Figure out JFIF/JPEG ALFA blocks.

[LazyDuchess]

The game generates thumbnails as JPEG images, and embeds a custom ALFA block in them to achieve transparency:

The image above is a Sim thumbnail opened with a HEX editor. They don't look terribly complicated to figure out, they're Run Length Encoded.

[LazyDuchess]

For the time being I'll be doing color picking and some other prost processing to get rid of the black bg. Obvs not ideal but will do for now.

[ammaraskar]

Got it :)

Raw

Transparency Mask

Details

Python Parsing Code

```python def setTransparencyFromALFA(im, alfa): im2 = im.convert('RGBA').copy() transparency = [] i = 0 while i < len(alfa): rle_byte = int.from_bytes(alfa[i:i+1], byteorder='big', signed=True) i += 1 if rle_byte < 0: # Repeat the next transparency n times. t = alfa[i] transparency.extend([t] * ((-rle_byte) + 1)) i += 1 else: # Read n unique transparency values. for _ in range(rle_byte + 1): transparency.append(alfa[i]) i += 1 print(len(transparency)) ctr = 0 for j in range(im.size[0]): for i in range(im.size[1]): pixel = im2.getpixel((i, j)) pixel = (transparency[ctr], pixel[1], pixel[2], pixel[3]) ctr += 1 im2.putpixel((i, j), pixel) return im2 ```

It uses a similar RLE scheme to the .reia files:

• Read a single signed byte n.
• If n is negative:
  • Repeat the next byte (-n) + 1 times.
• If n is positive:
  • Read n + 1 unique bytes.
• The bytes read give the transparency channel values.

Let me see if I can send a pull request to add this to IMGCodec.cs to get familiarized with the codebase.

[LazyDuchess]

Awesome! Cool that you figured out the RLE - I did notice there was some different behavior depending on if the byte was positive or negative but I hadn't thought of this.