SampleFrames ignore tail frames

[makecent]

Reproduce

from mmaction.datasets.pipelines import SampleFrames
from matplotlib import pyplot as plt
import numpy as np

f = dict(total_frames=1000, start_index=0)
s = SampleFrames(clip_len=1, num_clips=128)

# Sample 1000 times
a = []
for i in range(1000):
    a.append(s(f)['frame_inds'])
a = np.stack(a)

# Count occurance of each index
u, counts = np.unique(a, return_counts=True)
b = []
for i in range(1000):
    try:
        b.append(counts[u.tolist().index(i)])
    except ValueError:
        b.append(0)

# Plot
plt.plot(b)
plt.show()

outs:

Comments
SampleFrames will ignore tail frames because the // used:

mmaction2/mmaction/datasets/pipelines/loading.py

Lines 142 to 143 in b8115d9

	ori_clip_len = self.clip_len * self.frame_interval
	avg_interval = (num_frames - ori_clip_len + 1) // self.num_clips

[kennymckormick]

Use '/' instead of '//' can be more reasonable, we will add an option to do so.