how to let png have the transparent property

Question

I have use libpng to generate png file. for RGB png, there is no problem. but I want to give the png a transparent property.

I reference some code, and add the places where use * to flaged

*row++ = 230; *PNG_COLOR_TYPE_RGBA,*

to make it have transparent property, the code is run sucessfully, but I did't see the transparent effect.

can anyone familiar with libpng or png operation help me?

In this code, where the pixel_t,& bitmap_t is some data with rgb data.

/* A colored pixel. */
typedef struct {
    uint8_t red;
    uint8_t green;
    uint8_t blue;
} pixel_t;


/* A picture. */
typedef struct  {
    pixel_t *pixels;
    size_t width;
    size_t height;
} bitmap_t;


static int save_png_to_file (bitmap_t *bitmap, const char *path)
{
    FILE * fp;
    png_structp png_ptr = NULL;
    png_infop info_ptr = NULL;
    size_t x, y;
    png_byte ** row_pointers = NULL;
    /* "status" contains the return value of this function. At first
       it is set to a value which means 'failure'. When the routine
       has finished its work, it is set to a value which means
       'success'. */
    int status = -1;
    /* The following number is set by trial and error only. I cannot
       see where it it is documented in the libpng manual.
    */
    int pixel_size = 3;
    int depth = 8;

    fp = fopen (path, "wb");
    if (! fp) {
        goto fopen_failed;
    }

    png_ptr = png_create_write_struct (PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if (png_ptr == NULL) {
        goto png_create_write_struct_failed;
    }

    info_ptr = png_create_info_struct (png_ptr);
    if (info_ptr == NULL) {
        goto png_create_info_struct_failed;
    }


   // png_set_invert_alpha(png_ptr);



    /* Set up error handling. */

    if (setjmp (png_jmpbuf (png_ptr))) {
        goto png_failure;
    }

    /* Set image attributes. */


    png_set_IHDR (png_ptr,
                  info_ptr,
                  bitmap->width,
                  bitmap->height,
                  depth,
                  *PNG_COLOR_TYPE_RGBA,*
                  PNG_INTERLACE_NONE,
                  PNG_COMPRESSION_TYPE_DEFAULT,
                  PNG_FILTER_TYPE_DEFAULT);

    /* Initialize rows of PNG. */
    //(png_byte **) added by li
    row_pointers = (png_byte **)png_malloc (png_ptr, bitmap->height * sizeof (png_byte *));

    for (y = 0; y < bitmap->height; ++y) {
        //png_byte * added by li
//         png_byte *row = (png_byte *)
//             png_malloc (png_ptr, sizeof (uint8_t) * bitmap->width * pixel_size);
        png_byte *row = (png_byte *)
            png_malloc (png_ptr, sizeof (uint8_t) * bitmap->width * 4);


        row_pointers[y] = row;
        for (x = 0; x < bitmap->width; ++x) {
            pixel_t * pixel = pixel_at (bitmap, x, y);
            *row++ = pixel->red;
            *row++ = pixel->green;
            *row++ = pixel->blue;
            **row++ = 230;*

        }
    }

    /* Write the image data to "fp". */

    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);

    /* The routine has successfully written the file, so we set
       "status" to a value which indicates success. */

    status = 0;

    for (y = 0; y < bitmap->height; y++) {
        png_free (png_ptr, row_pointers[y]);
    }
    png_free (png_ptr, row_pointers);

 png_failure:
 png_create_info_struct_failed:
    png_destroy_write_struct (&png_ptr, &info_ptr);
 png_create_write_struct_failed:
    fclose (fp);
 fopen_failed:
    return status;
}

Answer 1

A truecolor PNG image with alpha , with bitdepth=8, stores the transparency as an extra channel, in RGBA order, in the range 0-255 (0=fully transparent; 255=fully opaque).

What you are doing looks correct to me. Only that a value of 230 means "almost opaque", it might be difficult to detect visually the trasnparency. Try with other values.

BTW, bear in mind that there are other ways to add transparency to a PNG image, see my answer here.