libjpeg 和 libpng 中的这些属性是什么？

Question

我实际上是在使用 libjpeg 来读取和保存 JPEG 图像。
那么首先像素大小 (info.output_components;) 的可能性是多少？
颜色 space (info.out_color_space;) 的可能性有哪些?
JPEG 图像可以有 alpha 通道吗？

我也在用libpng。
那么首先位深度是多少 (png_get_bit_depth(png, info);) ?
什么是颜色类型(png_get_color_type(png, info);)？

谢谢！

Answer 1

So first what are the possibilities of pixel size (info.output_components;) ?

来自doc

output_components is 1 (a colormap index) when quantizing colors; otherwise it equals out_color_components. It is the number of JSAMPLE values that will be emitted per pixel in the output arrays.

  int out_color_components;     /* # of color components in out_color_space */
  int output_components;        /* # of color components returned */
  /* output_components is 1 (a colormap index) when quantizing colors;
   * otherwise it equals out_color_components.

What are the possibilities of color space (info.out_color_space;) ?

来自source

  JCS_UNKNOWN,            /* error/unspecified */
  JCS_GRAYSCALE,          /* monochrome */
  JCS_RGB,                /* red/green/blue as specified by the RGB_RED,
                             RGB_GREEN, RGB_BLUE, and RGB_PIXELSIZE macros */
  JCS_YCbCr,              /* Y/Cb/Cr (also known as YUV) */
  JCS_CMYK,               /* C/M/Y/K */
  JCS_YCCK,               /* Y/Cb/Cr/K */
  JCS_EXT_RGB,            /* red/green/blue */
  JCS_EXT_RGBX,           /* red/green/blue/x */
  JCS_EXT_BGR,            /* blue/green/red */
  JCS_EXT_BGRX,           /* blue/green/red/x */
  JCS_EXT_XBGR,           /* x/blue/green/red */
  JCS_EXT_XRGB,           /* x/red/green/blue */
  /* When out_color_space it set to JCS_EXT_RGBX, JCS_EXT_BGRX, JCS_EXT_XBGR,
     or JCS_EXT_XRGB during decompression, the X byte is undefined, and in
     order to ensure the best performance, libjpeg-turbo can set that byte to
     whatever value it wishes.  Use the following colorspace constants to
     ensure that the X byte is set to 0xFF, so that it can be interpreted as an
     opaque alpha channel. */
  JCS_EXT_RGBA,           /* red/green/blue/alpha */
  JCS_EXT_BGRA,           /* blue/green/red/alpha */
  JCS_EXT_ABGR,           /* alpha/blue/green/red */
  JCS_EXT_ARGB,           /* alpha/red/green/blue */
  JCS_RGB565              /* 5-bit red/6-bit green/5-bit blue */

And can a JPEG image have an alpha channel ?

从上面的源代码可以看出，libjpeg-turbo 确实支持 jpeg 的 alpha 通道。

So first what is the bit depth (png_get_bit_depth(png, info);) ?

简单地说，就是用来表示图像中每个像素的位数。位深度越高，每个像素可以包含的颜色越多。

来自PNG Spec：

Color type is a single-byte integer that describes the interpretation of the image data. Color type codes represent sums of the following values: 1 (palette used), 2 (color used), and 4 (alpha channel used). Valid values are 0, 2, 3, 4, and 6.
  Color    Allowed    Interpretation
  Type    Bit Depths
  
  0       1,2,4,8,16  Each pixel is a grayscale sample.
  
  2       8,16        Each pixel is an R,G,B triple.
  
  3       1,2,4,8     Each pixel is a palette index;
                      a PLTE chunk must appear.
  
  4       8,16        Each pixel is a grayscale sample,
                      followed by an alpha sample.
  
  6       8,16        Each pixel is an R,G,B triple,
                      followed by an alpha sample.

libjpeg 和 libpng 中的这些属性是什么？

What are this properties in libjpeg and in libpng?

c++

libpng

libjpeg