[BUG] `video_set_format_callbacks` cannot negotiate chroma

[sxsong1207]

I have struggled on this problem for about 1 day. I tried so hard on many ways, like this, but I still cannot make it use RV24 chroma.

@vlc.CallbackDecorators.VideoSetupCb  
def setup_cb(opaque, chroma, width, height, pitch, lines):
    _ctx = ct.cast(opaque, ct.POINTER(VideoContext)).contents
    
    w = width.contents.value
    h = height.contents.value
    
    # Try to negotiate RV24
    requested_chroma = b'RV24'
    
    # Check if RV24 is supported by trying to set it
    chroma.contents = ct.c_char_p(requested_chroma)
    
    # Configure for RV24
    pitch[0] = w * 3  # 3 bytes per pixel for RGB24
    lines[0] = h
    
    _ctx.width = w
    _ctx.height = h
    
    print(f"Negotiated format: {requested_chroma} {w}x{h}")
    return 1

When i compare the c api and python api. I noticed that, ctypes.c_char_p is interpreted by python with bytes class. With bytes, I cannot pass my message back to VLC. Thats why it doesn't know what I want.

    VideoFormatCb = ctypes.CFUNCTYPE(
        ctypes.c_uint,
        ctypes.POINTER(ctypes.c_void_p),
        ctypes.c_char_p,
        ctypes.POINTER(ctypes.c_uint),
        ctypes.POINTER(ctypes.c_uint),
        ctypes.POINTER(ctypes.c_uint),
        ctypes.POINTER(ctypes.c_uint),
    )

=> I have tried to modify the callback type, but unfortunately, it still not working as expected.

VideoFormatCb = ctypces.CFUNCTYPE(
        ctypes.c_uint,
        ctypes.POINTER(ctypes.c_void_p),
        ctypes.POINTER(ctypes.c_char),
        ctypes.POINTER(ctypes.c_uint),
        ctypes.POINTER(ctypes.c_uint),
        ctypes.POINTER(ctypes.c_uint),
        ctypes.POINTER(ctypes.c_uint),
    )

[sxsong1207]

Update! This code worked, just changing the decorator of the callback.

import ctypes as ct
import ctypes
VideoFormatCb_NOTWORKING = ctypes.CFUNCTYPE(
    ctypes.c_uint,
    ctypes.POINTER(ctypes.c_void_p),
    ctypes.c_char_p,
    ctypes.POINTER(ctypes.c_uint),
    ctypes.POINTER(ctypes.c_uint),
    ctypes.POINTER(ctypes.c_uint),
    ctypes.POINTER(ctypes.c_uint),
)

VideoFormatCb_WORK = ct.CFUNCTYPE(
        ct.c_uint,
        ct.POINTER(ct.c_void_p),
        ct.POINTER(ct.c_char),
        ct.POINTER(ct.c_uint),
        ct.POINTER(ct.c_uint),
        ct.POINTER(ct.c_uint),
        ct.POINTER(ct.c_uint),
    )

@VideoFormatCb_WORK
# @vlc.CallbackDecorators.VideoFormatCb
def format_cb(opaque, chroma_ptr, width, height, pitch, lines):
    _ctx = ct.cast(opaque[0], ct.POINTER(VideoContext)).contents

    input_chroma = ct.string_at(chroma_ptr, 4)
    w = width.contents.value
    h = height.contents.value
    _ctx.width = w
    _ctx.height = h

    print(f"Format callback:")
    print(f"  Chroma: {input_chroma}")
    print(f"  Width: {w}")
    print(f"  Height: {h}")
    

    ct.memmove(chroma_ptr, b'RV24', 4)

    # Configure RV24
    pitch[0] = w * 3
    lines[0] = h
    return w*h*3