4 triangles in rectangle displayed in PPM

Question

I am trying to generate 4 triangles based on user inputted parameters for a rectangle. My idea is to get the slope from the center of the rectangle to the origin, which on a ppm file is in the top left. This causes everything to be reversed essentially. Anyway, I believe the trouble I am having is from an embedded loop in my main function but I am not certain. My output is of 3 triangles where the triangle on the left overruns the bottom triangle as if it were constantly hugging the slope.

The output is piped to file.ppm and then viewed.

#include <stdio.h>
#include <math.h>

void make_header (int width, int height);
void make_pixel (unsigned char r, unsigned char g, unsigned char b);
void print_Row (float yValue, int width, int height);
void print_Row2 (float yValue, int width, int height);

void make_header(int width, int height)
{
    fprintf(stdout,"P6\n");
    fprintf(stdout,"%d %d 255\n",width,height);
}

void make_pixel (unsigned char r, unsigned char g, unsigned char b)
{
    fprintf(stdout,"%c%c%c", r,g,b);
}

void print_Row(float yValue, int width, int height)
{
    int x;

    for(x=0;x<yValue;x++)   
    {
        make_pixel(255,0,0); // left triangle
    }


    for(x=x; x<width-yValue; x++)
    {
        make_pixel(0,255,0); //top triangle
    }

    for(x=x;x<width;x++)
    {
        make_pixel(0,0,255);  // right triangle
    }
}   

void print_Row2(float yValue, int width, int height)
{
    int x;

    for(x=0;x<yValue;x++)
    {
        make_pixel(255,0,0);
    }

    for(x=x;x<width-yValue;x++)
    {
        make_pixel(0,0,0); //bottom triangle
    }

    for(x=x;x<width;x++)
    {
        make_pixel(0,0,255);
    }
}   

int main()
{
    float slope, inv_slope, width_midpoint, height_midpoint, yValue ;
    int width,height,x,b;

    fprintf(stderr, "\nEnter width: ");
    scanf("%d", &width);
    fprintf(stderr, "\nEnter height: ");
    scanf("%d", &height);
    make_header(width, height);

    width_midpoint = width / 2;
    height_midpoint = height / 2;
    slope = (height_midpoint -0) / (width_midpoint - 0);
    inv_slope = pow(slope,-1);


    for(b=0;b<height_midpoint;b++)
    {   
        for(x=0;x<width;x++)
        {
            yValue = (inv_slope)*(x) + (b);
            print_Row(yValue, width, height);
        }
    }

    for(b=height_midpoint; b<=height; b++)
    {
        for(x=0;x<width;x++)
        {
            yValue = (-1*(slope))*(x)+(b);
            print_Row2(yValue,width,height);
        }
    }

    return 0;
}

score 1 · Accepted Answer · answered Oct 11 '12 at 08:35

You are actually writing way too much data. Replace the main loops with this, and everything should be ok ;)

for(b=0;b<height_midpoint;b++)
{   
    for(x=0;x<width;x++)
    {
        if(x < (b / slope)) { 
            make_pixel(255,0,0);
        }
        else if( x > (width - (b / slope))) {
            make_pixel(0,255,0);
        }
        else
            make_pixel(0,0,255);
    }
}

for(b=height_midpoint; b<=height; b++)
{
    for(x=0;x<width;x++)
    {
        if( x > (b / slope)) {
            make_pixel(0,255,0);
        }
        else if(x > (width - (b / slope))) { 
            make_pixel(0,0,0);
        }
        else
            make_pixel(255,0,0);
    }
}

No worries. You were on the right track anyway by splitting the problem in upper and lower halves of the rectangle. Just be careful to do the "if( x > (foo / bar))" in the correct order, or you will end up with the same problem. — wldsvc, Oct 11 '12 at 16:43

4 triangles in rectangle displayed in PPM

1 Answers1