Ambient occlusion improves the realism of synthetic images by accounting for the overall light attenuation due to local occlusions. This technique recently became a popular tool to produce realistic images, either for offline rendering or in interactive applications such as games. For offline rendering, ambient occlusion (AO) is traditionally computed by casting many rays from each visible point and analyzing the amount of rays hitting the geometry in a close vicinity around the considered point. To achieve interactive frame rates, AO is approximated in screen-space, using the depth buffer to represent the scene geometry, and built upon analytical or numerical tricks to replace the ray-traced occluders search. We present a novel method for computing screen-space ambient occlusion (SSAO) at interactive rates, while preserving the original formulation of the standard ray-traced ambient occlusion. Based on ray-marching on graphics hardware, our method achieves interactive frame rates with physically consistent results, even with few samples. Our SSAO computation is settled within a standard deferred shading framework, making it easily usable into post-production / pre-visualization workflows, or in computer games.