Create an RGBA image in C++

Question

I have a matrix of 640x480x4 values, i.e. 640x480 pixels each with 4 channels (RGBA). I want to create a png file which shows the transparency. I have tried using opencv, but apparently transparency is not supported. What is the quickest and easiest way I can do this, I just need to save a few images?

Accepted Answer

As other said, you can't easily write a PNG image file in C++ without using an external library.

`libpng` is the smallest and most portable solution, it is the official PNG reference library and is used by most of the image library to write and read PNG files. It's a fairly easy to use C library but may not be the best solution for a C++ novice.

Exemple of writing a PNG image with `libpng` :

```
FILE *fp = fopen("file.png", "wb");
png_structp png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, 0, 0, 0);
png_infop info_ptr = png_create_info_struct(png_ptr);
png_init_io(png_ptr, fp);

/* write 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 */
png_bytep * row_pointers;
<allocate row_pointers and store each row of your image in it>
png_write_image(png_ptr, row_pointers);
png_write_end(png_ptr, NULL);

/* cleanup heap allocation */
for (y=0; y<height; y++) free(row_pointers[y]);
free(row_pointers);

fclose(fp);
```

As you can see it needs quite some lines to simply initialize the writing context, and I voluntarily removed the error management which is a bit tricky with `libpng` (use of `setjmp` as instance) and definitly not suitable for a novice C programmer.

You may take a look at higher level image library such as `ImageMagick` which has a C++ API called `Magick++`, here is an example of writing an image in memory to a PNG file :

```
Magick::Blob blob(data, length); 
Magick::Image image; 
image.size("640x480") 
image.magick("RGBA"); 
image.read(blob);
image.write("file.png");
```

As you can see, it's much easier with a higher level library.

Create an RGBA image in C++

Solution

Problem