Using a delicate multi-directional quantitative schlieren optical system, the 20-directional observation of high-speed premixed turbulent flame, non-uniform density flow, by employing non-scanning 3D-CT (three-dimensional computer-tomography) technique has been conducted. The 3D density distributions data of target flame by using ML-EM (Maximum Likelihood-Expectation Maximization) an appropriate CT (computer-tomography) algorithm are successfully reconstructed. In the previous report, a novel technique "multi-path integration" is proposed for noise reduction in projections (density thickness) images of CT (computer-tomography). Multi-path integration is required schlieren brightness gradient in both horizontal (x-directional) and vertical (z-directional) directions, or in other words, in two orthogonal directions (e.g. horizontal and vertical or two perpendicular diagonal directions). The 20-directional quantitative schlieren optical system presents only images of schlieren brightness in the horizontal gradient (x-directional) and another 20-directional optical system seems necessary to obtain vertical (z-directional) schlieren brightness gradient, simultaneously. In the present investigation, the report deals with introducing a new technique for obtaining vertical schlieren brightness gradient from horizontal experimental data without the necessity of a new optical system. This new approach is a simple and inexpensive solution. First, for a deep understanding of complex schlieren brightness, a numerical simulation is performed. Next, the initial density thickness (Dt) images are obtained from images of schlieren brightness in the horizontal gradient (x-directional). Finally, using inverse process from density thickness (Dt) images, the vertical (z-directional) and diagonal schlieren brightness gradient data of target flame successfully obtained. The mentioned technique can be used for obtaining any optional directions of schlieren brightness gradient.