Adaptive undersampling is a method for accelerating the rendering process by replacing the calculation of a volume integral with an interpolation procedure for a number of pixels. In this paper, we propose a method for accelerating the volume integral calculation for the rest of the pixels, i.e. those pixels for which interpolation cannot be done with sufficient accuracy. This method requires two passes through the input data. On the first pass, rendering is done into a low-resolution texture.

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... t this stage, the values of the volume integral on a set of intervals of a given length are calculated and saved into a special G-buffer alone with the pixel's color. On the second pass, these values are used to determine colors of the pixels. For those pixels whose result is not precise enough, the volume integral is calculated on one or several intervals, rather than the whole ray. The proposed method allows one to accelerate adaptive undersampling by a factor of 1.5 on average, depending on the input data.