Slobone...a crescendo does not have to have a one-dimensional direction. You will limit yourself if you think that, I promise.
Well, that's not what I was trying to say. I guess I'll rephrase it (and agree with some of what's been said) by saying the word
crescendo is hopelessly inadequate to express what's really going on in a musical performance. Like
rubato, it's only a starting point for understanding, that ends up being not especially helpful. And beginners can really be misled by it.
If you want to say there's more than one dimension, or there are different types of crescendos, that's fine. But I don't know if that's all that helpful either.
For me, playing the piano is about
phrases that are played by
voices. It's as if your fingers were an orchestra and you were the conductor. Sometimes the voices are clearly differentiated, other times they blend into a whole. But they never completely lose their individuality.
Let's take a simple Chopin waltz or mazurka, where the left hand is going oom pah pah, oom pah pah. But already, if you play the "ooms" by themselves, you will see they form a melody of sorts. It may be rudimentary, and not the sort of thing you find yourself humming, but each of the notes relates to the others. There is a direction to them. One of the ooms is the most important, and that's the one the others lead up to or away from.
And even the "pahs". If there are two of them, they do something different. You don't play them the same. One of the pahs is more interesting than the other.
So already you have 3 separate voices: the ooms, the pahs, and the melody in the right hand. And each voice is divided into successive phrases, whose beginnings and endings may or may not coincide in the different voices. Within each phrase, you rise to the climax of the phrase, then descend.
But "rising" and "descending" can be accomplished in many ways. By getting louder, certainly, but sometimes by getting softer. Or by accelerating, then retarding. Or perhaps by articulation: accents, staccato, marcato, slurring, whatever.
So it's only in the process of deciding how to shape these phrases that crescendos and diminuendos come into the picture. Regardless of what the composer has written in the score: that's an essential clue, but not adequate to tell you what to do.
And that's without even going into your understanding of the place of the phrase within the overall architecture of the piece, what feeling Chopin meant you to convey, what feeling
you want to convey, how you feel that day, how the audience is responding, etc.
Most musicians, I think, hear what they want the music to sound like in their minds, then try to create that sound with their fingers. (A process that definitely includes a knowledge of what their colleagues have done or are doing with the same piece.) It's actually easier to approach it from this end (although in practice it's not easy at all).
All that is much more important than the mechanics of "here I have to make a crescendo or a diminuendo." Which no amount of words can ever describe adequately anyway.
And PS: that's the difference between an amateur and a professional. A professional (hopefully, although there are some surprising exceptions) automatically plays this way, even when he/she is sight-reading or improvising. An amateur has to laboriously construct it from first principles, and rarely succeeds in getting it right. It's like trained actors, who automatically start creating a believable character every time they read from a script, or artists, who incorporate perspective every time they sketch something, without having to give it any conscious thought.
And also why "virtuoso" music is such a challenge: anybody (who's graduated from conservatory) can cram the correct number of notes into each second, but to really say something meaningful through it can only be accomplished by a handful of artists in any generation. Which is why there are so few Liszt performances worth listening to...
Topic: A Masterclass video