Purpose of FRM files
FRM files are unpaletted 256-color image files containing either one or several images in one file. The palette used for FRM files come in the form of external palette files.
All values within the FRM file are stored in Big-Endian (Motorola) format as opposed to Little-Endian (Intel) format. For those who don't know what the difference is, Big-Endian stores the most-significant (largest digit place) digits closest to the number's starting address, whereas Little-Endian stores values with the least-significant digits closest to the starting address. Ex:
- With hex number
The FRM File Format is used to store the images used in both Fallout and Fallout 2. It stores all the images and animations, with the exception of those used on the splash screen and the movies. Each FRM file contains one or more frames of image data.
The FRM file uses an index colour model to store the image data. This means that each pixel is represented by an offset into a palette of colours. A colour index of 0, means that the pixel is transparent.
The palette file to use is generally color.pal, but certain FRM files have their own palette files. These specific palette files have the same name as the FRM file, but with the extension .pal. For example: the palette file of helpscrn.frm is helpscrn.pal.
A FRM file may contain image data for one or all of the 6 orientations used in Fallout. If a FRM file has the extension .fr[0-5] instead of the usual .frm, then that file contains image data for the orientation given by the last digit in the extension. Otherwise, the FRM file contains image data for either orientation 0, or all 6 orientations.
The FRM file contains info to correctly align (or centre) the image data. For example: Image data should be aligned so that each critter's feet should be on the same level.
Each frame contained within the FRM file has an offset from the previous frame which must be applied before rendering. This offset ensures that in an animation sequence each frame is correctly aligned.
The centre of the image data is the centre of the bottom edge of the frame. To find the position of the top left corner of the frame (needed for rendering):
left = centre_x - (frame_width / 2) top = centre_y - frame_height
FRM header & data
|Offset||Size||Datatype||Possible values in Fallout 1/2 .FRM's||Description|
|0x0000||4 byte||unsigned||0x04||Version number of the FRM file format|
|0x0004||2 byte||unsigned||0x04||FPS - Frames per second rate of the animation|
|0x0006||2 byte||unsigned||0x01 → 0xFF (if the .FRM got 255 single frames which is highly unlikely)||Action frame - Frame of the animation on which actions occur (shot, open doors, etc.)|
|0x0008||2 byte||unsigned||0x01 → 0xFF||Number of frames per direction in this FRM file - This gives the number of frames for a particular orientation. For a static image, this number will be 1. For an animation of say 10 frames, this value will be 10, even though it is likely that there will be a total of 60 frames stored (10 for each of the 6 orientations)|
|0x000A||2 byte||signed||Unknown||Required shift in the X direction, of frames with orientation 0 - This is the shift required so that the frames with orientation 0 are centred correctly. This shift is needed to correctly align objects when rendering, as the logical centre of the frame will not necessarily be the absolute centre of the frame (width/2, height/2). For example, the centre of a critter will be the position of its feet, while the centre of a tree will be its base. A positive value means shift the frame to the right of the screen when rendering|
|0x000C||2 byte||signed||Unknown||Required shift in the X direction, of frames with orientation 1|
|0x000E||2 * (6 - 2) byte||signed||Unknown||Required shift in the X direction, of frames with orientations [2-5]|
|0x0016||2 * 6 byte||signed||Unknown||Required shift in the Y direction, of frames with orientations [0-5] - A positive value means shift the frame to the bottom of the screen when rendering|
|0x0022||4 byte||unsigned||Unknown||Offset of first frame for direction 0 from beginning of frame area - Frame area start from offset 0x003E|
|0x0026||4 byte||unsigned||Unknown||Offset of first frame for direction 1 from beginning of frame area|
|0x002A||4 byte||unsigned||Unknown||Offset of first frame for direction 2 from beginning of frame area|
|0x002E||4 byte||unsigned||Unknown||Offset of first frame for direction 3 from beginning of frame area|
|0x0032||4 byte||unsigned||Unknown||Offset of first frame for direction 4 from beginning of frame area|
|0x0036||4 byte||unsigned||Unknown||Offset of first frame for direction 5 from beginning of frame area|
|0x003A||4 byte||unsigned||Unknown||Size of frame area - Could be used to allocating memory for frames|
|0x003E||2 byte||unsigned||Unknown||FRAME-0-WIDTH: Width of frame 0 - The width (in pixels) of the 0th frame in the orientation 0|
|0x0040||2 byte||unsigned||Unknown||FRAME-0-HEIGHT: Height of frame 0 - The height (in pixels) of the 0th frame in the orientation 0|
|0x0042||4 byte||unsigned||FRAME-0-WIDTH * FRAME-0-HEIGHT||FRAME-0-SIZE: Number of pixels for frame 0 - The total area, and hence number of bytes taken up by the 0th frame|
|0x0046||2 byte||signed||FRAME-0-WIDTH * FRAME-0-HEIGHT||Offset in X direction of frame 0 - The offset of this frame from the previous frame|
|0x0048||2 byte||signed||FRAME-0-WIDTH * FRAME-0-HEIGHT||Offset in Y direction of frame 0|
|0x004A||FRAME-0-SIZE byte||unsigned||0x00 → 0x3F(hex) / 63(dec)||colorIndex for frame 0 - Contains the colorIndex for the 0th frame. Each index is represented by one byte; there are 256 possible indexes and their RGB values can be found in the appropriate .pal file. Pixel data starts at the top left corner of the frame and increases left to right, then top to bottom|
|0x004A + (FRAME-0-SIZE)||flexible||unsigned||0x00 → 0xFF||Frame data for all the following frames - Frames are stored for all the rest of the frames of orientation 0, then moves on to orientation 1 up to 5 (if these orientations are stored in the file)|
Information about palette
Frame data comes in the form of byte color indexes to a 256-color palette. Hence a byte with the number 137 would correspond to color 137 in the palette.