不支持 BC 的设备（A16）上 iOS 版闪退：贴图视图 R8Unorm → RGBA16Uint 断言失败
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Assessment
- Difficulty
- 5/5
- Estimated time
- Over a week
- Newbie friendliness
- 35/100
Research direction
Start in nvn_mtl.m at attach() around line 1858 and tex_by_id(); the report traces the crash to a BC4 GPU encoded texture using an RGBA16Uint view when BC compression is unavailable. Read the texture format handling and reproduce on Mac with DM_TEST_NO_BC=1. Done means the texture view is handled without the Metal assertion while preserving the encoded texture content; also inspect the reported g_red_once bounds issue.
Written by the indexing model from the issue text.
Description
设备： iPhone 14 Pro（A16，supportsBCTextureCompression = NO），iOS 27.0
版本： main @ 9579030，档案 01007EF00011E000（1.9.0 更新）
现象
启动几秒后闪退，日志最后一行 SetDepthClamp 开：宿主暂忽略 之后紧跟：
_mtlValidateArgumentsForTextureViewOnDevice:1877: failed assertion `Texture Creation
source texture pixelFormat (MTLPixelFormatR8Unorm) not compatible with texture view pixelFormat (MTLPixelFormatRGBA16Uint).'
Mac 上可复现：DM_TEST_NO_BC=1（不用开调试层）。调用栈：
_mtlValidateArgumentsForTextureViewOnDevice
-[AGXTexture initWithTexture:pixelFormat:resourceIndex:]
attach nvn_mtl.m:1858
rp_begin_ex_ nvn_mtl.m:1888
draw nvn_mtl.m:4868
mtl_exec nvn_mtl.m:5747
h_nvnQueueSubmitCommands hle_nvn.c:846
原因
游戏在 GPU 上编码一张 BC4 贴图：通过 RGBA16Uint 视图往里画（NVN 贴图格式 0x49，视图格式 0x2c，一个像素对应一个 8 字节块）。无 BC 时 tex_desc 把这张贴图建成解压格式（R8Unorm），原始块视图没有对应的存储，Metal 直接断言。
我本地的绕过办法（nvn_mtl.m）
attach()：bc_unpack(t->d.format)为真且视图格式不是 BC 时，不挂这个附件（和上面压缩格式附件的处理一样），打一条红行。tex_by_id()：采样视图做同样判断，按空槽处理。
改完后 DM_TEST_NO_BC=1 下能读档进到野外，肉眼没看出画面差异；但 GPU 编码的那张 BC4 贴图内容丢了，正式修法可能要把写入的块解码回 R8 贴图（或保留一份原始块的影子贴图）。
顺带：g_red_once 声明为 84 个元素，但代码里用了 RED_ONCE(84, …)，越界一位。
另外无 BC 时野外的内存占用约 3.9 GB（有 BC 时约 2.8 GB），在 6 GB 内存的 iPhone 上可能超过单个 App 的上限。
- Dominant language
- C
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- Avg merge
- 11h 32m
- Merged PRs (30d)
- 30
Getting set up
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First steps
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