libpng output is not the expected one - c++

Question

I'm trying to get a grasp of basic features in libpng. To do this, I've used this snippet and adapted to my own example.
int x, y;
png_byte color_type = PNG_COLOR_TYPE_RGBA;
png_byte bit_depth = 16;
png_structp png_ptr;
png_infop info_ptr;
auto create_image(const int height, const int width) {
png_byte **rows = new png_byte *[height];
for (auto i = 0; i < height; i++)
rows[i] = new png_byte[width * 4];
return rows;
}
auto modify_image(const int height, const int width, png_byte **rows) {
for (auto i = 0; i < height; i++) {
for (auto j = 0; j < width; j++) {
// Red channel
rows[i][j * 4 + 0] = (j * 127.) / width;
// Blue channel
rows[i][j * 4 + 2] = (i * 127.) / height;
// Alpha channel
rows[i][j * 4 + 3] = 127;
}
}
}
void write(const std::string& filename, const int height, const int width, png_byte** rows)
{
/* create file */
FILE *fp = fopen(filename.c_str(), "wb");
if (!fp)
abort_("[write_png_file] File %s could not be opened for writing", filename);
/* initialize stuff */
png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
if (!png_ptr)
abort_("[write_png_file] png_create_write_struct failed");
info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
abort_("[write_png_file] png_create_info_struct failed");
if (setjmp(png_jmpbuf(png_ptr)))
abort_("[write_png_file] Error during init_io");
png_init_io(png_ptr, fp);
/* write header */
if (setjmp(png_jmpbuf(png_ptr)))
abort_("[write_png_file] Error during writing header");
png_set_IHDR(png_ptr, info_ptr, width, height, bit_depth, color_type, PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
png_write_info(png_ptr, info_ptr);
/* write bytes */
if (setjmp(png_jmpbuf(png_ptr)))
abort_("[write_png_file] Error during writing bytes");
png_write_image(png_ptr, rows);
/* end write */
if (setjmp(png_jmpbuf(png_ptr)))
abort_("[write_png_file] Error during end of write");
png_write_end(png_ptr, NULL);
fclose(fp);
}
Though, when I run my code:
void render(const int height, const int width, const std::string &filename) {
png_byte **rows = create_image(height, width);
modify_image(height, width, rows);
write(filename, height, width, rows);
}
The problem is... I don't get the expected result at all. While I was expecting a square image with some kind of gradient I get... two rectangular rectangles.
Also, I've noticed that these two rectangles are stretched: while trying to render a circle, I've found that the circle was distorted and doubling its width makes it an actual circle...
Finally, I've seen that on the second rectangle, the last row seems to be random data (which is not the case on the first rectangle).
Please suggest if you have any ideas.

Answer 1

You create image with 16 bit depth yet use 1 byte per channel. Output image consists of odd / even rows of your original image being put on the same row. Basically each line of the right rectangle is caused by a buffer overrun. You need to allocate buffers that are twice as big, that is width * 4 * 2 and fill higher and lower bytes of each channel separately.