First pick a point where you'll draw around a circle.
Then there are 2 ways:
-Using cosine and sine
calculate the pixels in 360 degrees (for a smooth circle) and draw lines between them.

-Using square root
Try drawing sqrt(9) in a graph and you know what I mean.

If you want examples E-mail me at bosjoh@fcmail.com.

hi,
It's simple there is an API function
called Ellipse which gets a HDC and some
parameters and draws a circle or ellipse,
there is also a DX function which returns
a HDC to a surface !! So it's simple to
draw circles in DX surfaces, Get a HDC to
your DX surface and pass it to the Ellipse
API function...

If you want to draw with your own routine,
there's a fast circle drawing algo called
Middle Point Algorithm which uses only
addition to draw a circle (no mul or divs).

------------------
--Ali Seyedof (It's all dark !)

Thanks for the replies guys. Over the last day or so I have tried the look-up tables, square root, and mid-point algorithms.

I finally settled on mid-point ... but I am still not very satisfied with the circle I get. Just hoping there was something out there that I hadn't found, that would work better.

- Chris

There is a way to draw DDA "circles" (really, they are not circles, but very close to...)

The circle equation is x^2+y^2=r^2
We take derivative: (real coders don't afraid of math! )

dx/dy=1/(2*sqrt(R^2 - x^2) * (-2*x) = -x/y
(sqrt is changed to y)

Then think that dx is delta_x and dy is delta_y.
This way we can draw 1/8 of circle, other 7/8 can be founded easily by mirroring calculated points. Here is the sample code:

; Digital Difference Algorithm demonstration
.386
a segment byte public use16
assume cs:a, ds:a
org 100h
start:
mov ax,13h
int 10h
push 0A000h
pop es
next: mov di,281
sub di,word ptr R+2 ; screen addr starting
;===== 8< ===========================================
xor ecx,ecx ; y starting
mov ebx,R ; x starting
mov bp,bx
circ: mov al,color
mov byte ptr es:[di],al
mov eax,ecx
cdq
shld edx,eax,16
div ebx ; delta x
sub ebx,eax ; next x
sub bp,ax ; looking 4 CF
adc di,320
add ecx,10000h
cmp ecx,ebx
jb circ
;===== 8< ===========================================
dec color
sub R,17935 ; just a number
ja next

xor ah,ah
int 16h
mov ax,3
int 10h
retn

R dd 281*65536
color db ?

a ends
end start

-------------------------------------------
This example was taken from DemoDesign FAQ and translated to english by me

------------------
FlyFire/CodeX
http://codexorg.webjump.com

hi dear chris
Hope you'll be satisfied with one of the
circle drawing algos
But what is more important for you?
the speed of the drawing routine or precision
of the circle shape?

------------------
--Ali Seyedof (It's all dark !)

Does anybody know of a fairly fast way to draw cirlces in DX?

Algorithm or code will work.

- Chris

With the feature that I 'was' going to add, speed would have been the most important. However, now that I decided not to implement that particular effect, quality is the priority. I can speed up any algorithm ... I just want something that is accurate, and fairly fast.

- Chris