IMP sprites and RLE algorithm

[snv]

To whom it may concern, here is the structure of IMP files and decoding alghorithm (variation of ILBM RLE). Have a nice day. --- SNV


IMP_File {
 u2 Type; // 8=uncompressed, 9=RLE
 u2 Unknown1;
 u2 Width;   // virtual Width
 u2 Height;  // virtual Height
 u4 PaletteOffset;

 u4 Unknown2; // checksum or id?
 u4 Unknown3;
 u4 Unknown4;
 u2 Unknown5;

 u2 NumSequences;
 u4 SequencesOffset;

 //Sequence Sequences[NumSequences];
 // Sequence represents specific animation (like movement or spellcasting),
 // together with facings

 //Facing Facings[]; // directions, sprite can face (some are mirrored)
 //Facing Frames[];  // frame headers

 //u1 Palette[256*4]; // RGBA palette:
                      //    index 0 used for transparency,
                      //    index 1 is shadow
                      //    index 0xff is RLE special value

 //Data[] // all frames compressed or uncompressed data

 // hotspot (selection) at the end of the file
 //u1 HotSpot[];
}


Sequence {
 u4 Unknown1;
 u4 Unknown2;
 u1 Unknown3;
 u2 Unknown4;
 u1 Size;   // length of sequence
 u4 Offset; // offset of facing groups (directions)
}

// Facings go clockwise
Facing {
 u2 Unknown1;
 u2 Size;
 u4 Offset;
}

Frame {
 u1 Type; // 0=normal, 8=duplicate-back-reference
 u1 Unknown1; // delay?
 u2 Width;
 u2 Height;
 u2 Size; // compressed size, or uncompressed, if Type=8 or IMP_File->Type==9
          // should be aligned to 4-bytes

 u4 Unknown2; // checksum?
 u4 Offset; // offset or number of duplicating frame
 // Note: duplicates used for animation delay. Don't simply strip them.
}


for (L = 0; L < H->Width*H->Height; ) {
 int C = (char)*I++;
 if (C<0) {
   C = -C;
   while(C--) O[L++] = *I++;
 } else {
   C =  C + 3;
   while(C--) O[L++] = *I;
   P++;
 }
}