在C中将ppm图像向右旋转90度

Question

I have the following problem with rotating the PPM image to the right The first two lines in the result image are black (or some color from the rainbow)

Here's the code that sets the buffer for the image (the variables g_Width and g_height are set by a function)

struct pixel *image = malloc(sizeof(struct pixel) * g_width * g_height);

here's the function with the pointer passed into it

void rotate90(struct pixel *img) {
    int i, j, size, th;
    size = sizeof(struct pixel) * g_width * g_height;
    struct pixel *buffer = malloc(size);

    if (buffer == NULL) {
        fprintf(stderr, "Unable to allocate memory\n");
        exit(EXIT_FAILURE);
    }

    for (i = 0; i < g_height; i++) {
        for (j=0; j < g_width; j++) {
            buffer[(g_height*j)+(g_height-i)] = img[(g_width*i) + j];
        }
    }

    //copy the buffer into the image pointer
    memcpy(img, buffer, size);

    //free the buffer and swap the width and height around
    free(buffer);
    th = g_height;
    g_height = g_width;
    g_width = th;
}

If I print the image buffer it comes out just fine, but if I rotate it it comes out like this (note the first 2 lines of pixels)

https://www.dropbox.com/s/vh8l6s26enbxj42/t3.png?dl=0

it's as if the last 2 lines aren't being swapped at all, please help

EDIT: I solved the second black line at least, but I still need help with the last line

Answer 1

As said you mix the first line (and overflow)

void rotate90(struct pixel *img) {
    int i, j, size, th;
    size = sizeof(struct pixel) * g_width * g_height;
    struct pixel *buffer = malloc(size);

    if (buffer == NULL) {
        fprintf(stderr, "Unable to allocate memory\n");
        exit(EXIT_FAILURE);
    }

    for (i = 0; i < g_height; i++) {
        for (j=0; j < g_width; j++) {
            buffer[(g_height*j)+(g_height-i -- 1)] = img[(g_width*i) + j];
        }
    }

    //copy the buffer into the image pointer
    memcpy(img, buffer, size);

    //free the buffer and swap the width and height around
    free(buffer);
    th = g_height;
    g_height = g_width;
    g_width = th;
}

Answer 2

This would rotate it one way (removing unnecessary brackets)

for (i=0; i<g_height; i++) {
    for (j=0; j<g_width; j++) {
        buffer[g_height * j + i] = img[g_width * i + j];
    }
}

But your code suggest you want it the other way, and the code lacks a -1 , resulting in clipping a line at one edge and an undefined line at the other edge.

for (i=0; i<g_height; i++) {
    for (j=0; j<g_width; j++) {
        buffer[g_height * j + g_height - i - 1] = img[g_width * i + j];
    }
}