you need the frame before the f-throw to properly compare positional shifts lol. The example is misleading.
&& on the dair: that's all well and good, but Dair has been changed from Melee. Put the active frames of both Melee and PM dairs side by side and you'll note two things:
1. PM dair hits higher on its final frame, meaning that it can clip higher opponents more easily
2. Every PM frame has more range—note the overlap between the sourspot and sweetspots.
You also fail to consider that Brawl's engine changes aerial interruptible frames. You can use special moves during these frames in brawl—that plainly cannot happen in Melee. That puts the dair in spots that it couldn't be in melee. It's never when I'm "below marth" that I get dair spiked. It's when I'm level with stage and Marth can just jump out, dair, and still get back because of this engine difference.
And do note that I'm talking about "low percents."
I was also equating playing against Marths often to "knowing the matchup." Knowing how to DI throws is level 1 matchup knowledge. I shouldn't have to explain that.
Any positional shift before the f throw is meaningless because the opponent will shift with you. The "catch wait" animation is the animation you are in while holding an opponent. It is 60 frames long, and all frames share the same basic position. There are two images below, the first is a frame of the the "catch wait" animation. As soon as you enter fthrow, you go from the "catch wait" animation to the "ThrowF" animation. The second picture is the first frame of fthrow. Just to be clear, you transition straight from "catch wait" to " ThrowD", there is nothing in between. There is no movement. If you want to be sure, have a look at the position of the front foot on the floor.
[collapse="Frames"]
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As for Dair, where to begin?
Below are the last active frames for Melee and PM, taken from the respective frame data threads
[collapse="Dair frames"]
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They look pretty similar to me. In fact, PM Marth lost some hitboxes on his left arm which were in Melee. I can't say one way or the other about pm dair hitting higher because they look too similar to me, and obviously they were captured by different means and the different animations mean that the "centre" is in different positions. However, even if it is true, the difference is so minimal that it is nearly irrelevant.
As for the claim of having more range... I don't see it. If someone knows how I can do range comparisons, please let me know and I'll do them. All I can say is that in PM, the hitbox is 5.54 units away from the base of the sword with a size (radius) of 3.52 units, giving it max reach of 9.06 units away from the base of the sword. I have no idea how to calculate this in Melee.
Also, I have literally never heard of aerial interruptible frames, can you point me to somewhere where I can read about it? I searched to no avail. The only thing I know about Marth's dair is it can be interrupted with ANY move on frame 60 in both games. If you can interrupt it earlier, please let me know. However, from my interpretation of your words, this seems universal and intrinsic to the physics engine, not just a Marth thing. I don't see how changing Marth but leaving every other character the same would fix it.
And "low percents", do you mean 0? because you can DI out of the follow up at 4-8% (character dependent) and honestly, that's pretty low. If you get grabbed at 0% you deserve to take more than just the 4% fthrow damage.
As you said, knowing how to DI throws is the most basic of basics. Since you don't seem to know how to do that, it is a fair assumption that you don't know the match up very well.