Triangular properties which Blank (1961) proposed are useful in investigating the geometry of visual space. With this method, however, it is not possible to estimate quantitatively the curvature of visual space. In the present article, hyperbolic and elliptic triangles are described with mathematical equations in the two and three dimensional Euclidean maps, and a method is proposed to estimate the curvature with triangular properties. Further, one experiment is conducted to find how the curvature changes according to the experimental conditions. Visual triangles are constructed in an eye level plane, a slanted plane and a horopter plane. The result shows that the curvature is negative in the eye level plane and in the slanted plane, but positive in the horopter plane.
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