In this post I mainly plan to go into a brief mathematical explanation as to when the trick will most definitely work, without having to make assumptions as to where the block you are clipping is (like going to the furthest possible right and falling off to find where the right of the block is at). Also, many people use something such as Masterjun's script without actually having an understanding of how a corner clip works.
There are a couple variables that you must first keep in mind before attempting the trick (especially if you are not equipped with a proper lua script). The reference point of where you are actually clipping is perhaps the most important. To help explain this, I have brought along my lua script.
The two main variables are the "theoretical" right and left of the block. This just means that we'll be able to reference the two in the same way whether or not we are coming from the left or right side.
To begin, you need to find the grid position of the block. To do this, subtract the modulus of a position within the block (both x and y) and 16 so that each becomes a multiple of 16.
Just for clarity, let's find the grid position of this vine block:
Getting out our lua script
As you can see, we are at position 3203.00 on the x axis. Subtracting the modulus of our value (3203 - 3203 % 16), we get 3200.
Doing the same on the y axis (even though we wouldn't need to if we were clipping the block):
We are actually at the grid y position of the block here, which is 256.00.
So our grid position for the vine block will be (3200, 256)
Now, to find the theoretical left and right sides of the block. The theoretical left of the block we are looking at will always be the grid's x position - 14, and the theoretical right of the block will always be the grid's x position + 13. The same goes on the y axis if you were to find those values accordingly for top and bottom, but we do not need those for this specific example.
For our vine block, the theoretical left side of the block will be 3186(.00) and the theoretical right will be 3213(.f0).
Onto the actual checking of the clip. In order to successfully do a corner clip, the player needs to reach exactly
2.f0 pixels into the theoretical left/right of the block, so that on the next frame, with 49 speed or higher, you will be able to clip the block perfectly.
As you can see here, one of Mario's predicted positions in the future meets our requirements of reaching 2.f0 subpixels into the block (in this case, 3211.00) and he has (-)49 speed. Also, with the simple formula time = (predicted position - player position) / speed
we can see how many frames it will be before I can make that clip.
And that's it. The largest part of this is actually finding where the block is at, which can be done with a lua script, and from there all you must do is look for one specific value, making sure that your speed is 49 (and you are going downwards).
* Also note that corner clipping to the right is impossible if Mario is small or ducking, unless he is on yoshi.
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