Sample Code
This code assumes that the texture is square (an extremely common occurrence), that no mipmapping comes into play, and that there is only one channel of data (not so common. Nearly all textures are in color so they have red, green, and blue channels, and many have an alpha transparency channel, so we must make three or four calculations of y, one for each channel). The location of UV-coordinates is at center of texel. For example, {(0.25,0.25), (0.75,0.25), (0.25,0.75), (0.75,0.75)} are values for 2x2 texture.
double getBilinearFilteredPixelColor(Texture tex, double u, double v) { u = u * tex.size - 0.5; v = v * tex.size - 0.5; int x = floor(u); int y = floor(v); double u_ratio = u - x; double v_ratio = v - y; double u_opposite = 1 - u_ratio; double v_opposite = 1 - v_ratio; double result = (tex * u_opposite + tex * u_ratio) * v_opposite + (tex * u_opposite + tex * u_ratio) * v_ratio; return result; }Read more about this topic: Bilinear Filtering
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