Continue with the affine transformation (i.e. after transformation, the mid-point of the line segment is no longer the mid-point), this will result in some distortion and this artifact is even more noticeable when the triangle is large:
Condition for linear interpolation
When interpolating the attributes in a linear way, we are saying that given a set of vertices, vi (where i is any integer>=0) with a set of attributes ai (such as texture coordinates), we have a function mapping a vertex to the corresponding attributes, i.e.
Say, to interpolate a vertex inside a triangle in a linear way, the function f need to have the following properties:
which means that we can calculate the interpolated attributes using the same weight taffine function with the following form:
When a vertex is projected from view space to normalized device coordinates(NDC), we will have the following relation (ratio of the triangles) between the view space and NDC space: