XImage is the X client side storage mechanism for a X Window System

X Window System

The X window system is a computer software system and network protocol that provides a basis for graphical user interfaces and rich input device capability for networked computers...

 pixel map. The structure of an XImage as defined by the X Window core protocol

X Window core protocol

The X Window System core protocol is the base protocol of the X Window System, which is a networked windowing system for bitmap displays used to build graphical user interfaces on Unix, Unix-like, and other operating systems...

 is the following:

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typedef struct XImage XImage;
struct XImage {
int width, height; /* size of image */
int xoffset; /* number of pixels offset in X direction */
int format; /* XYBitmap, XYPixmap, ZPixmap */
char *data; /* pointer to image data */
int byte_order; /* data byte order, LSBFirst, MSBFirst */
int bitmap_unit; /* quant. of scanline 8, 16, 32 */
int bitmap_bit_order; /* LSBFirst, MSBFirst */
int bitmap_pad; /* 8, 16, 32 either XY or ZPixmap */
int depth; /* depth of image */
int bytes_per_line; /* accelerator to next scanline */
int bits_per_pixel; /* bits per pixel (ZPixmap) */
unsigned long red_mask; /* bits in z arrangement */
unsigned long green_mask;
unsigned long blue_mask;
XPointer obdata; /* hook for the object routines to hang on */
struct funcs { /* image manipulation routines */
XImage *(*create_image);
int (*destroy_image);
unsigned long (*get_pixel);
int (*put_pixel);
XImage *(*sub_image);
int (*add_pixel);
} f;
};