Raw CGA Data

Raw CGA Data

Format type	Image
Hardware	CGA
Colour depth	2-bit (CGA)
Minimum size (pixels)	0×0
Maximum size (pixels)	Unlimited
Palette	External
Plane count	1-2
Transparent pixels?	No
Hitmap pixels?	No
Games	None

This page describes a number of generic arrangements for storing CGA graphics data. The graphics in many CGA games are stored based on one of these formats.

Linear CGA data

For CGA tiles, one byte stores the color information for four pixels. Since the tile has 64 pixels (8×8), each tile takes up 16 bytes.

There are four possible combinations of 2 bits (2^2). The actual colors for each pixel depend on the active CGA palette, of which there are three basic ones. See wp:Color Graphics Adapter for details. Every game can use the four color palette differently. Catacomb uses color palette 1 with high intensity.

This data represents one tile from Catacomb:

FF FF FF FF FD 7F F6 9F F6 9F FD 7F FF FF FF FF

Expanded into bits for an 8×8 tile:

11 11 11 11 11 11 11 11 
11 11 11 11 11 11 11 11
11 11 11 01 01 11 11 11 
11 11 01 10 10 01 11 11
11 11 01 10 10 01 11 11 
11 11 11 01 01 11 11 11
11 11 11 11 11 11 11 11 
11 11 11 11 11 11 11 11

With color palette 1i, each pair of bits represents a different color:

Bits	Color
00	black
01	light cyan
10	light magenta
11	white

Resulting Image (actual size):

Resulting Image (8× the size):

Interlaced CGA Data

Offsets into file data

Line (Y)	Plane	Column (X)
		0-3	4-7	8-11	12-15
0	0	00	01	02	03
1	1	10	11	12	13
2	0	04	05	06	07
3	1	14	15	16	17
4	0	08	09	0A	0B
5	1	18	19	1A	1B
6	0	0C	0D	0E	0F
7	1	1C	1D	1E	1F

This arrangement stores the image data in two halves - even lines and odd lines.

It is essentially two black-and-white (1bpp) images one after the other, each half the height of the total picture. The first image is drawn on lines 0, 2, 4, etc. and the second image is drawn on lines 1, 3, 5, etc. to produce the final picture. Each row of data is in the same arrangement as #Linear CGA data above.

A 16×8 image in this arrangement is described in the table on the right. The byte at index 00 contains the first eight-pixels in the image, and the byte at offset 15 contains the second four pixels on the fourth line.

Full-Screen Images

Full Screen PIC Image Format

File Offset	Size	Description
0x0000	0x1F40	Pixel data (even lines)
0x1F40	0x00C0	Padding Block
0x2000	0x1F40	Pixel data (odd lines)
0x3F40	0x00C0	Padding Block

Many full screen CGA images store their data in interlaced format, so that they can be loaded directly into video memory without any additional processing. In some of these cases however, the data is loaded in one operation for the entire image rather than two operations (one per plane.)

This is significant because in 320×200 mode, each plane occupies 8000 bytes of memory, leading to 16000 bytes for the entire image. However because the planes are aligned in CGA memory, the second plane actually starts at offset 8192 into memory, not at offset 8000.

This means that file formats which load an image in a single operation will have 192 bytes of padding between the planes (and possibly 192 bytes after the last plane), which should be ignored when processing the image.

A quick way to tell is to look at the image size. If it is 16000 bytes, it contains two planes with no padding, and if it is 16192 or 16384 bytes then it contains padding between planes.

CGA palettes

The actual RGB values that correspond to each colour value depend on the palette in use by the game. This table lists the RGB values for all the CGA palettes.

RGB values of CGA palettes

Colour index	Palette
	0	0i	1	1i	2	2i
0	Application-specific (default #000000)
1	#00AA00	#55FF55	#00AAAA	#55FFFF	#00AAAA	#55FFFF
2	#AA0000	#FF5555	#AA00AA	#FF55FF	#AA0000	#FF5555
3	#AA5500	#FFFF55	#AAAAAA	#FFFFFF	#AAAAAA	#FFFFFF

For more detailed information, see the CGA Palette article.

Source Code

Interlaced Viewer

This FreeBASIC program will render a full-screen CGA graphic which uses linear encoding.

' Renders a full-screen CGA graphic using linear encoding.

' Put the full file path to the necessary graphic here.
Open "H:\DOS\Ultima\castle.4" For Binary As #1

' Choose the appropriate CGA color palette.
' 0 - Green, Red, Brown
' 1 - Cyan, Magenta, Gray
' 2 - Lt. Green, Lt. Red, Yellow
' 3 - Lt. Cyan, Lt. Magenta, White
Dim As UByte CGAPalette = 1

Screen 1
Color 0, CGAPalette

Dim As UByte X
Dim As UByte Y
Dim As UByte Pixels
Dim As UByte ColorBlock
Dim As UByte Pixel
Dim As UByte Offset

Y = 0
Do
    For X = 0 To 79
        Get #1, , Pixels

        ColorBlock = 7
        For Offset = 0 To 3
            If Bit(Pixels, ColorBlock) = -1 Then
                If Bit(Pixels, ColorBlock - 1) = -1 Then
                    Pixel = 3
                Else
                    Pixel = 2
                End If
            Else
                If Bit(Pixels, ColorBlock - 1) = -1 Then
                    Pixel = 1
                Else
                    Pixel = 0
                End If
            End If

            PSet(Offset + (X * 4), Y), Pixel
            ColorBlock = ColorBlock - 2
        Next Offset
    Next X

    ' The CGA image is interlaced, so we skip a line as we read. When we 
    ' reach the bottom of the screen, we will wrap back up to the top.
    Y = Y + 2
    If Y = 200 Then
        ' When we reach the end of the first pass, there are 192 bytes 
        ' (768 pixels worth) of data that isn't displayed. This is just 
        ' filler to match the size of the CGA card's memory. The 
        ' following code reads the data and throws it away before 
        ' back up to line 1 to draw all the odd lines.
        For Y = 0 To 191
            Get #1, , Pixels
        Next Y
        Y = 1
    End If
Loop Until Y = 201

Close #1
Sleep