XNA画一个圆圈

Question

In another thread on XNA, Callum Rogers wrote some code which creates a texture with the outline of a circle, but I'm trying to create a circle filled with a color. What I have to modify on this code to fill the circle with color?

public Texture2D CreateCircle(int radius)
{
    int outerRadius = radius*2 + 2; // So circle doesn't go out of bounds
    Texture2D texture = new Texture2D(GraphicsDevice, outerRadius, outerRadius);

    Color[] data = new Color[outerRadius * outerRadius];

    // Colour the entire texture transparent first.
    for (int i = 0; i < data.Length; i++)
        data[i] = Color.Transparent;

    // Work out the minimum step necessary using trigonometry + sine approximation.
    double angleStep = 1f/radius;

    for (double angle = 0; angle < Math.PI*2; angle += angleStep)
    {
        // Use the parametric definition of a circle: http://en.wikipedia.org/wiki/Circle#Cartesian_coordinates
        int x = (int)Math.Round(radius + radius * Math.Cos(angle));
        int y = (int)Math.Round(radius + radius * Math.Sin(angle));

        data[y * outerRadius + x + 1] = Color.White;
    }

    texture.SetData(data);
    return texture;
}

Answer 1

Don't use a texture for stuff like this (especially for things being in one single color!) - also don't try to do it pixel by pixel. You've got 3D acceleration for a reason.

Just draw the circle similar to a pie using a triangle fan. You'll need the following vertices.

• Center of the circle
• x points on the circle's border.

The first two points will define a line between the center of the circle and its border. The third vertex will define the first polygon. Vertices 1, 3 and 4 will then define the second polygon, etc.

To get the points on the circle's border use the formulas from your example. The first angle will be 0°, the following ones multiples of (360° / points on circle ). To get a full circle you'll need one additional point that matches the second point (the first point on the border).

Depending on the number of vertices on the circle you'll get different n-gons. The more vertices you use the rounder the shape will look (at some performance cost):

• (Less than 2 vertices aren't possible as a polygon requires at least 3 vertices to be drawn.)
• Total of 4 points (3 points on circle) will result in a triangle.
• Total of 5 points (4 point on circle) will result in a square.
• Total of 6 points (5 points on circle) will result in a pentagon
• ...

Actually the XNA example for drawing primites show how to draw a circle (or n-gon) using a triangle fan.

Answer 2

well for anyone who wants to do it pixel by pixel ... i made a solution based on the information given. In your 2d texture method add the following code to fill the circle. I'm making a game and wanted to be able to make circles different colors and sizes. So inside CreateCircle(int radius) method, add the following code after the outline has been created :

        bool finished = false;
        int firstSkip = 0;
        int lastSkip = 0;
        for (int i = 0; i <= data.Length - 1; i++)
        {
            if (finished == false)
            {
                //T = transparent W = White;
                //Find the First Batch of Colors TTTTWWWTTTT The top of the circle
                if ((data[i] == Color.White) && (firstSkip == 0))
                {
                    while (data[i + 1] == Color.White)
                    {
                        i++;
                    }
                    firstSkip = 1;
                    i++;
                }
                //Now Start Filling                       TTTTTTTTWWTTTTTTTT
                //circle in Between                       TTTTTTW--->WTTTTTT
                //transaparent blancks                    TTTTTWW--->WWTTTTT
                //                                        TTTTTTW--->WTTTTTT
                //                                        TTTTTTTTWWTTTTTTTT
                if (firstSkip == 1)
                {
                    if (data[i] == Color.White && data[i + 1] != Color.White)
                    {
                        i++;
                        while (data[i] != Color.White)
                        {
                                //Loop to check if its the last row of pixels
                                //We need to check this because of the 
                                //int outerRadius = radius * 2 + -->'2'<--;
                                for (int j = 1; j <= outerRadius; j++)
                                {
                                    if (data[i + j] != Color.White)
                                    {
                                        lastSkip++;
                                    }
                                }
                                //If its the last line of pixels, end drawing
                                if (lastSkip == outerRadius)
                                {
                                    break;
                                    finished = true;
                                }
                                else
                                {
                                    data[i] = Color.White;
                                    i++;
                                    lastSkip = 0;
                                }
                            }
                        while (data[i] == Color.White)
                        {
                            i++;
                        }
                        i--;
                    }


                }
            }
        }
        // Set the data when finished 
        //-- don't need to paste this part, already given up above
        texture.SetData(data);
        return texture;

Answer 3

If you need to do it from scratch (though I'm guessing there are easier ways), change the way you perform the rendering. Instead of iterating through angles and plotting pixels, iterate through pixels and determine where they are relative to the circle. If they are <R , draw as fill color. If they are ~= R , draw as border color.

Answer 4

I know that I'm a little late, but I modified your code to fill in the center

public static Texture2D CreateCircle(GraphicsDevice importedGraphicsDevice, int radius)
    {
        int outerRadius = radius * 2 + 2; // So circle doesn't go out of bounds
        Texture2D texture = new Texture2D(importedGraphicsDevice, outerRadius, outerRadius);

        Color[] data = new Color[outerRadius * outerRadius];

        // Colour the entire texture transparent first.
        for (int i = 0; i < data.Length; i++)
            data[i] = Color.Transparent;

        // Work out the minimum step necessary using trigonometry + sine approximation.
        double angleStep = 1f / radius;

        for (double angle = 0; angle < Math.PI * 2; angle += angleStep)
        {
            // Use the parametric definition of a circle: http://en.wikipedia.org/wiki/Circle#Cartesian_coordinates
            int x = (int)Math.Round(radius + radius * Math.Cos(angle));
            int y = (int)Math.Round(radius + radius * Math.Sin(angle));

            data[y * outerRadius + x + 1] = Color.White;
        }

                    //width
        for (int i = 0; i < outerRadius; i++)
        {
            int yStart = -1;
            int yEnd = -1;


            //loop through height to find start and end to fill
            for (int j = 0; j < outerRadius; j++)
            {

                if (yStart == -1)
                {
                    if (j == outerRadius - 1)
                    {
                        //last row so there is no row below to compare to
                        break;
                    }

                    //start is indicated by Color followed by Transparent
                    if (data[i + (j * outerRadius)] == Color.White && data[i + ((j + 1) * outerRadius)] == Color.Transparent)
                    {
                        yStart = j + 1;
                        continue;
                    }
                }
                else if (data[i + (j * outerRadius)] == Color.White)
                {
                    yEnd = j;
                    break;
                }
            }

            //if we found a valid start and end position
            if (yStart != -1 && yEnd != -1)
            {
                //height
                for (int j = yStart; j < yEnd; j++)
                {
                    data[i + (j * outerRadius)] = new Color(10, 10, 10, 10);
                }
            }
        }

        texture.SetData(data);
        return texture;
    }