Interpolation in Graphics Pipeline I am trying to program my own version of WEBGL graphics pipeline in python, however, I am confused as to where the color interpolation takes place in the pipeline. According to my research, it happens after the rasterizer has calculated the points covered by the primitive. Does that mean rasterizer has access to color attribute, if so, how is the color attribute passed to the rasterizer, as the inputs for rasterizer are vertices? Or is it happening in the fragment shader?
The rasterizer determines, which pixel is rendered depending on vertex positions that come out of the vertex shader. For each rasterized pixel, the rasterizer knows where it lies inside the triangle formed by its three positions. With the known position inside the triangle, the rasterizer can also interpolate the vertex values for each pixel. So it also calculates the interpolated color, in case your vertex shader passes a color down the pipeline. The interpolated value is then provided to the fragment shader. However, the fragment shader can still modify the color before it is finally rendered to the screen.


  Does that mean rasterizer has access to color attribute


It has full access to everything that comes out of the vertex shader. A vertex shader must always have a position as output (gl_Position). In some special cases also other outputs are required. However, if your initial vertices had a color, but the color is not passed down the pipeline by the vertex shader, the rasterizer won't produce an interpolated color. It only interpolates each variable/attribute, that comes out of the vertex shader.


  how is the color attribute passed to the rasterizer, as the inputs for rasterizer are vertices?


You can define arbitrary outputs in your vertex shader. Even if the initially submitted vertex, did not have a color attribute, you can add a color output in the vertex shader. The data the rasterizer gets per vertex is just the vertex shader outputs. A vertex is a data point containing a position and optional additional values/attributes. So if a vertex has a color attribute, the rasterizer can see it, since it is part of the vertex. 

Note that the vertices you pass into the vertex shader and the vertices that come out of it are not the same and might differ.


  Or is it happening in the fragment shader?


No, the fragment shader receives already interpolated values as input. But it can decide to pass it directly, ignore it or modify it as you wish. If you just pass it through, you will see the interpolated vertex colors. 

Further information can be found on this site