反转PPM图像的值

Question

I am writing in C and I have to write code that will invert the RGB values for each pixel in an image. It's a simple process in that you take the max color value and subtract the actual RGB values. I have successfully read in the max color value, but when trying to invert the values, everything is returned as 0 and when written to a new file is not readable. Below is the code, any ideas?

Inverting the picture

int i,j;
for(i=0; i<myPic->rows;++i) {
    //Moves up and down the columns reaching all Pixels
    //Moves across left to right across all columns
    for (j=0;j<myPic->cols;++j) {
    //Inverstion requires the actual value to be subtracted from the max
        myPic->pixels[i][j].red = myPic->colors - myPic->pixels[i][j].red;
        myPic->pixels[i][j].green = myPic->colors - myPic->pixels[i][j].green;
        myPic->pixels[i][j].blue = myPic->colors - myPic->pixels[i][j].blue;
        }
    }
return myPic;

}

Output of the image

fprintf(name,"P3\n%d %d\n%d\n",myPic->rows,myPic->cols,myPic->colors);
//The P3,rows,columns,and color values are all printed first
int i,j;
for(i=0; i< myPic->rows;++i) {
        for(j=0; j<myPic->cols;++j) { //Each Pixel is printed one at a time
        fprintf(name,"%d",myPic->pixels[i][j].red); //Red printed first
        fprintf(name,"%d",myPic->pixels[i][j].green); //Green printed second
        fprintf(name,"%d",myPic->pixels[i][j].blue); //Blue printed third
        fprintf("\n");
        }
    }

}

Thanks for the help guys, this is what I'm using now and it works

Answer 1

When writing the pixel data, this line

myPic->pixels[i] = malloc(sizeof(Pixel) *myPic->cols);

overwrites the existing pointer, and makes it point to new (and more importantly, uninitialized) data.

Copy-paste programming (which you seem to have been doing) can work sometimes, but you have to take care to modify the copied code properly.

---

On an unrelated note, you don't print a newline after each line, so the resulting PPM file will actually not be correct.

Answer 2

@Joachim is correct. so do something like this:

int i,j;
for(i=0; i<myPic->rows;++i) {

    Pixel[myPic->cols] temp = *myPic->pixels[i];

    //Moves up and down the columns reaching all Pixels
    myPic->pixels[i] = malloc(sizeof(Pixel) *myPic->cols);
    //Moves across left to right across all columns
    for (j=0;j<myPic->cols;++j) {
    //Inverstion requires the actual value to be subtracted from the max
        myPic->pixels[i][j].red = myPic->colors - temp[j].red;
        myPic->pixels[i][j].green = myPic->colors - temp[j].green;
        myPic->pixels[i][j].blue = myPic->colors - temp[j].blue;
        }
    }
    return myPic;
}