Mask off bits in displayed value

[XJDHDR]

I'm trying to write a pattern for a video container file format (Bink specifically). After the header, the file contains a sequence of u32 values that give the start address for that frame. The issue I'm having is that the 1st bit in that value is a flag that indicates whether or not that is a keyframe. To get the actual address, you have to mask out this 1st bit if it's set.

I am stuck at this part because I don't know how to do that. This is the closest I've gotten so far, but FrameAddress is right-shifted by 1.

bitfield FrameAddressData {
    bool    IsKeyframe: 1;
            FrameAddress: 31;   // TODO: This value needs to be left shifted by 1 bit.
};

union FrameAddress {
    u32 RawAddressData;
    FrameAddressData FrameAddressData;
};

[paxcut]

that should work as it stands. the problem may be when the first bit is zero.

[paxcut]

Don't use a union, the value for the address is read from FrameAddress. If you need the union for the case when first bit is zero you can still recover the adress without masking from the value of FrameAddress

[XJDHDR]

that should work as it stands. the problem may be when the first bit is zero.

It doesn't, sadly. If you compare each FrameAddress to the RawAddressData, the two values are supposed to be identical if the Keyframe bit is unset, and off by 1 otherwise.

This is not the case. Instead, the FrameAddress value is basically: FrameAddress = RawAddressData >> 1

[XJDHDR]

Don't use a union, the value for the address is read from FrameAddress.

I would like to get rid of the union but right now, it's the only way I can get a somewhat accurate address. If FrameAddress had the correct value, I wouldn't need RawAddressData.

[paxcut]

I see then you want to mask it using & 0x7FFF. you can create local variables and have them show using [[export]] to mask out the fist bit instead of the last use & 0xFFFE

[XJDHDR]

I see then you want to mask it using & 0x7FFF

I'm trying to mask out the 1st bit, not everything after the 15th.

to mask out the fist bit instead of the last use & 0xFFFE

That masks off not just the 1st bit, but also the last 16 bits. Remember, I am using a 32-bit int.

[XJDHDR]

Edit: My pattern now uses the following and works as it should:

bitfield FrameAddressData {
    bool    IsKeyframe: 1 [[no_unique_address]];
            FrameAddress: 32 [[transform("maskFirstBit")]];
};

fn maskFirstBit(u32 originalValue) {
    return (originalValue & 0xFFFFFFFE);
};

Original comment:
I changed my pattern to this now and it's seems to give me what I need:

bitfield FrameAddressData {
    bool    IsKeyframe: 1 [[no_unique_address]];
            RawData: 32 [[hidden]];
            
    u32 FrameAddress = RawData & 0xFFFFFFFE [[export]];
};

The only question I have now is whether it's possible to get rid of the local variable and have that bitwise AND applied directly to RawData? The reason I ask is because FrameAddress is just marked as having a Local start and end address in the Pattern Data viewer, which also means the data isn't highlighted in the Hex editor.

Here is what I see without hiding RawData. I would like FrameAddress to look like this.

[paxcut]

you can use a [[transform("transform_function")]] attribute that changes the value of the field but probably a format attribute is all you need

[XJDHDR]

you can use a [[transform("transform_function")]] attribute that changes the value of the field but probably a format attribute is all you need

Thanks, that did the trick. I saw those attributes before but nothing jumped out as what I needed.

Format would work if I just needed to modify what was displayed in the Pattern Data area. However, those addresses (with flag masked out) are also used elsewhere in the pattern to determine where frame data starts and ends.

[XJDHDR]

On a side note, I've finished creating my pattern and would like to contribute it to ImHex as a built-in pattern. I can't figure out where to add it in this repo or where in the repo the patterns that ImHex already comes with are stored.

[WerWolv]

Hey, you can create a PR to the WerWolv/ImHex-Patterns repo with your pattern. Read through the PR template and maybe take a look at the PRs other people opened in the past :)

[XJDHDR]

Thanks, I actually just found that repo myself. I'll sort out a PR when I have some time.