Hey WPLG,
Flat and Frontal are two completely different projection methods. Yes, Frontal is like using the frame of the editor view and place the texture there, the object receives the projection of it, but can move freely “inside”.
The projection method “Flat” on the other hand connects with the object (or should be, I go into details later on*). It projects like a flat plane. Imagine that each pixel of the texture is like a stick. In other words, if the image has a x and y dimension, then the pixel is just projected in z. If the pixel hits the surface of the object, then this pixel value is used there.
*There are generated or deformed objects which shows the effect that something is sliding. One remedy could be to use the Object Manager> Tags> CINEMA 4D Tags> Stick Texture.
Alternative to this, if the object has UV(W) coordinates, you might select all polygons and of the object and use the Object Manager> Tags> Assign UVW Coordinates, if there is no UV(W) Tag, then use the Generate UVW Coordinates. (Doesn’t work with primitives nor with generated objects, it needs to have the ability to use UV(W) Tags.)
If you can share a little example scene and what your target is, perhaps the answer is shorter. I like to start with a problem-solving from the very first idea, not half the way, as the solution is then (most-likely) only limited.
That is the reason why I ask to get the initial idea about the scene/project, anything else might fix other problems that would have been avoidable in the first place.
All the best
Sassi
