Converting bmp to png - Access violation writing location - c++

Question

How to convert bmp to png properly? I was using this code:
#define WIN32_LEAN_AND_MEAN
#define _CRT_SECURE_NO_DEPRECATE
#include <png.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
void GetDesktopResolution(int& horizontal, int& vertical)
{
RECT desktop;
// Get a handle to the desktop window
const HWND hDesktop = GetDesktopWindow();
// Get the size of screen to the variable desktop
GetWindowRect(hDesktop, &desktop);
// The top left corner will have coordinates (0,0)
// and the bottom right corner will have coordinates
// (horizontal, vertical)
horizontal = desktop.right;
vertical = desktop.bottom;
}
typedef struct _RGBPixel {
uint8_t blue;
uint8_t green;
uint8_t red;
} RGBPixel;
/* Structure for containing decompressed bitmaps. */
typedef struct _RGBBitmap {
RGBPixel *pixels;
size_t width;
size_t height;
size_t bytewidth;
uint8_t bytes_per_pixel;
} RGBBitmap;
/* Returns pixel of bitmap at given point. */
#define RGBPixelAtPoint(image, x, y) \
*(((image)->pixels) + (((image)->bytewidth * (y)) \
+ ((x) * (image)->bytes_per_pixel)))
/* Attempts to save PNG to file; returns 0 on success, non-zero on error. */
int save_png_to_file(RGBBitmap *bitmap, const char *path)
{
FILE *fp = fopen(path, "wb");
png_structp png_ptr = NULL;
png_infop info_ptr = NULL;
size_t x, y;
png_uint_32 bytes_per_row;
png_byte **row_pointers = NULL;
if (fp == NULL) return -1;
/* Initialize the write struct. */
png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
if (png_ptr == NULL) {
fclose(fp);
return -1;
}
/* Initialize the info struct. */
info_ptr = png_create_info_struct(png_ptr);
if (info_ptr == NULL) {
png_destroy_write_struct(&png_ptr, NULL);
fclose(fp);
return -1;
}
/* Set up error handling. */
if (setjmp(png_jmpbuf(png_ptr))) {
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(fp);
return -1;
}
/* Set image attributes. */
png_set_IHDR(png_ptr,
info_ptr,
bitmap->width,
bitmap->height,
8,
PNG_COLOR_TYPE_RGB,
PNG_INTERLACE_NONE,
PNG_COMPRESSION_TYPE_DEFAULT,
PNG_FILTER_TYPE_DEFAULT);
/* Initialize rows of PNG. */
bytes_per_row = bitmap->width * bitmap->bytes_per_pixel;
png_malloc(png_ptr, bitmap->height * sizeof(png_byte *));
for (y = 0; y < bitmap->height; ++y) {
uint8_t *row = (uint8_t *)png_malloc(png_ptr, sizeof(uint8_t)* bitmap->bytes_per_pixel);
row_pointers[y] = (png_byte *)row; /************* MARKED LINE ***************/
for (x = 0; x < bitmap->width; ++x) {
RGBPixel color = RGBPixelAtPoint(bitmap, x, y);
*row++ = color.red;
*row++ = color.green;
*row++ = color.blue;
}
}
/* Actually write the image data. */
png_init_io(png_ptr, fp);
png_set_rows(png_ptr, info_ptr, row_pointers);
png_write_png(png_ptr, info_ptr, PNG_TRANSFORM_IDENTITY, NULL);
/* Cleanup. */
for (y = 0; y < bitmap->height; y++) {
png_free(png_ptr, row_pointers[y]);
}
png_free(png_ptr, row_pointers);
/* Finish writing. */
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(fp);
return 0;
}
int main()
{
RGBBitmap rgbbitmap;
int w, h;
GetDesktopResolution(w, h);
rgbbitmap.height = h;
rgbbitmap.width = w;
rgbbitmap.bytes_per_pixel = 1;
rgbbitmap.bytewidth = w / 100;
RGBPixel rgbpixel;
rgbpixel.blue = 100;
rgbpixel.green = 100;
rgbpixel.red = 100;
rgbbitmap.pixels = &rgbpixel;
save_png_to_file(&rgbbitmap, "abc.bmp");
return 0;
}
But at runtime, I'm getting this error on the marked line :
Unhandled exception at 0x01258F04 in ProjectName.exe: 0xC0000005: Access violation writing location 0x00000000.
I've searched in the net, I've found this interesting small code, but I wasn't able to found its mother lib. And, I think that using libpng gives you more control on your output image.
How do I fix this issue? Or, maybe it's a wrong way to convert? If so, how may I do that?

Answer 1

You never allocate memory for row_pointers, so it's NULL when you do row_pointers[y] = (png_byte *)row;.