Raw VGA Image
| Format type | Image |
|---|---|
| Hardware | VGA |
| Colour depth | 8-bit |
| Minimum size (pixels) | 0×0 |
| Maximum size (pixels) | Unlimited |
| Palette | Varies |
| Plane count | 1 |
| Transparent pixels? | No |
| Hitmap pixels? | No |
| Games |
This format is an exact match for the memory layout of VGA standard mode 13.
Each byte in the image data is treated as an 8-bit number for that pixel. The number is an index into the palette, so a number of 1 refers to the second index in the palette (0 being the first.) The bytes/pixels are arranged on the screen from left-to-right, top-to-bottom.
To render an image correctly, some more information is required which is not part of the raw data:
- Image width
- Image height (optional; can be deduced from file size and width)
- Colour palette
Examples
A raw image decoded with the incorrect width (50px)
The same raw image decoded with the correct width (32px)
Full-screen images
Full-screen images are 320×200 pixels in size, so at 8bpp they are also 64000 bytes long - in this case often with an accompanying 768-byte palette (256 colours × 24-bits).
In real-mode DOS, the 64000 bytes of image data are loaded into address A000:0000 and the image will appear on the screen, subject to the correct palette being loaded.
Palette
Very few images use the default VGA Palette, and a number of schemes are possible for storing the palette.
- As a separate file (e.g. example.img and example.pal), typically the palette file is 768-bytes in length
- Before or after the image data (resulting in a file of 64768 bytes in size for a full-screen image)
- Stored inside the game executable
- As part of another image
Unfortunately since there is no standard in this regard, the palette must be located manually.
Source Code
The following code will read and display a raw VGA image, where the 768-byte palette follows the image data at offset 64,000.
' Load needed variables.
Dim sFile As String
Dim sX As UShort, sY As UShort
Dim bPixel As UByte
' Set the file path to the graphic image.
sFile = "E:\Games\SPISPOPD\map.256"
Open sFile For Binary As #1
' Set to 320x200 256 color mode.
Screen 13
' Load and Draw Image.
For sY = 0 To 199
For sX = 0 To 319
Get #1, , bPixel
PSet(sX, sY), bPixel
Next sX
Next sY
Dim bColor As UShort
Dim bR As UByte, bG As UByte, bB As UByte
' Load and change palette.
For bColor = 0 To 255
Get #1, , bR
Get #1, , bG
Get #1, , bB
Palette bColor, (bR * 4), (bG * 4), (bB * 4)
Next bColor
' Close the graphic file.
Close #1
' Wait for a key press.
Sleep
Credits
FreeBASIC example code by TheAlmightyGuru. If you find this information helpful in a project you're working on, please give credit where credit is due. (A link back to this wiki would be nice too!)