What is a PCX file?
A PCX file, Picture Exchange file, is a raster image file format that was used as native file format for PC Paintbrush application. It was developed by ZSoft Corporation, USA for DOS and Windows platforms and was adopted as the main imaging file format before the arrival of BMP, JPEG, and PNG file formats. PCX files are smaller in size as they are compressed using the RLE encoding. It is used in the multi-page DCX file that is primarily used to create digital fax files.
PCX File Format
PCX files are saved to disc in binary file format. The internal file format structure follows little endian byte ordering and has the following three sections:
PCX Header - A PCX Header is 128 bytes long. It contains an identifier, a version number, image dimensions, 16 palette colors, number color planes and bit depth of each plane, and a value for compression method.
PCX Header is as shown below with reference from Encyclopedia of graphics file formats (2nd ed.).
typedef struct _PcxHeader
{
BYTE Identifier; /* PCX Id Number (Always 0x0A) */
BYTE Version; /* Version Number */
BYTE Encoding; /* Encoding Format */
BYTE BitsPerPixel; /* Bits per Pixel */
WORD XStart; /* Left of image */
WORD YStart; /* Top of Image */
WORD XEnd; /* Right of Image
WORD YEnd; /* Bottom of image */
WORD HorzRes; /* Horizontal Resolution */
WORD VertRes; /* Vertical Resolution */
BYTE Palette[48]; /* 16-Color EGA Palette */
BYTE Reserved1; /* Reserved (Always 0) */
BYTE NumBitPlanes; /* Number of Bit Planes */
WORD BytesPerLine; /* Bytes per Scan-line */
WORD PaletteType; /* Palette Type */
WORD HorzScreenSize; /* Horizontal Screen Size */
WORD VertScreenSize; /* Vertical Screen Size */
BYTE Reserved2[54]; /* Reserved (Always 0) */
} PCXHEAD;
Image Data - PCX image data follows just after the header. The image data may be compressed depending upon the field setting in the header. The storage of data in the PCX file depends on the number of specified colour planes. Image data is organized in rows. In case of multiple planes, these are stored by plane within row in sequential arrangement of red, green and blue data. This pattern is repeated for each line in the plane.