Thank you for your help, my progress is very fast.

But there is another problem.On the functions provided by ALLEGRO.

al_draw_rotated_bitmap()

This is a function that can be used to rotate the picture.
However, my angle add, then my speed reduction, even if only 0.1 angle.

In the end, I became a tortoise.:(

I repeatedly checked my code.No speed reduction.

This is bug？Or function characteristic？
-------------------------------------------------
I just simply turn al_draw_bitmap into al_draw_rotated_bitmap.

My first blind guess would be you're doing a unit conversion (from degrees to radians) BUT you forgot to add a .0 to the number so it's automatically converting it to an integer (chopping off all the precision!).

 al_draw_rotated_bitmap(my_bitmap, 0, 0, x, y, my_angle*360/2, 0); //no
 al_draw_rotated_bitmap(my_bitmap, 0, 0, x, y, my_angle*360/2.0, 0); //yes
 al_draw_rotated_bitmap(my_bitmap, 0, 0, x, y, my_angle*360.0/2.0, 0); //yes
 al_draw_rotated_bitmap(my_bitmap, 0, 0, x, y, my_angle*360.0/2, 0); //yes

where my_angle was originally in degrees, and you are converting it to radians using *360/2.

Well I hope you're not converting like that. ;P

double deg = rad*180.0/M_PI;
double rad = deg*M_PI/180.0;

Whoops!

Katko,

You're gonna have to show more code. We can't guess what you're doing.

@Chris Katko
That means his principle？
I need to keep the accuracy of the arc.
Using float, the precision will be lost.
It will still reduce the speed.
Float only guarantees six bits.
------------------------------
Anyway, my speed will still be reduced.
I can only use a stupid way.
Revise his speed at a time.
------------------------------
old code：

const float pi = 3.14;
const float Vx_min = 4.2;
const float Vy_min = 4.2;
float arc = pi / 2.0;

if (key[W] && !key[S] && !key[A] && !key[D]) {
        Vx = Vx*cos(arc);
        Vy = Vy*sin(arc);
        y -= Vy;
        x -= Vx;
      }

al_draw_rotated_bitmap(wt, wtW / 2, wtH / 2, x + wtW / 2, y + wtH / 2, arc - pi / 2.0, 0);

------------------------------
Maybe,pi=3.1415926.Speed reduction is not recognized by eyes.
But in that way, the efficiency is down.

Sorry to drop in abruptly but the expression

arc - pi / 2.0

Should evaluate always to 0, as I don't see arc being changed anywhere in the (small) code you posted

If you're having a speed problem, on a modern system the al_draw_rotated_bitmap alone shouldn't be causing that.