01-14-2023, 11:19 AM
I am working in an environment where I can't pull in any external modules/libraries, so I have to create my own image decoder for the BCn format. I am encountering a problem with BC7 in particular - or rather, something that everything is claiming is BC7, but fundamentally cannot be.
I can't attach the image itself because .dds isn't an allowed file type, but here's how it begins:
Each "30 ff 03 00 c0 ff af aa aa aa 4a 92 24 49 92 24" is a solid chunk of red pixels, as evidenced by opening the file in Visual Studio or XNViewMP (top left corner). In addition, the entire image is completely opaque and has no blue at all. Multiple programs including Visual Studio list the image as being BC7.
I'm using these references to understand the format:
According to all of these, this data is a "mode 2" block, and thus should be broken up like this:
The problem is that this is impossible. There are two pieces of proof:
So this image claims to be BC7, but the only way it can be is if all documentation on the format is extremely incorrect. Therefore, I ask: What is the actual format of this image, and how do I decode it?
I can't attach the image itself because .dds isn't an allowed file type, but here's how it begins:
Code:
44 44 53 20 7c 00 00 00 07 10 00 00 00 02 00 00
00 02 00 00 00 00 04 00 00 00 00 00 01 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 20 00 00 00
04 00 00 00 44 58 31 30 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
62 00 00 00 03 00 00 00 00 00 00 00 01 00 00 00
00 00 00 00 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24 30 ff 03 00 c0 ff af aa aa aa 4a 92
24 49 92 24Each "30 ff 03 00 c0 ff af aa aa aa 4a 92 24 49 92 24" is a solid chunk of red pixels, as evidenced by opening the file in Visual Studio or XNViewMP (top left corner). In addition, the entire image is completely opaque and has no blue at all. Multiple programs including Visual Studio list the image as being BC7.
I'm using these references to understand the format:
- https://learn.microsoft.com/en-us/window...bc7-format
- https://learn.microsoft.com/en-us/window...-reference
- https://registry.khronos.org/DataFormat/...l#bptc_bc7
- https://github.com/python-pillow/Pillow/...cnDecode.c
According to all of these, this data is a "mode 2" block, and thus should be broken up like this:
Code:
00110000111111110000001100000000110000001111111110101111101010101010101010101010010010101001001000100100010010011001001000100100
mode: 001
partition: 100001
red endpoints: 11111 11000 00011 00000 00011 00000
green endpoints: 01111 11111 01011 11101 01010 10101
blue endpoints: 01010 10101 00100 10101 00100 10001
indexes: 0 01 00 0 10 01 00 11 00 10 01 00 01 00 10 0The problem is that this is impossible. There are two pieces of proof:
- None of the blue endpoints are zero. Every pixel in the image has a blue channel of zero. Contradiction.
- When I use a hex editor to mess with the bits early in the "red endpoints" section, the pixels instead become partly transparent. Mode 2 has no alpha components, and in none of the BC7 modes does alpha ever come before colour. It can't even be a "red <-> alpha" swap, because the later red endpoints do affect the red, and no possible swap can result in the order "a,r,g,b".
So this image claims to be BC7, but the only way it can be is if all documentation on the format is extremely incorrect. Therefore, I ask: What is the actual format of this image, and how do I decode it?