What are this properties in libjpeg and in libpng?

I'm actually using the libjpeg to read and save JPEG images.
So first what are the possibilities of pixel size (info.output_components;) ?
What are the possibilities of color space (info.out_color_space;) ?
And can a JPEG image have an alpha channel ?

I'm using the libpng too.
So first what is the bit depth (png_get_bit_depth(png, info);) ?
And what is the color type (png_get_color_type(png, info);) ?

Thanks !

Solution

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

From the 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;) ?

From the 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 ?

As you can see from the source code above, libjpeg-turbo does support alpha channel for jpegs.

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

Simply put, number of bits used to represent each pixel in an image. The higher the bit depth, the more colors each pixel can contain.

From the 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.