如何让png具有透明属性

Question

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

我参考一些代码，并添加地方使用*到已检举

* 行++ = 230; * PNG_COLOR_TYPE_RGBA，*

，使其具有透明特性， 代码成功地运行，但余did't看到透明效果。

任何熟悉libpng或png操作的人都能帮助我吗？

在此代码中， 其中pixel_t，& 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

用α-甲真彩色PNG图像，与位深度= 8，存储透明度作为额外信道，在RGBA顺序，在0-255范围内（0 =完全透明; 255 =完全不透明）。

你在做什么看起来对我来说是正确的。只有230的值意味着“几乎不透明”，可能难以直观地检测到透明度。尝试其他值。

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。