How to rotate a pixel/point around (0, 0) in C? [closed] - c

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I already have the formula for rotating (px, py) around (ox, oy) by angle theta:
p'x = cos(theta) * (px-ox) - sin(theta) * (py-oy) + ox
p'y = sin(theta) * (px-ox) + cos(theta) * (py-oy) + oy
But when I set theta to:
double theta = 5 * PI / 180;
My point stays on the starting point link: (5, 5), whilst when I set theta to:
double theta = 6 * PI / 180;
It starts rotating like a link: square. I know that's supposed to happen because command prompt is like a huge monitor and there can't be a circle, but how can I make it rotate any point I set it to? For example the 6 * PI / 180 above doesn't work when my dot is link: (2, 2).
EDIT: ox, oy are (0, 0).

You don't show your code, but I guess that your code looks like this:
int x = 5;
int y = 5;
double a = 6 * PI / 180;
int i;
for (i = 0; i < 72; i++) {
int xx = round(x);
int yy = round(y);
putpixel(xx, yy, '*');
x = round(xx * cos(a) - yy * sin(a));
y = round(xx * sin(a) + yy * cos(a));
}
The variables could also be double, but the important thing here is that you round an thereby, depending on your radius and angle, cancel the distance you have covered by rotating. I can reproduce the screenshots you show with the code above.
The solution is to keep the fractional floating-point values throughout and to convert to integer only for printing:
double x = 5.0; // unrounded actual coordinates
double y = 5.0;
double a = 5 * PI / 180;
int i;
for (i = 0; i < 72; i++) {
int ix = rint(x); // temporary integers for plotting
int iy = rint(y);
double nx, ny; // temporary variables for update
putpixel(x, y, '*');
nx = x * cos(a) - y * sin(a);
ny = x * sin(a) + y * cos(a);
x = nx;
y = ny;
}
This gives a nice, round circle. If you want to draw a pixel circle with integer coordinates, have a look at the Midpoint algorithm.

Related

Why does my parallel projection appear inverted?

I have the following parallel projection (Row major):
Before I apply the projection I use the following matrix to apply a transformation on the original points:
EDIT correct MATRIX:Matrix
Where a is z rotation,b is y rotation and g is x rotation and s scales my wireframe height.
And it is giving me the following output:
When in reality it should be giving me this (image not centered):
n is 0.1
and f is 100
The origin is in the middle of the wireframe and the 42 wireframe height is in the +z direction.
My matrix multiplication:
typedef struct s_point
{
float x;
float y;
float z;
int c; //color
} t_point;
void multiply_matrix_vector(t_point *i, t_point *o, t_pmatrix *m)
{
float w;
o->x = i->x * m->m[0][0] + i->y * m->m[1][0] + i->z * m->m[2][0] + m->m[3][0];
o->y = i->x * m->m[0][1] + i->y * m->m[1][1] + i->z * m->m[2][1] + m->m[3][1];
o->z = i->x * m->m[0][2] + i->y * m->m[1][2] + i->z * m->m[2][2] + m->m[3][2];
w = i->x * m->m[0][3] + i->y * m->m[1][3] + i->z * m->m[2][3] + m->m[3][3];
if (w != 0.0f)
{
o->x /= w;
o->y /= w;
o->z /= w;
}
o->c = i->c;
}
What am I doing wrong?
I remembered that I was having a hard time with matrix multiplication before and that probably I might had screwed a sign or a trigonometric function. I double checked everything and but all was ok. Still the result was flipped on the y axis so what I did was this:
Matrix
On the first matrix at [1][1] I just changed the 1 for -1
Matrices are tricky because the multiplication order matters more than one would think

Clock Coordinates

I am new to c programming and I had received an assignment not too long ago and ended up doing good enough (points were taken off for not using a for loop). Here is what is from my assignment. "Write a program that prompts the user for the radius of an analog clock, and then prints the x and y coordinates of the 12 hour marks for that clock.You may assume that the origin for the x and y axes is in the center of the clock"
I was confused on how to use the for loop because I was trying to figure out a way how. I know it easy but I struggle with little things. This ended up to copy-pasting code, which led to massive amounts of code. I was wondering if I can get any hints on how to do this program with a for loop rather than having to write all this code Here is my code, but I will not display all of it because there is not point. (Also wondering if I do need arrays? Let me know that too, sorry for the weird format, I have not figured out how to make the code blend in).
int main () {
float x [12];
float y [12];
float rad;
int i, theta = 0;
printf("Enter the radius of the clock\n");
scanf("%f", &rad);
x[0] = rad * cos(90*M_PI/180);
y[0] = rad * sin(90*M_PI/180);
x[1] = rad * cos(60*M_PI/180);
y[1] = rad * sin (60*M_PI/180);
x[2] = rad * cos (30*M_PI/180);
y[2] = rad * sin (30*M_PI/180);
x[3] = rad * cos (0 * M_PI/180);
y[3] = rad * sin (0* M_PI/180);
x[4]= rad * cos (330*M_PI/180);
y[4]= rad * sin (330*M_PI/180);
x[5] = rad * cos (300*M_PI/180);
y[5] = rad * sin (300*M_PI/180);
x[6] = rad * cos (270*M_PI/180);
y[6] = rad * sin (270*M_PI/180);
x[7] = rad * cos (240*M_PI/180);
y[7] = rad * sin (240*M_PI/180);
x[8] = rad * cos (210*M_PI/180);
y[8] = rad * sin (210*M_PI/180);
x[9] = rad * cos (180*M_PI/180);
y[9] = rad * sin (180*M_PI/180);
x[10] = rad * cos (150*M_PI/180);
y[10] = rad * sin (150*M_PI/180);
x[11] = rad * cos (120*M_PI/180);
y[11] = rad * sin (120*M_PI/180);
printf("The x and y coordinates of the 12 o'clock mark are (%f , %f)\n", x[0], y[0]);
I use printf statement for all 12 arrays.
The output is suppose to look like this "The x and y coordinates of the 12 o'clock mark are (0.000000 , 1.000000) and it changes depending on the radius." Thank you!
Just taking sequential statements like this and abstracting them into a loop is a task that requires factoring out the parts that are common.
Start with these two pairs of statements:
x[0] = rad * cos(90*M_PI/180);
y[0] = rad * sin(90*M_PI/180);
x[1] = rad * cos(60*M_PI/180);
y[1] = rad * sin (60*M_PI/180);
What changes here, between the first pair and the second? Only the index of x/y and the input angle.
We can see from scanning the rest of the statements that the index increments sequentially (value increases by one) with each subsequent pair. So we could use a simple loop index as the array indices:
for (int i = 0 ; i < 12; i++) {
x[i] = rad * cos(90*M_PI/180);
y[i] = rad * sin(90*M_PI/180);
}
Okay, good start, but now all 12 values will use 90 degrees. How can we abstract out the angle to make our loop work equivalent to the statements you used? Start by deciding what the increment should be -- in this case, it's not just by 1 like i was. We want 30 degrees, which is also one twelfth of 360.
const size_t divisions = 12;
const size_t degrees_per_iter = 360 / divisions;
for (int i = 0 ; i < divisions; i++) {
x[i] = rad * cos(i*degrees_per_iter*M_PI/180);
y[i] = rad * sin(i*degrees_per_iter*M_PI/180);
}
Now, we're off to a good start. We will cover all 12 values of x and y this way. But we're not 100% identical to the original code. Is the order critical? Let's assume that it is critical, and that will raise the bar a bit.
In order to stay within the valid degree range and keep the output function continuous, we need to use modulus or remainder division.
const size_t divisions = 12;
const size_t degrees_per_iter = 360 / divisions;
const size_t start_angle_deg = 90;
for (int i = 0 ; i < divisions; i++) {
const size_t angle_deg = (start_angle_deg + (i * degrees_per_iter)) % 360;
const float x = rad * cos(angle_deg * M_PI/180);
const float y = rad * sin(angle_deg * M_PI/180);
printf("The x and y coordinates of the 12 o'clock mark are (%f , %f)\n", x, y);
}
Also wondering if I do need arrays
Yes, indeed you do! But you were already using them for x and y. No further arrays are necessary. No arrays are necessary.
I would not have used arrays, I would've done something more like this:
int dir = 0;
char hour = 3;
for(int i=0; i<12; i++){
float xcoord = rad * cos(dir*M_PI/180);
float ycoord = rad * sin(dir*M_PI/180);
printf(...);
dir += 30;
hour++;
hour = (hour > 12) ? 1 : hour+1;
}

Animating a sine wave infinitely

I need a function to animate a sine wave infinitely over time. The sine wave moves to the left.
My sine wave is built using the following equation:
A * sin(B * x + C) + D
Now to animate the sine wave as if it is moving to the left, I simply increase C by 1 everytime I refresh the screen. Now this is all fine and dandy for a few minutes but I need to have that animation run for hours. I can't just have an integer build up 60 times a second forever. How does someone deal with this? Do I just try to find a point where the sine wave crosses 0 and then restart the animation from 0?
I just need to have the logic of something like this explained.
EDIT #1
I forgot to mention that there's a randomized component to my sine. The sine is not continuously the same. A and D are sinusoidal functions tied to that integer at the moment. The sine needs to look random with varying periods and amplitudes.
EDIT #2
Edited see Edit 3
EDIT #3
#Potatoswatter I tried implementing your technique but I don't think I'm getting it. Here's what I got:
static double i = 0;
i = i + (MPI / 2);
if ( i >= 800 * (MPI / 2) ) i -= 800 * (MPI / 2);
for (k = 0; k < 800; ++k)
{
double A1 = 145 * sin((rand1 * (k - 400) + i) / 300) + rand3; // Amplitude
double A2 = 100 * sin((rand2 * (k - 400) + i) / 300) + rand2; // Amplitude
double A3 = 168 * sin((rand3 * (k - 400) + i) / 300) + rand1; // Amplitude
double B1 = 3 + rand1 + (sin((rand3 * k) * i) / (500 * rand1)); // Period
double B2 = 3 + rand2 + (sin((rand2 * k) * i) / 500); // Period
double B3 = 3 + rand3 + (sin((rand1 * k) * i) / (500 * rand3)); // Period
double x = k; // Current x
double C1 = 10 * i; // X axis move
double C2 = 11 * i; // X axis move
double C3 = 12 * i; // X axis move
double D1 = rand1 + sin(rand1 * x / 600) * 4; // Y axis move
double D2 = rand2 + sin(rand2 * x / 500) * 4; // Y axis move
double D3 = rand3 + cos(rand3 * x / 400) * 4; // Y axis move
sine1[k] = (double)A1 * sin((B1 * x + C1) / 400) + D1;
sine2[k] = (double)A2 * sin((B2 * x + C2) / 300) + D2 + 100;
sine3[k] = (double)A3 * cos((B3 * x + C3) / 500) + D3 + 50;
}
How do I modify this to make it work?
Halp!
Sine has a period of 2 pi, meaning that sin(x) = sin(x + 2 * M_PI), for any x.
So, you could just increase C by, say, pi/n where n is any integer, as you refresh the screen, and after 2n refreshes, reset C (to 0, or whatever).
Edit for clarity: the integer n is not meant to change over time.
Instead, pick some n, for example, let's say n = 10. Now, every frame, increase x by pi / 10. After 20 frames, you have increased x by a total of 20 * pi / 10 = 2 * pi. Since sin(x + 2 * pi) = sin(x), you may as well just reset your sin(...) input to just x, and start the process over.
sin is periodic, with a period of 2π. Therefore, if the argument is greater than 2π, you can subtract 2 * M_PI from it and get the same answer.
Instead of using a single variable k to compute all waves of various speeds, use three variables double k1, k2, k3, and keep them bound in the range from 0 to 2π.
if ( k2 >= 2 * M_PI ) k2 -= 2 * M_PI;
They may be individually updated by adding some value each frame. If the increment may be more than 2π then subtracting a single 2π won't bring them back into range, but you can use fmod() instead.
I decided to change my course of action. I just drive i with the system's monotonic clock like so:
struct timespec spec;
int ms;
time_t s;
static unsigned long long etime = 0;
clock_gettime(CLOCK_MONOTONIC, &spec);
s = spec.tv_sec;
ms = spec.tv_nsec / 10000000;
etime = concatenate((long)s, ms);
Then I simply changed i to etime in my sine equations. Here's the concatenating function I used for this purpose:
unsigned concatenate(unsigned x, unsigned y) {
x = x * 100;
return x + y;
}

fast algorithm for drawing filled circles?

I am using Bresenham's circle algorithm for fast circle drawing. However, I also want to (at the request of the user) draw a filled circle.
Is there a fast and efficient way of doing this? Something along the same lines of Bresenham?
The language I am using is C.
Having read the Wikipedia page on Bresenham's (also 'Midpoint') circle algorithm, it would appear that the easiest thing to do would be to modify its actions, such that instead of
setPixel(x0 + x, y0 + y);
setPixel(x0 - x, y0 + y);
and similar, each time you instead do
lineFrom(x0 - x, y0 + y, x0 + x, y0 + y);
That is, for each pair of points (with the same y) that Bresenham would you have you plot, you instead connect with a line.
Just use brute force. This method iterates over a few too many pixels, but it only uses integer multiplications and additions. You completely avoid the complexity of Bresenham and the possible bottleneck of sqrt.
for(int y=-radius; y<=radius; y++)
for(int x=-radius; x<=radius; x++)
if(x*x+y*y <= radius*radius)
setpixel(origin.x+x, origin.y+y);
Here's a C# rough guide (shouldn't be that hard to get the right idea for C) - this is the "raw" form without using Bresenham to eliminate repeated square-roots.
Bitmap bmp = new Bitmap(200, 200);
int r = 50; // radius
int ox = 100, oy = 100; // origin
for (int x = -r; x < r ; x++)
{
int height = (int)Math.Sqrt(r * r - x * x);
for (int y = -height; y < height; y++)
bmp.SetPixel(x + ox, y + oy, Color.Red);
}
bmp.Save(#"c:\users\dearwicker\Desktop\circle.bmp");
You can use this:
void DrawFilledCircle(int x0, int y0, int radius)
{
int x = radius;
int y = 0;
int xChange = 1 - (radius << 1);
int yChange = 0;
int radiusError = 0;
while (x >= y)
{
for (int i = x0 - x; i <= x0 + x; i++)
{
SetPixel(i, y0 + y);
SetPixel(i, y0 - y);
}
for (int i = x0 - y; i <= x0 + y; i++)
{
SetPixel(i, y0 + x);
SetPixel(i, y0 - x);
}
y++;
radiusError += yChange;
yChange += 2;
if (((radiusError << 1) + xChange) > 0)
{
x--;
radiusError += xChange;
xChange += 2;
}
}
}
Great ideas here!
Since I'm at a project that requires many thousands of circles to be drawn, I have evaluated all suggestions here (and improved a few by precomputing the square of the radius):
http://quick-bench.com/mwTOodNOI81k1ddaTCGH_Cmn_Ag
The Rev variants just have x and y swapped because consecutive access along the y axis are faster with the way my grid/canvas structure works.
The clear winner is Daniel Earwicker's method ( DrawCircleBruteforcePrecalc ) that precomputes the Y value to avoid unnecessary radius checks. Somewhat surprisingly that negates the additional computation caused by the sqrt call.
Some comments suggest that kmillen's variant (DrawCircleSingleLoop) that works with a single loop should be very fast, but it's the slowest here. I assume that is because of all the divisions. But perhaps I have adapted it wrong to the global variables in that code. Would be great if someone takes a look.
EDIT: After looking for the first time since college years at some assembler code, I managed find that the final additions of the circle's origin are a culprit.
Precomputing those, I improved the fastest method by a factor of another 3.7-3.9 according to the bench!
http://quick-bench.com/7ZYitwJIUgF_OkDUgnyMJY4lGlA
Amazing.
This being my code:
for (int x = -radius; x < radius ; x++)
{
int hh = (int)std::sqrt(radius_sqr - x * x);
int rx = center_x + x;
int ph = center_y + hh;
for (int y = center_y-hh; y < ph; y++)
canvas[rx][y] = 1;
}
I like palm3D's answer. For being brute force, this is an amazingly fast solution. There are no square root or trigonometric functions to slow it down. Its one weakness is the nested loop.
Converting this to a single loop makes this function almost twice as fast.
int r2 = r * r;
int area = r2 << 2;
int rr = r << 1;
for (int i = 0; i < area; i++)
{
int tx = (i % rr) - r;
int ty = (i / rr) - r;
if (tx * tx + ty * ty <= r2)
SetPixel(x + tx, y + ty, c);
}
This single loop solution rivals the efficiency of a line drawing solution.
int r2 = r * r;
for (int cy = -r; cy <= r; cy++)
{
int cx = (int)(Math.Sqrt(r2 - cy * cy) + 0.5);
int cyy = cy + y;
lineDDA(x - cx, cyy, x + cx, cyy, c);
}
palm3D's brute-force algorithm I found to be a good starting point. This method uses the same premise, however it includes a couple of ways to skip checking most of the pixels.
First, here's the code:
int largestX = circle.radius;
for (int y = 0; y <= radius; ++y) {
for (int x = largestX; x >= 0; --x) {
if ((x * x) + (y * y) <= (circle.radius * circle.radius)) {
drawLine(circle.center.x - x, circle.center.x + x, circle.center.y + y);
drawLine(circle.center.x - x, circle.center.x + x, circle.center.y - y);
largestX = x;
break; // go to next y coordinate
}
}
}
Next, the explanation.
The first thing to note is that if you find the minimum x coordinate that is within the circle for a given horizontal line, you immediately know the maximum x coordinate.
This is due to the symmetry of the circle. If the minimum x coordinate is 10 pixels ahead of the left of the bounding box of the circle, then the maximum x is 10 pixels behind the right of the bounding box of the circle.
The reason to iterate from high x values to low x values, is that the minimum x value will be found with less iterations. This is because the minimum x value is closer to the left of the bounding box than the centre x coordinate of the circle for most lines, due to the circle being curved outwards, as seen on this image
The next thing to note is that since the circle is also symmetric vertically, each line you find gives you a free second line to draw, each time you find a line in the top half of the circle, you get one on the bottom half at the radius-y y coordinate. Therefore, when any line is found, two can be drawn and only the top half of the y values needs to be iterated over.
The last thing to note is that is that if you start from a y value that is at the centre of the circle and then move towards the top for y, then the minimum x value for each next line must be closer to the centre x coordinate of the circle than the last line. This is also due to the circle curving closer towards the centre x value as you go up the circle. Here is a visual on how that is the case.
In summary:
If you find the minimum x coordinate of a line, you get the maximum x coordinate for free.
Every line you find to draw on the top half of the circle gives you a line on the bottom half of the circle for free.
Every minimum x coordinate has to be closer to the centre of the circle than the previous x coordinate for each line when iterating from the centre y coordinate to the top.
You can also store the value of (radius * radius), and also (y * y) instead of calculating them
multiple times.
Here's how I'm doing it:
I'm using fixed point values with two bits precision (we have to manage half points and square values of half points)
As mentionned in a previous answer, I'm also using square values instead of square roots.
First, I'm detecting border limit of my circle in a 1/8th portion of the circle. I'm using symetric of these points to draw the 4 "borders" of the circle. Then I'm drawing the square inside the circle.
Unlike the midpoint circle algorith, this one will work with even diameters (and with real numbers diameters too, with some little changes).
Please forgive me if my explanations were not clear, I'm french ;)
void DrawFilledCircle(int circleDiameter, int circlePosX, int circlePosY)
{
const int FULL = (1 << 2);
const int HALF = (FULL >> 1);
int size = (circleDiameter << 2);// fixed point value for size
int ray = (size >> 1);
int dY2;
int ray2 = ray * ray;
int posmin,posmax;
int Y,X;
int x = ((circleDiameter&1)==1) ? ray : ray - HALF;
int y = HALF;
circlePosX -= (circleDiameter>>1);
circlePosY -= (circleDiameter>>1);
for (;; y+=FULL)
{
dY2 = (ray - y) * (ray - y);
for (;; x-=FULL)
{
if (dY2 + (ray - x) * (ray - x) <= ray2) continue;
if (x < y)
{
Y = (y >> 2);
posmin = Y;
posmax = circleDiameter - Y;
// Draw inside square and leave
while (Y < posmax)
{
for (X = posmin; X < posmax; X++)
setPixel(circlePosX+X, circlePosY+Y);
Y++;
}
// Just for a better understanding, the while loop does the same thing as:
// DrawSquare(circlePosX+Y, circlePosY+Y, circleDiameter - 2*Y);
return;
}
// Draw the 4 borders
X = (x >> 2) + 1;
Y = y >> 2;
posmax = circleDiameter - X;
int mirrorY = circleDiameter - Y - 1;
while (X < posmax)
{
setPixel(circlePosX+X, circlePosY+Y);
setPixel(circlePosX+X, circlePosY+mirrorY);
setPixel(circlePosX+Y, circlePosY+X);
setPixel(circlePosX+mirrorY, circlePosY+X);
X++;
}
// Just for a better understanding, the while loop does the same thing as:
// int lineSize = circleDiameter - X*2;
// Upper border:
// DrawHorizontalLine(circlePosX+X, circlePosY+Y, lineSize);
// Lower border:
// DrawHorizontalLine(circlePosX+X, circlePosY+mirrorY, lineSize);
// Left border:
// DrawVerticalLine(circlePosX+Y, circlePosY+X, lineSize);
// Right border:
// DrawVerticalLine(circlePosX+mirrorY, circlePosY+X, lineSize);
break;
}
}
}
void DrawSquare(int x, int y, int size)
{
for( int i=0 ; i<size ; i++ )
DrawHorizontalLine(x, y+i, size);
}
void DrawHorizontalLine(int x, int y, int width)
{
for(int i=0 ; i<width ; i++ )
SetPixel(x+i, y);
}
void DrawVerticalLine(int x, int y, int height)
{
for(int i=0 ; i<height ; i++ )
SetPixel(x, y+i);
}
To use non-integer diameter, you can increase precision of fixed point or use double values.
It should even be possible to make a sort of anti-alias depending on the difference between dY2 + (ray - x) * (ray - x) and ray2 (dx² + dy² and r²)
If you want a fast algorithm, consider drawing a polygon with N sides, the higher is N, the more precise will be the circle.
I would just generate a list of points and then use a polygon draw function for the rendering.
It may not be the algorithm yo are looking for and not the most performant one,
but I always do something like this:
void fillCircle(int x, int y, int radius){
// fill a circle
for(int rad = radius; rad >= 0; rad--){
// stroke a circle
for(double i = 0; i <= PI * 2; i+=0.01){
int pX = x + rad * cos(i);
int pY = y + rad * sin(i);
drawPoint(pX, pY);
}
}
}
The following two methods avoid the repeated square root calculation by drawing multiple parts of the circle at once and should therefore be quite fast:
void circleFill(const size_t centerX, const size_t centerY, const size_t radius, color fill) {
if (centerX < radius || centerY < radius || centerX + radius > width || centerY + radius > height)
return;
const size_t signedRadius = radius * radius;
for (size_t y = 0; y < radius; y++) {
const size_t up = (centerY - y) * width;
const size_t down = (centerY + y) * width;
const size_t halfWidth = roundf(sqrtf(signedRadius - y * y));
for (size_t x = 0; x < halfWidth; x++) {
const size_t left = centerX - x;
const size_t right = centerX + x;
pixels[left + up] = fill;
pixels[right + up] = fill;
pixels[left + down] = fill;
pixels[right + down] = fill;
}
}
}
void circleContour(const size_t centerX, const size_t centerY, const size_t radius, color stroke) {
if (centerX < radius || centerY < radius || centerX + radius > width || centerY + radius > height)
return;
const size_t signedRadius = radius * radius;
const size_t maxSlopePoint = ceilf(radius * 0.707106781f); //ceilf(radius * cosf(TWO_PI/8));
for (size_t i = 0; i < maxSlopePoint; i++) {
const size_t depth = roundf(sqrtf(signedRadius - i * i));
size_t left = centerX - depth;
size_t right = centerX + depth;
size_t up = (centerY - i) * width;
size_t down = (centerY + i) * width;
pixels[left + up] = stroke;
pixels[right + up] = stroke;
pixels[left + down] = stroke;
pixels[right + down] = stroke;
left = centerX - i;
right = centerX + i;
up = (centerY - depth) * width;
down = (centerY + depth) * width;
pixels[left + up] = stroke;
pixels[right + up] = stroke;
pixels[left + down] = stroke;
pixels[right + down] = stroke;
}
}
This was used in my new 3D printer Firmware, and it is proven the
fastest way for filled circle of a diameter from 1 to 43 pixel. If
larger is needed, the following memory block(or array) should be
extended following a structure I wont waste my time explaining...
If you have questions, or need larger diameter than 43, contact me, I
will help you drawing the fastest and perfect filled circles... or
Bresenham's circle drawing algorithm can be used above those
diameters, but having to fill the circle after, or incorporating the
fill into Bresenham's circle drawing algorithm, will only result in
slower fill circle than my code. I already benchmarked the different
codes, my solution is 4 to 5 times faster. As a test I have been
able to draw hundreds of filled circles of different size and colors
on a BigTreeTech tft24 1.1 running on a 1-core 72 Mhz cortex-m4
https://www.youtube.com/watch?v=7_Wp5yn3ADI
// this must be declared anywhere, as static or global
// as long as the function can access it !
uint8_t Rset[252]={
0,1,1,2,2,1,2,3,3,1,3,3,4,4,2,3,4,5,5,5,2,4,5,5,
6,6,6,2,4,5,6,6,7,7,7,2,4,5,6,7,7,8,8,8,2,5,6,7,
8,8,8,9,9,9,3,5,6,7,8,9,9,10,10,10,10,3,5,7,8,9,
9,10,10,11,11,11,11,3,5,7,8,9,10,10,11,11,12,12,
12,12,3,6,7,9,10,10,11,12,12,12,13,13,13,13,3,6,
8,9,10,11,12,12,13,13,13,14,14,14,14,3,6,8,9,10,
11,12,13,13,14,14,14,15,15,15,15,3,6,8,10,11,12,
13,13,14,14,15,15,15,16,16,16,16,4,7,8,10,11,12,
13,14,14,15,16,16,16,17,17,17,17,17,4,7,9,10,12,
13,14,14,15,16,16,17,17,17,18,18,18,18,18,4,7,9,
11,12,13,14,15,16,16,17,17,18,18,18,19,19,19,19,
19,7,9,11,12,13,15,15,16,17,18,18,19,19,20,20,20,
20,20,20,20,20,7,9,11,12,14,15,16,17,17,18,19,19
20,20,21,21,21,21,21,21,21,21};
// SOLUTION 1: (the fastest)
void FillCircle_v1(uint16_t x, uint16_t y, uint16_t r)
{
// all needed variables are created and set to their value...
uint16_t radius=(r<1) ? 1 : r ;
if (radius>21 ) {radius=21; }
uint16_t diam=(radius*2)+1;
uint16_t ymir=0, cur_y=0;
radius--; uint16_t target=(radius*radius+3*radius)/2; radius++;
// this part draws directly into the ILI94xx TFT buffer mem.
// using pointers..2 versions where you can draw
// pixels and lines with coordinates will follow
for (uint16_t yy=0; yy<diam; yy++)
{ ymir= (yy<=radius) ? yy+target : target+diam-(yy+1);
cur_y=y-radius+yy;
uint16_t *pixel=buffer_start_addr+x-Rset[ymir]+cur_y*buffer_width;
for (uint16_t xx= 0; xx<=(2*Rset[ymir]); xx++)
{ *pixel++ = CANVAS::draw_color; }}}
// SOLUTION 2: adaptable to any system that can
// add a pixel at a time: (drawpixel or add_pixel,etc_)
void FillCircle_v2(uint16_t x, uint16_t y, uint16_t r)
{
// all needed variables are created and set to their value...
uint16_t radius=(r<1) ? 1 : r ;
if (radius>21 ) {radius=21; }
uint16_t diam=(radius*2)+1;
uint16_t ymir=0, cur_y=0;
radius--; uint16_t target=(radius*radius+3*radius)/2; radius++;
for (uint16_t yy=0; yy<diam; yy++)
{ ymir= (yy<=radius) ? yy+target : target+diam-(yy+1);
cur_y=y-radius+yy;
uint16_t Pixel_x=x-Rset[ymir];
for (uint16_t xx= 0; xx<=(2*Rset[ymir]); xx++)
{ //use your add_pixel or draw_pixel here
// using those coordinates:
// X position will be... (Pixel_x+xx)
// Y position will be... (cur_y)
// and add those 3 brackets at the end
}}}
// SOLUTION 3: adaptable to any system that can draw fast
// horizontal lines
void FillCircle_v3(uint16_t x, uint16_t y, uint16_t r)
{
// all needed variables are created and set to their value...
uint16_t radius=(r<1) ? 1 : r ;
if (radius>21 ) {radius=21; }
uint16_t diam=(radius*2)+1;
uint16_t ymir=0, cur_y=0;
radius--; uint16_t target=(radius*radius+3*radius)/2; radius++;
for (uint16_t yy=0; yy<diam; yy++)
{ ymir= (yy<=radius) ? yy+target : target+diam-(yy+1);
cur_y=y-radius+yy;
uint16_t start_x=x-Rset[ymir];
uint16_t width_x=2*Rset[ymir];
// ... then use your best drawline function using those values:
// start_x: position X of the start of the line
// cur_y: position Y of the current line
// width_x: length of the line
// if you need a 2nd coordinate then :end_x=start_x+width_x
// and add those 2 brackets after !!!
}}
I did pretty much what AlegGeorge did but I changed three lines. I thought that this is faster but these are the results am I doing anything wrong? my function is called DrawBruteforcePrecalcV4. here's the code:
for (int x = 0; x < radius ; x++) // Instead of looping from -radius to radius I loop from 0 to radius
{
int hh = (int)std::sqrt(radius_sqr - x * x);
int rx = center_x + x;
int cmx = center_x - x;
int ph = center_y+hh;
for (int y = center_y-hh; y < ph; y++)
{
canvas[rx][y] = 1;
canvas[cmx][y] = 1;
}
}

Filling a polygon

I created this function that draws a simple polygon with n number of vertexes:
void polygon (int n)
{
double pI = 3.141592653589;
double area = min(width / 2, height / 2);
int X = 0, Y = area - 1;
double offset = Y;
int lastx, lasty;
double radius = sqrt(X * X + Y * Y);
double quadrant = atan2(Y, X);
int i;
for (i = 1; i <= n; i++)
{
lastx = X; lasty = Y;
quadrant = quadrant + pI * 2.0 / n;
X = round((double)radius * cos(quadrant));
Y = round((double)radius * sin(quadrant));
setpen((i * 255) / n, 0, 0, 0.0, 1); // r(interval) g b, a, size
moveto(offset + lastx, offset + lasty); // Moves line offset
lineto(offset + X, offset + Y); // Draws a line from offset
}
}
How can I fill it with a solid color?
I have no idea how can I modify my code in order to draw it filled.
The common approach to fill shapes is to find where the edges of the polygon cross either each x or each y coordinate. Usually, y coordinates are used, so that the filling can be done using horizontal lines. (On framebuffer devices like VGA, horizontal lines are faster than vertical lines, because they use consecutive memory/framebuffer addresses.)
In that vein,
void fill_regular_polygon(int center_x, int center_y, int vertices, int radius)
{
const double a = 2.0 * 3.14159265358979323846 / (double)vertices;
int i = 1;
int y, px, py, nx, ny;
if (vertices < 3 || radius < 1)
return;
px = 0;
py = -radius;
nx = (int)(0.5 + radius * sin(a));
ny = (int)(0.5 - radius * cos(a));
y = -radius;
while (y <= ny || ny > py) {
const int x = px + (nx - px) * (y - py) / (ny - py);
if (center_y + y >= 0 && center_y + y < height) {
if (center_x - x >= 0)
moveto(center_x - x, center_y + y);
else
moveto(0, center_y + y);
if (center_x + x < width)
lineto(center_x + x, center_y + y);
else
lineto(width - 1, center_y + y);
}
y++;
while (y > ny) {
if (nx < 0)
return;
i++;
px = nx;
py = ny;
nx = (int)(0.5 + radius * sin(a * (double)i));
ny = (int)(0.5 - radius * cos(a * (double)i));
}
}
}
Note that I only tested the above with a simple SVG generator, and compared the drawn lines to the polygon. Seems to work correctly, but use at your own risk; no guarantees.
For general shapes, use your favourite search engine to look for "polygon filling" algorithms. For example, this, this, this, and this.
There are 2 different ways to implement a solution:
Scan-line
Starting at the coordinate that is at the top (smallest y value), continue to scan down line by line (incrementing y) and see which edges intersect the line.
For convex polygons you find 2 points, (x1,y) and (x2,y). Simply draw a line between those on each scan-line.
For concave polygons this can also be a multiple of 2. Simply draw lines between each pair. After one pair, go to the next 2 coordinates. This will create a filled/unfilled/filled/unfilled pattern on that scan line which resolves to the correct overall solution.
In case you have self-intersecting polygons, you would also find coordinates that are equal to some of the polygon points, and you have to filter them out. After that, you should be in one of the cases above.
If you filtered out the polygon points during scan-lining, don't forget to draw them as well.
Flood-fill
The other option is to use flood-filling. It has to perform more work evaluating the border cases at every step per pixel, so this tends to turn out as a slower version. The idea is to pick a seed point within the polygon, and basically recursively extend up/down/left/right pixel by pixel until you hit a border.
The algorithm has to read and write the entire surface of the polygon, and does not cross self-intersection points. There can be considerable stack-buildup (for naive implementations at least) for large surfaces, and the reduced flexibility you have for the border condition is pixel-based (e.g. flooding into gaps when other things are drawn on top of the polygon). In this sense, this is not a mathematically correct solution, but it works well for many applications.
The most efficient solution is by decomposing the regular polygon in trapezoids (and one or two triangles).
By symmetry, the vertexes are vertically aligned and it is an easy matter to find the limiting abscissas (X + R cos(2πn/N) and X + R cos(2π(+1)N)).
You also have the ordinates (Y + R sin(2πn/N) and Y + R sin(2π(+1)N)) and it suffices to interpolate linearly between two vertexes by Y = Y0 + (Y1 - Y0) (X - X0) / (X1 - X0).
Filling in horizontal runs is a little more complex, as the vertices may not be aligned horizontally and there are more trapezoids.
Anyway, it seems that I / solved / this myself again, when not relying on assistance (or any attempt for it)
void polygon (int n)
{
double pI = 3.141592653589;
double area = min(width / 2, height / 2);
int X = 0, Y = area - 1;
double offset = Y;
int lastx, lasty;
while(Y-->0) {
double radius = sqrt(X * X + Y * Y);
double quadrant = atan2(Y, X);
int i;
for (i = 1; i <= n; i++)
{
lastx = X; lasty = Y;
quadrant = quadrant + pI * 2.0 / n;
X = round((double)radius * cos(quadrant));
Y = round((double)radius * sin(quadrant));
//setpen((i * 255) / n, 0, 0, 0.0, 1);
setpen(255, 0, 0, 0.0, 1); // just red
moveto(offset + lastx, offset + lasty);
lineto(offset + X, offset + Y);
} }
}
As you can see, it isn't very complex, which means it might not be the most efficient solution either.. but it is close enough.
It decrements radius and fills it by virtue of its smaller version with smaller radius.
On that way, precision plays an important role and the higher n is the less accuracy it will be filled with.

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