This function must be called for any image constructed by the client before passing it to any other Xlib function. Image structures created or returned by Xlib do not need to be initialized in this fashion.
This function returns a nonzero status if initialization of the structure is successful. It returns zero if it detected some error or inconsistency in the structure, in which case the image is not changed.
The XCreateImage function allocates the memory needed for an XImage structure for the specified display but does not allocate space for the image itself. Rather, it initializes the structure byte-order, bit-order, and bitmap-unit values from the display and returns a pointer to the XImage structure. The red, green, and blue mask values are defined for Z format images only and are derived from the Visual structure passed in. Other values also are passed in. The offset permits the rapid displaying of the image without requiring each scanline to be shifted into position. If you pass a zero value in bytes_per_line, Xlib assumes that the scanlines are contiguous in memory and calculates the value of bytes_per_line itself.
Note that when the image is created using XCreateImage, XGetImage, or XSubImage, the destroy procedure that the XDestroyImage function calls frees both the image structure and the data pointed to by the image structure.
The basic functions used to get a pixel, set a pixel, create a subimage, and add a constant value to an image are defined in the image object. The functions in this section are really macro invocations of the functions in the image object and are defined in <X11/Xutil.h>.
The XGetPixel function returns the specified pixel from the named image. The pixel value is returned in normalized format (that is, the least significant byte of the long is the least significant byte of the pixel). The image must contain the x and y coordinates.
The XPutPixel function overwrites the pixel in the named image with the specified pixel value. The input pixel value must be in normalized format (that is, the least significant byte of the long is the least significant byte of the pixel). The image must contain the x and y coordinates.
The XSubImage function creates a new image that is a subsection of an existing one. It allocates the memory necessary for the new XImage structure and returns a pointer to the new image. The data is copied from the source image, and the image must contain the rectangle defined by x, y, subimage_width, and subimage_height.
The XAddPixel function adds a constant value to every pixel in an image. It is useful when you have a base pixel value from allocating color resources and need to manipulate the image to that form.