How to get credits-mode working
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You have to adjust the information in 'xvid2.cfg' as this:

Common information:

# use credits at the beginning of the movie (0 = no, 1 = yes)
credits_start = 1

# set XXX/YYY to begin/end frame of the start credits
credits_start_begin = XXX
credits_start_end   = YYY

# use credits at the end of the movie (0 = no, 1 = yes)
credits_end = 1

# set XXX/YYY to begin/end frame of the end credits
credits_end_begin = XXX
credits_end_end   = YYY

The frame numbers are relative to your encoding range; so if you start 
encoding at frame 100 and your credits start at frame 150, you have to 
set 'credits_start_begin = 50' !

Credits mode:

Via 'credits_mode' you can select from several modes for credit 
handling, each using different values:

VBR_CREDITS_MODE_QUANT:
# Use a fixed quant for all credit frames

# use this when doing VBR_MODE_FIXED_QUANT
credits_fixed_quant = 20

# use this in all other VBR_MODEs...
# ...for I-frames
credits_quant_i = 20
# ...for P-frames
credits_quant_p = 20

Annotation: If and only if credits_quant_i does not equal 
credits_quant_p, then I-frames appear in credits. All other modes do not 
generate I-frames within credits. This conflicts with this comment:
/* We only care about Keyframes when not in credits */
Maybe this does also need to be fixed, but I did never try this combination.

VBR_CREDITS_MODE_RATE:
# Use only 'credits_quant_ratio' % of the full encoding rate as
# encoding rate for credit frames, e.g. with encoding rate 700kpbs
# this uses 140kps for the credits
credits_quant_ratio = 20

VBR_CREDITS_MODE_SIZE:
# Use a fixed size XXX/YYY for all opening/ending credit frames together
credits_start_size = XXX
credits_end_size   = YYY
