c++ - 如何让png具有透明属性

Question

我已经使用 libpng 生成了 png 文件。对于 RGB png，没有问题。但我想给 png 一个透明的属性。

我引用了一些代码，并在使用 * 的地方添加了标记

*行++ = 230；*PNG_COLOR_TYPE_RGBA,*

使其具有透明属性，代码运行成功，但是没有看到透明效果。

熟悉libpng或png操作的人可以帮帮我吗？

在这段代码中，其中 pixel_t 和 bitmap_t 是一些带有 rgb 数据的数据。

/* 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

具有 alpha 和位深度 = 8 的真彩色 PNG 图像将透明度存储为额外 channel ，按 RGBA 顺序，范围为 0-255(0=完全透明；255=完全不透明)。

在我看来，您所做的是正确的。只有 230 的值表示“几乎不透明”，可能难以通过视觉检测透明度。尝试使用其他值。

顺便说一句，请记住还有其他方法可以为 PNG 图像添加透明度，请参阅我的回答 here .