OpenGL - tex coord in fragment shader outside of specified range

Question

I'm trying to draw a rectangle with a texture in OpenGL. I'm simply trying to render an entire .jpg image, so I specify the texture coordinates as [0, 0] to [1, 1] in the vertex buffer. I expect all the interpolated texture coordinates in the fragment shader to be between [0, 0] and [1, 1], however, depending on where the texture is drawn, I sometimes get a texture coordinate that is less than 0 (I know this is the case because I tried outputting red from the fragment shader if the tex coord is less than 0).

How come I get an interpolated value outside of the specified range? I currently visualize vertices/fragments like the following image (https://learnopengl.com/Advanced-OpenGL/Anti-Aliasing):

If I imagine a rectangle instead, then if the pixel sample is inside the rectangle, then the interpolated texture coord must be at least 0, since the very left of the rectangle represents 0, right? So how do I end up with a value less than 0?

Edit: after some basic testing, it looks like the fragment shader is called if a shape simply intersects that pixel, not if the pixel sample point is inside the shape. I tested this by placing the start of the rectangle slightly before and slightly after the middle of a pixel - when slightly behind the middle of the pixel, I don't get a negative value, but if I place it slightly after the middle, then I do get a negative value. This contradicts what the website I linked to said - perhaps it's driver-dependent?

Edit: the previous test I did was with multisampling on. If I turn multisampling off, then even if the shape is past the middle, I don't get a negative value...

Answer 1

Turns out I just needed to keep reading the article I linked:

This is where multisampling becomes interesting. We determined that 2 subsamples were covered by the triangle so the next step is to determine a color for this specific pixel. Our initial guess would be that we run the fragment shader for each covered subsample and later average the colors of each subsample per pixel. In this case we'd run the fragment shader twice on the interpolated vertex data at each subsample and store the resulting color in those sample points. This is (fortunately) not how it works, because this basically means we need to run a lot more fragment shaders than without multisampling, drastically reducing performance.

How MSAA really works is that the fragment shader is only run once per pixel (for each primitive) regardless of how many subsamples the triangle covers. The fragment shader is run with the vertex data interpolated to the center of the pixel and the resulting color is then stored inside each of the covered subsamples. Once the color buffer's subsamples are filled with all the colors of the primitives we've rendered, all these colors are then averaged per pixel resulting in a single color per pixel. Because only two of the 4 samples were covered in the previous image, the color of the pixel was averaged with the triangle's color and the color stored at the other 2 sample points (in this case: the clear color) resulting in a light blue-ish color.

So I was getting a negative value because the fragment shader was being run on a pixel that had at least one of its sub-sample points covered by the shape, but the shape was slightly after the mid-point of the pixel, and since "the fragment shader is run with the vertex data interpolated to the center of the pixel", I was getting a negative value.