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PPM Vectors

This is a discussion on PPM Vectors within the C Programming forums, part of the General Programming Boards category; I'm having a rough time trying to make a PPM image of a triangle given three points. If I can ...

  1. #1
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    PPM Vectors

    I'm having a rough time trying to make a PPM image of a triangle given three points. If I can get the points into vectors, I can just say that

    if (((x > AC) && (x < BC) && (x < AB)) && ((y > AB) && (y < AC) && (y < BC)))
    make_pixel(purple);
    else
    make_pixel(black);

    I understand how to get the slopes, but how can I form that into vectors? I've tried to get it into y=mx+b format... My three points are

    A (400,100)
    B (600,240)
    C (525,450)

    All in a 640 * 480W image.

    Really lost here...

  2. #2
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    Is your array to hold the pixels, an array of structs with rows and columns, 640 rows x 480 columns, all able to hold a full pixel (red, green, and blue), so 3 chars?

  3. #3
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    Quote Originally Posted by Adak View Post
    Is your array to hold the pixels, an array of structs with rows and columns, 640 rows x 480 columns, all able to hold a full pixel (red, green, and blue), so 3 chars?
    I believe so, except 640 columns and 480 rows. The background is longer than it is tall.

    I meant a vector in the Geometric sense. Like a line segment. If I can print the line segments and just set a pixel 'i' to greater than, less than each vector then I can just use a for loop to fill in the triangle. Or at least, I think I can. My question is either how to (1) code line segments between each point or (2) how to draw a triangle. It isn't a right triangle, so that kinda complicates things.

    My dad emailed me something called the Barycentric Coordinate System, but the math is just beyond me.

    Barycentric coordinate system - Wikipedia, the free encyclopedia

    Barycentric Coordinates, Areal coordinates

  4. #4
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    Since I did most of my beer drinking at home, your dad's Barycentric coordinate system, is well beyond me, also.

    I believe I could do it with an array, however. I've drawn triangles with PPM images before, but I just wrote them out from start to finish, directly to the file, which I don't think is workable in your case.

    Let me mess with that a bit.
    Matticus likes this.

  5. #5
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    This is what I came up with. This is my first time working with PPM files, using an array. Study it at your own risk!!

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    #define WIDTH  120
    #define HEIGHT 100
    
    typedef struct pixels {
       unsigned char r;
       unsigned char g;
       unsigned char b;
    }pxl;
    
    typedef struct points {
       int x;
       int y;
    }point;
    
    void triangles(pxl px[HEIGHT][WIDTH]);
    void output(pxl px[][WIDTH],FILE *fp); 
    
    int main(void) {
       pxl px[HEIGHT][WIDTH]={0};
       FILE *fp;   
       fp=fopen("out.ppm","wb");
       
       if(!fp) {
          printf("Error: file failed to open\n");
          return 0;
       }
      
       triangles(px);
       output(px,fp);
    
       system("out.ppm");
       return 0;
    }
    
    //puts the pixels into the array px[][]
    void triangles (pxl px[][WIDTH]) {
       int r1,c1,rm; //rm=right margin
       point A,B,C;      //where A,B,C are the corners of the triangle
       A.y=10;           // A|-----------B
       A.x=10;           //  |          /
       B.y=10;           //  |         / 
       B.x=60;           //  |        /
       C.y=80;           //  |       /
       C.x=10;           //  |      /
                         //  |     /
                         //  |    /
                         //  |   /
                         //  |  /
                         //  | /
                         //  |/
       rm=B.x;           //  C
       for(r1=A.y;r1<=C.y;r1++) {             
          
          for(c1=A.x;c1<=rm;c1++) {
             
             px[r1][c1].r=0;
             px[r1][c1].g=255;
             px[r1][c1].b=0;      
          }
          rm-=WIDTH/HEIGHT;   
    
       }
    }
    void output(pxl px[][WIDTH],FILE *fp) {
       int r,c;
       
       fprintf(fp,"P6\n%d %d %d\n", WIDTH, HEIGHT, 255); 
       
       for(r=0;r<HEIGHT;r++) {
          for(c=0;c<WIDTH;c++) {
             fprintf(fp, "%c%c%c",px[r][c].r,px[r][c].g,px[r][c].b);
          }
       }
       fclose(fp);
    }

  6. #6
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    Quote Originally Posted by Adak View Post
    This is what I came up with. This is my first time working with PPM files, using an array. Study it at your own risk!!

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    #define WIDTH  120
    #define HEIGHT 100
    
    typedef struct pixels {
       unsigned char r;
       unsigned char g;
       unsigned char b;
    }pxl;
    
    typedef struct points {
       int x;
       int y;
    }point;
    
    void triangles(pxl px[HEIGHT][WIDTH]);
    void output(pxl px[][WIDTH],FILE *fp); 
    
    int main(void) {
       pxl px[HEIGHT][WIDTH]={0};
       FILE *fp;   
       fp=fopen("out.ppm","wb");
       
       if(!fp) {
          printf("Error: file failed to open\n");
          return 0;
       }
      
       triangles(px);
       output(px,fp);
    
       system("out.ppm");
       return 0;
    }
    
    //puts the pixels into the array px[][]
    void triangles (pxl px[][WIDTH]) {
       int r1,c1,rm; //rm=right margin
       point A,B,C;      //where A,B,C are the corners of the triangle
       A.y=10;           // A|-----------B
       A.x=10;           //  |          /
       B.y=10;           //  |         / 
       B.x=60;           //  |        /
       C.y=80;           //  |       /
       C.x=10;           //  |      /
                         //  |     /
                         //  |    /
                         //  |   /
                         //  |  /
                         //  | /
                         //  |/
       rm=B.x;           //  C
       for(r1=A.y;r1<=C.y;r1++) {             
          
          for(c1=A.x;c1<=rm;c1++) {
             
             px[r1][c1].r=0;
             px[r1][c1].g=255;
             px[r1][c1].b=0;      
          }
          rm-=WIDTH/HEIGHT;   
    
       }
    }
    void output(pxl px[][WIDTH],FILE *fp) {
       int r,c;
       
       fprintf(fp,"P6\n%d %d %d\n", WIDTH, HEIGHT, 255); 
       
       for(r=0;r<HEIGHT;r++) {
          for(c=0;c<WIDTH;c++) {
             fprintf(fp, "%c%c%c",px[r][c].r,px[r][c].g,px[r][c].b);
          }
       }
       fclose(fp);
    }
    How do you compile that? I tried gcc triangle.c ; ./a.out > out.ppm gave me an error out.ppm not found...

    The way we compile PPM images is
    gcc *.c
    ./a.out > *.ppm (this is where you name it whatever you want.)
    display *.ppm

    This is for a class and I have classmates that frequent these forums, so I'll post my code tomorrow for a future reference for anyone. It's due midnight tonight.

    At this point I have it done, and it works, but I'm just curious how yours works.

  7. #7
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    Don't redirect the output - the program makes it's own output file. Just display it. (In Windows you can call system(filename.ext), and if the ext has a program associated with it (in this case a view program [IrfanView], pops up and handles the actual display.)

    Name:  GreenTriangle.PNG
Views: 81
Size:  2.0 KB

    The actual size of the pic is not known because the background is black in IrvanView, and I left the background in the ppm file black also, so they have no precise separation here.

  8. #8
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    Nice

  9. #9
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    I'm using uBuntu instead of windows, that could be why it laughs at my './a.out' and calls me a retard...

  10. #10
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    The default output from the program IS out.ppm, so you could change that to "triangle.ppm" or some such.

    I like Ubuntu (although I have practiced imitating a cussing sailor's vocabulary with it a few times). I program on a Windows PC, however.

    The main difference between our programs is mine doesn't do the math yours does. Thank Gawd! :ROFL:

    It uses the A,B,&C x and y values so it only colors green pixels within the triangle. (not tested yet, but more or less).

    Thanks xArt.

  11. #11
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    Quote Originally Posted by Adak View Post
    The main difference between our programs is mine doesn't do the math yours does. Thank Gawd! :ROFL:
    But your program only works with triangles which have a rectangle between A and B and B must be right of A and C must be below A.
    [Edit]It doesn't work for the points in OP's post[/Edit]

    For a more general solution (i.e. no restrictions on the positions of A, B and C) you need the math (e.g. one of these two methods).

    Bye, Andreas
    Last edited by AndiPersti; 03-14-2013 at 01:21 PM.

  12. #12
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    I found a way to work with area's, to test if a point was inside a triangle or not, so the triangle function became:

    Code:
    void triangles (pxl px[][WIDTH]) {
       int r,c,lox,hix,loy,hiy; 
       double areaABC,areaPTAB,areaPTBC,areaPTAC,areaPTotal;
    
       point A,B,C;       //where A,B,C are the corners of the triangle
       A.y=100;           
       A.x=400;           
       B.y=240;            
       B.x=600;           
       C.y=450;           
       C.x=525;           
                          
       lox=400;           
       hix=600;           
       loy=100;
       hiy=450;
       areaABC = abs((A.x*(B.y - C.y) + B.x*(C.y - A.y) + C.x*(A.y-B.y))/2.0);
    
       printf("areaABC: %.2f \n",areaABC); //getchar(); 
    
       for(r=hiy;r>=loy;r--) {             
          
          for(c=lox;c<=hix;c++) {
             areaPTAB= abs((A.x*(B.y-r) + B.x*(r-A.y) + c*(A.y-B.y))/2.0);
             areaPTBC= abs((B.x*(C.y-r) + C.x*(r-B.y) + c*(B.y-C.y))/2.0);
             areaPTAC= abs((A.x*(C.y-r) + C.x*(r-A.y) + c*(A.y-C.y))/2.0);
             areaPTotal=areaPTAB+areaPTBC+areaPTAC;
             //printf("areaPTAB: %.2f areaPTBC: %.2f  areaPTAC: %.2f\n",areaPTAB,areaPTBC,areaPTAC); 
             //printf("areaPTotal: %.2f\n",areaPTotal); //getchar();
             if(areaABC>areaPTotal-2.0 && areaABC<areaPTotal+2.0) { 
                px[r][c].r=0;
                px[r][c].g=255;
                px[r][c].b=0;      
             }
             
          }
       }
    }
    Which has a sloppy "fudge" factor necessary - I'm using some int's where I would need doubles in those expressions, apparently.

    Despite the arithmetic, it's not too slow.

  13. #13
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    Ha! The bug was abs() -- the little devil!

    Code:
    //puts the pixels into the array px[][]
    void triangles (pxl px[][WIDTH]) {
       int r,c, lox,hix,loy,hiy; 
       double areaABC,areaPTAB,areaPTBC,areaPTAC,areaPTotal;
    
       point A,B,C;       //where A,B,C are the corners of the triangle
       A.y=100;           
       A.x=400;           
       B.y=240;            
       B.x=600;           
       C.y=450;           
       C.x=525;           
                          
       lox=400;           
       hix=600;           
       loy=100;
       hiy=450;
       areaABC = (A.x*(B.y - C.y) + B.x*(C.y - A.y) + C.x*(A.y-B.y))/2.0;
    
       printf("areaABC: %.2f \n",areaABC); //getchar(); 
    
       for(r=hiy;r>=loy;r--) {             
          
          for(c=lox;c<=hix;c++) {
             areaPTAB= (A.x*(B.y-r) + B.x*(r-A.y) + c*(A.y-B.y))/2.0;
             areaPTBC= (B.x*(C.y-r) + C.x*(r-B.y) + c*(B.y-C.y))/2.0;
             areaPTAC= (A.x*(C.y-r) + C.x*(r-A.y) + c*(A.y-C.y))/2.0;
    
             if(areaPTAB<0.000) areaPTAB*=-1;
             if(areaPTBC<0.000) areaPTBC*=-1;
             if(areaPTAC<0.000) areaPTAC*=-1;
    
             areaPTotal=areaPTAB+areaPTBC+areaPTAC;
             //printf("areaPTAB: %.2f areaPTBC: %.2f  areaPTAC: %.2f\n",areaPTAB,areaPTBC,areaPTAC); 
             //printf("areaPTotal: %.2f\n",areaPTotal); //getchar();
     
             if(areaABC==areaPTotal) {
                px[r][c].r=0;
                px[r][c].g=255;
                px[r][c].b=0;      
             }
             
          }
       }
    }

  14. #14
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    Okay so I'm doing this as a service to future people with problems coding a triangle in C.. Here is my full code for the program.

    I included a .h file for extra credit, I know it was useless in getting the triangle to print but it does make it a little easier to change the points around and see what happens.. There are two files, Shapes.c and Shapes.h

    Shapes.c:
    Code:
    /* ||__This is programming assignment 3__||
       ||__ Robert Harp CPSC 101 Section 1 __||
       ||__          **Shapes.c**          __||
       ||__********************************__||
       ||__        **Due 03.14.13**        __||
       ||__ This is a program that creates __||
       ||__   a PPM image containing an    __||
       ||__  circle and a purple triangle  __||
       ++====================================++ */
    
    #include<stdio.h>
    #include<stdlib.h>
    #include<math.h>
    #include<stdbool.h>
    #include "Shapes.h"
    
    void make_header(int, int);
    void make_image(int, int);
    void make_pixel(unsigned char, unsigned char, unsigned char);
    double area(int, int, int, int, int, int);
    bool is_inside(int, int, int, int, int, int, int, int);
    
    /*
     *
     */
    
    int main(void)
    {
        int height, width;
    
        height = 480;
        width = 640;
        
        make_header(width, height);
        make_image(width, height);
        fprintf(stderr, "creating an image with a width %d pixels by a height"
            " %d pixels\n", width, height);
    
    return (0);
    }
    
    /*
     * function to make a PPM header for the file
     */
    
    void make_header(int width, int height)
    {
        fprintf(stdout,"P6\n%d %d %d\n", width, height, 255);
    }
    
    /*
     * function to make the image and all of its shapes
     */
    
    void make_image(int width, int height)
    {
        int x, y; //values respective to each axis
    
        for(y = 1; y <= height; y++)
        {
            for(x = 1; x <= width; x++)
            { //if <circle>, else if <triangle>, else <background>
                if ((((x-CCX) * (x-CCX) + (y-CCY) * (y-CCY)) <= 5625))
                {    
                    make_pixel(255,125,1); //Orange circle
                }
                else
                {
                    if (is_inside(TAX, TAY, TBX, TBY, TCX, TCY, x, y))
                    {
                        make_pixel(148, 0, 212); // Purple triangle
                    }
                    else
                    {
                        make_pixel(0, 0, 0); //Black background
                    }
                }
            }
        }
    }
    
    /*
     * Function to print each individual pixel
     */
    
    void make_pixel(unsigned char r, unsigned char g, unsigned char b)
    {
        printf("%c%c%c", r, g, b);
    }
    
    /*
     * Function to find the area of each 
     */
    
    double area(int x1, int y1, int x2, int y2, int x3, int y3)
    {
        return abs((x1*(y2-y3) + x2*(y3-y1)+ x3*(y1-y2))/2.0);
    }
    
    /* 
     * A function to check whether point P(x, y) lies inside the triangle formed
     * by A(x1, y1), B(x2, y2) and C(x3, y3) 
     */
    
    bool is_inside(int x1, int y1, int x2, int y2, int x3, int y3, int x, int y)
    { 
        /* Calculate area of triangle ABC */
        double long A = area (x1, y1, x2, y2, x3, y3);
    
        /* Calculate area of triangle PBC */  
        double long A1 = area (x, y, x2, y2, x3, y3);
    
        /* Calculate area of triangle PAC */  
        double long A2 = area (x1, y1, x, y, x3, y3);
    
        /* Calculate area of triangle PAB */   
        double long A3 = area (x1, y1, x2, y2, x, y);
      
        /* Check if sum of A1, A2 and A3 is same as A */
    return (A >= A1 + A2 + A3);
    }
    /*
     * Coded by Robbo
     */

    And Shapes.h:
    Code:
    #define CCX 225
    #define CCY 175
    #define TAX 400
    #define TAY 100
    #define TBX 600
    #define TBY 240
    #define TCX 525
    #define TCY 450
    
    /*
     * To define my #define's
     * CCX, CCY - Circle Center X value, Y value, respectively
     * TAX, TAY - Triangle Point (A)'s X and Y values, respectively
     * TBX, TBY - Triangle Point (B)'s X and Y values, respectively
     * TCX, TCY - Triangle Point (C)'s X and Y values, respectively
     */
    You are welcome, world.

    Do note that in the last line of Shapes.c, the return value of "bool is_inside()" is >= to the sum of the areas. If you have it as ==, you will get a distorted triangle. <= doesn't print a triangle at all.

    So if you steal this from the geeksforgeeks.org page like I did, you HAVE to change the last line to >= from == to get a full triangle.

    Of course there are other ways to make a triangle, this is just an example to help with site traffic in the coming semesters of CPSC101 students at Clemson.

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