Gpu derivatives. How it's done across 2x2 boundary? This question bothers me since the first time I got to know about derivatives in pixel shader. Suppose we have a 2d texture:

5   10  20  30
6   11  50  100


If I correctly understand, the derivatives for the pixel [0, 0] will be:


dx = pixel[1, 0] - pixel[0, 0] = 10 - 5 = 5
dy = pixel[0, 1] - pixel[0, 0] = 6 - 5 = 1


And this will be 100% guaranteed because gpu executes pixels in 2x2 block. But what is the dx-derivative for the pixel[1, 0]? The pixel right to it is from another block which maybe not executed yet? And what about pixel[3, 0]? It doesn't have a right neighbor at all!
In Vulkan the shader only looks at each 2x2 and won't attempt to look beyond the neighbourhood: http://vulkan-spec-chunked.ahcox.com/ch15s05.html

$$dPdx_{0,0}=dPdx_{1, 0} = P_{1,0}−P_{0,0}\\
dPdx_{2,0}=dPdx_{3, 0} = P_{3,0}−P_{2,0}$$

For times when a pixel would fall off the geometry the shader is invoked for the values it would have if the triangle extended a pixel further.