 0);
    gVertices[2] = VECTOR3F(x + w - 1, y + h - 1, 0);
    gVertices[3] = VECTOR3F(x, y + h - 1, 0);

    gTexCoords[0] = VECTOR2F(0, 1);
    gTexCoords[1] = VECTOR2F(1, 1);
    gTexCoords[2] = VECTOR2F(1, 0);
    gTexCoords[3] = VECTOR2F(0, 0);

    gIndexes[0] = 2;
    gIndexes[1] = 1;
    gIndexes[2] = 0;

    gIndexes[3] = 0;
    gIndexes[4] = 3;
    gIndexes[5] = 2;

    glVertexPointer(3, GL_FLOAT, 0, gVertices); 
    glTexCoordPointer(2, GL_FLOAT, 0, gTexCoords);
    glDrawElements(GL_TRIANGLES, 3 * 2, GL_UNSIGNED_SHORT, gIndexes);

    glDisable(GL_TEXTURE_2D);


    glPopMatrix();

    glMatrixMode(GL_PROJECTION);
    glPopMatrix();
    glMatrixMode(GL_MODELVIEW);

    if (depth)
        glEnable(GL_DEPTH_TEST);
}

This is my Render::RenderDepth() : it doesn't work / I have a texture with depth = 0.0f instead to use the depth of each texel. I want only to change the depth buffer and do nothing to the color buffer Render::RenderDepth() is similar to Render::RenderImage() : I render 2 triangles

void Render::RenderDepth(U32 tex, int x, int y, int w, int h, float anchorX, float anchorY)
{
    glClear(GL_DEPTH_BUFFER_BIT);


    /*glTexParameteri(GL_TEXTURE_2D_ARRAY, GL_TEXTURE_COMPARE_MODE, GL_COMPARE_REF_DEPTH_TO_TEXTURE_EXT);
    glTexParameteri(GL_TEXTURE_2D_ARRAY, GL_TEXTURE_COMPARE_FUNC, GL_ALWAYS);
    glTexParameteri(GL_TEXTURE_2D_ARRAY, GL_DEPTH_TEXTURE_MODE, GL_ALPHA);
*/
    GLboolean depth = glIsEnabled(GL_DEPTH_TEST);
    //if (depth)
    glEnable(GL_DEPTH_TEST);

    glMatrixMode(GL_PROJECTION);
    glPushMatrix();
    glLoadIdentity();
    gluOrtho2D(0, s_width, s_height, 0);

    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();

    glColor3ub(255, 255, 255);

    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, tex);

    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glDisableClientState(GL_COLOR_ARRAY);

    x -= (int)(anchorX * w);
    y -= (int)(anchorY * h);

    gVertices[0] = VECTOR3F(x, y, 0.0);
    gVertices[1] = VECTOR3F(x + w - 1, y, 0.0);
    gVertices[2] = VECTOR3F(x + w - 1, y + h - 1, 0.0);
    gVertices[3] = VECTOR3F(x, y + h - 1, 0.0);

    gTexCoords[0] = VECTOR2F(0, 1);
    gTexCoords[1] = VECTOR2F(1, 1);
    gTexCoords[2] = VECTOR2F(1, 0);
    gTexCoords[3] = VECTOR2F(0, 0);

    gIndexes[0] = 2;
    gIndexes[1] = 1;
    gIndexes[2] = 0;

    gIndexes[3] = 0;
    gIndexes[4] = 3;
    gIndexes[5] = 2;

    glVertexPointer(3, GL_FLOAT, 0, gVertices);
    glTexCoordPointer(2, GL_FLOAT, 0, gTexCoords);
    glDrawElements(GL_TRIANGLES, 3 * 2, GL_UNSIGNED_SHORT, gIndexes);

    glDisable(GL_TEXTURE_2D);


    glPopMatrix();

    glMatrixMode(GL_PROJECTION);
    glPopMatrix();
    glMatrixMode(GL_MODELVIEW);

}