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idk31
Cheater
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Joined: 11 Jan 2016
Posts: 42
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 Posted: Sat Jul 29, 2017 9:07 pm Post subject: Using fpatan and restoring registers correctly? Cam tracker |
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Goal: Aim camera at player (tracking cam). Up/down, Z not important. 64-bit UE4 game. My code works until crash on level change. Values are "???" briefly. Would like a way to account for that and know if my code makes sense, is efficient, and I'm restoring everything I need to correctly. Borrowed code from Dark Byte here to restore xmm0. Is it even necessary? Am I messing something up when I use fld?
Available to use:
[rbx+120] player X
[rbx+124] player Y
[rax+120] camera X
[rax+124] camera Y
[rax+154] camera yaw (degrees)
I made the following:
| Code: |
push ecx
sub esp,#20
movdqu [esp],xmm0 //save xmm0. this necessary?
mov ecx,[rbx+124] //player Y
mov [esp+10],ecx
movss xmm0,[esp+10]
subss xmm0,[rax+124] //subtract camera Y
movss [esp+10],xmm0
fld dword ptr [esp+10] //load
mov ecx,[rbx+120] //player X
mov [esp+10],ecx
movss xmm0,[esp+10]
subss xmm0,[rax+120] //subtract camera X
movss [esp+10],xmm0
fld dword ptr [esp+10] //load
fpatan //inverse tan
mov [esp+10],(float)57.29577951308232
fmul dword ptr [esp+10] //multiply to get degrees
fstp dword ptr [rax+154] //set camera yaw
movss xmm0,[esp] //restore xmm0
add esp,#20
pop ecx
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panraven
Grandmaster Cheater
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Joined: 01 Oct 2008
Posts: 958
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| Posted: Sun Jul 30, 2017 1:21 am Post subject: |
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The code run on CE's createThread, and seems give correct answer.
but
| Code: |
sub esp,#20 /// it is decimal 20, in hex 0x14, which not stack align
...
add esp,#20
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It may be ok in CE's createThread enviroment, but may cause problem in actual game? ie. 12 bytes (0x20-0x14 = 0xc) in stack may be overwritten.
Also may change all register in memory [ eax + * ] to r-version rax?
Although ce accept them, it may look less confuse.
It seems ok to use all fpu math, (same result as yours)
| Code: |
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dd (float)3.14151692,(float)180
...
fld dword ptr[rbx+124]
fsub dword ptr[rax+124]
fld dword ptr[rbx+120]
fsub dword ptr[rax+120]
fpatan
fmul dword ptr[_+4] // 180
fdiv dword ptr[_+0] // pi
fstp dword ptr[rax+154]
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ADDED:
save a link : https://godbolt.org/g/eusJoj generate assembler from c
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Last edited by panraven on Sun Jul 30, 2017 7:34 am; edited 1 time in total |
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Dark Byte
Site Admin
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Joined: 09 May 2003
Posts: 25805
Location: The netherlands
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| Posted: Sun Jul 30, 2017 4:11 am Post subject: |
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you are mixing 32-bit and 64-bit together. My example was for 32-bit code
But i think it may work.
But do add some tests for the validity of RBX and RAX, they could be 0
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idk31
Cheater
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Joined: 11 Jan 2016
Posts: 42
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| Posted: Sun Jul 30, 2017 12:04 pm Post subject: |
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Panraven, thanks! That's MUCH better than what I was doing  I googled for days "fpatan example" and "assembly subtract/multiply float." Nothing was making sense. Then I just cobbled something together. Was surprised when it worked.
Dark Byte, I'm still crashing when values go to "??" I am not sure what it means or how to handle it. The address appears to always be valid. I'm watching it from pointer [_camera+120] I get from "mov [_camera],rax." Do I need isbadreadptr?
I tried (with no luck)...
| Code: | cmp [_player],0 //rbx was never more than symbol "_player"
je return
cmp [rax],0
je return
test rax,rax
je return
push rbx
mov rbx,[_player]
//stuff
pop rbx |
Complete table. Point of interest: script "CAMERA," label "doTracking"
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