I cannot seem to modify the framebuffer

Question

I am trying to modify the framebuffer on Linux. I am running the program through a virtual terminal (tty). I cannot seem to modify the pixel I want. Here is my code:

#include <stdio.h>

    unsigned char buffer[4 * 1366 * 768];
    const int framewidth = 1366;
void placepixel(int x, int y, int r, int g, int b, int a){
    buffer[(framewidth * y) + x] = b;
    buffer[(framewidth * y) + x+1] = g;
    buffer[(framewidth * y) + x+2] = r;
    buffer[(framewidth * y) + x+3] = a;
}

void placepixelbynum(int i, int r, int g, int b, int a){
    buffer[i] = b;
    buffer[i+1] = g;
    buffer[i+2] = r;
    buffer[i+3] = a;
}
int main(){
    for(int i = 0; i < 4 * 1366 * 768; i+=4){
        placepixelbynum(i, 50,50,50,0);
    }
    FILE *write_ptr;

    write_ptr = fopen("/dev/fb0","wb");

    int x, y, z, xr, yr, zr;

    while(true){

    for(int i = 0; i < 128; i++){

    placepixel(128+i,128,255,0,0,0);
    }
    fwrite(buffer,sizeof(buffer),1,write_ptr);

    }
    return 0;

}

When I run this, the screen turns gray (as expected), but places the line not where I expected. (I want it to start at 128x128 and end at 256x128), but it is near the right end of the screen.

Answer 1

The problem appears it would seem, from a failure to account for 4 bytes of buffer-space for each pixel. Consider the following desk-check

pixel(0,0)  
(framewidth*y) + x + 0  -> 0    
(framewidth*y) + x + 1  -> 1    
(framewidth*y) + x + 2  -> 2    
(framewidth*y) + x + 3  -> 3

pixel(1,0)  
(framewidth*y) + x + 0  -> 1    
(framewidth*y) + x + 1  -> 2    
(framewidth*y) + x + 2  -> 3    
(framewidth*y) + x + 3  -> 4

I suspect you'll have slightly more success with something like this:

void placePixel(int x, int y, int r, int g, int b, int a)
{
    int index = 4 * ((frameWidth*y) + x)
    buffer[index+0] = r;
    buffer[index+1] = g;
    buffer[index+2] = b;
    buffer[index+3] = a;
}