Differences

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--- formats:frm16 [2010-09-26 21:26] – edheldil
+++ formats:frm16 [2010-09-26 23:12] – edheldil
@@ Line 9: / Line 9: @@
   * header
   * Opaque layer
-    * Scanline LUT for the opaque layer
+    * Bitmap
+    * Scanline LUT
   * 80% opacity layer
-    * Scanline LUT for the 80% layer
+    * Bitmap
+    * Scanline LUT
   * 60% opacity layer
-    * Scanline LUT for the 60% layer
+    * Bitmap
+    * Scanline LUT
   * 40% opacity layer
-    * Scanline LUT for the 40% layer
+    * Bitmap
+    * Scanline LUT
   * 20% opacity layer
-    * Scanline LUT for the 20% layer
+    * Bitmap
+    * Scanline LUT
-Any (FIXME: even the fully opaque) layer might be missing
-[FIXME: quotes on FRM/FRM16 and JPEG-like compression]
-[FIXME: links to convertors by Balder, Miloch, ...]
+Any (FIXME: even the fully opaque) layer might be missing.
 There are at least two types of FRM16 files - type 64 and type 68. Type 64 uses uncompressed pixels, while type 68 uses RLE.
@@ Line 38: / Line 39: @@
 | 0x0c | 2 | Type (64 or 68) |
 | 0x0e | 2 | Unknown, possibly padding, always 0 in standalone files |
-| 0x10 | 4 | Data size for opaque layer |
+| 0x10 | 4 | Bitmap size for opaque layer |
-| 0x14 | 4 | Data size for % layer |
+| 0x14 | 4 | Bitmap size for 80?% layer |
-| 0x18 | 4 | Data size for % layer |
+| 0x18 | 4 | Bitmap size for 60?% layer |
-| 0x1c | 4 | Data size for % layer |
+| 0x1c | 4 | Bitmap size for 40?% layer |
-| 0x20 | 4 | Data size for % layer |
+| 0x20 | 4 | Bitmap size for 20?% layer |
 ==== Layers ====
-Layer
+Each layer consists of a bitmap and a scanline lookup table (LUT).
-^ Offset ^ Size ^ Description  ^
-==== Pixel Encoding, RLE ====
+==== Bitmap ====
 Pixel color is encoded as a 16bit word, probably with a common 5-6-5 RGB? encoding. FIXME: How are fully transparent pixels encoded?
-Type 64 files do not use any compression. Each layer contains all Width x Height pixels.
+Type 64 files do not use any compression. Each bitmap consists of width x height pixels of 2 bytes each.
-Type 68 FRM16 files use Run Length Encoding, RLE. Each image scanline consists of a series of pixel runs. The scheme uses two highest bits in the first byte of each run. If bit 7 is set, bits 0-6 contain number of pixels to skip. If bit 7 is clear and bit 6 is set, bits 0-5 contain number of 16bit pixels which follow. If both bits 7 and 6 are cleared, bits 0-5 contain a number of repetitions of the following 16 bit pixel.
+Type 68 FRM16 files use Run Length Encoding, RLE. Each bitmap line consists of variable number of pixel runs. The RLE scheme uses two highest bits in the first byte of each run. If bit 7 is set, bits 0-6 contain number of pixels to skip in the resulting image. If bit 7 is clear and bit 6 is set, bits 0-5 contain number of 16bit pixels which follow. If both bits 7 and 6 are cleared, bits 0-5 contain a number of repetitions of the following 16 bit pixel.
+For example:
+  42 ff fe ff fc  05 00 00
+would result in
+  __ __  ff fe  ff fc  00 00  00 00  00 00  00 00  00 00
+(First one pixel (0x81 & 0x7f) is skipped, then two (0x42 & 0x3f) whitish pixels are copied, then a black pixel is copied 5 times)
-==== Color Encoding ====
+In type 68 files, the first 4 bytes in a bitmap is a DWORD containing bitmap's size in bytes. Since LUT's size is known (2 * height), it allows to read or skip the bitmap or whole layer without decoding it. However, the same number is already in the header, so its usefullness is unclear.
-Pixels' color is encoded as a 16bit word, probably with a common 5-6-5 RGB? encoding. Transparent pixels ...
+==== Scanline Lookup Table, LUT ====
+Array of 32bit indices into bitmap pointing to beginnings of each bitmap scanline. The first index is 0 in type 64 files, 4 in type 68 files.
+^ Offset ^ Size ^ Description ^
+| 0x00 | 2 | Offset of the 1st scanline (0 in type 64 files, 4 in type 68 files) |
+| 0x02 | 2 | Offset of the 2nd scanline |
+| ...|||
+| height * 2 | 2 | Offset of the last scanline |
+==== References ====
+[FIXME: quotes on FRM/FRM16 and JPEG-like compression]
+[FIXME: links to convertors by Balder, Miloch, ...]