GRPO with Inter-Group Comparison Reward
Vanilla GRPO computes relative advantages only within intra-group rollouts and lacks cross-sample
reference signals, leading to a “direction blindness” issue. We introduce an inter-group comparison
reward while still computing relative advantages within intra-group rollouts, so that the model can
better distinguish vlog edits of varying quality and more effectively identify the optimal editing plan.
The final model uses \(m = 4\) candidate editing plans per asset, \(G = 8\) rollouts, and
\(\lambda = 0.25\) unless otherwise stated.
Reward Function Setup
Format reward. The rollout must output integer scores from 1 to 5 and textual feedback for all
six dimensions. If the required schema is satisfied, \(r_{\text{format}}\) is 1; otherwise it is 0,
which masks out malformed answers in the total reward.
Score reward. For predicted scores
\(s = \{s_1, \ldots, s_k\}\) and ground-truth scores
\(\hat{s} = \{\hat{s}_1, \ldots, \hat{s}_k\}\) over \(k = 6\) dimensions, the reward is the
exact-match ratio with a nominal correctly-formatted floor \(\alpha = 0.1\):
\[
r_{\text{score}} =
(1 - \alpha)\frac{1}{k}\sum_{i=1}^{k}\mathbb{1}[s_i = \hat{s}_i] + \alpha .
\]
Comparison reward. Let \(S_i^{[j]}\) be the total predicted score of rollout \(i\) for plan
\(j\), and let \(\hat{S}^{[j]}\) be the ground-truth total score. For each alternative plan \(l\),
the pairwise comparison reward is 1 when the predicted score difference has the same sign as the
ground-truth quality difference, and 0 otherwise. The final \(r_{\text{comp}}\) averages these pairwise
rewards over the other \(m - 1\) plans.
\[
r_{\text{comp}}(o_i^{[j]}, l) =
\mathbb{1}\left[
\left(S_i^{[j]} - \frac{1}{G}\sum_{i'=1}^{G} S_{i'}^{[l]}\right)
\left(\hat{S}^{[j]} - \hat{S}^{[l]}\right) > 0
\right],
\]
\[
r_{\text{comp}}(o_i^{[j]}) =
\frac{1}{m - 1}\sum_{l \ne j} r_{\text{comp}}(o_i^{[j]}, l).
\]
\[
r_{\text{total}} =
r_{\text{format}}\left[(1 - \lambda)r_{\text{score}} + \lambda r_{\text{comp}}\right].
\]
Figure 6 (left). Illustration of the role of comparison reward (CR) beyond score reward (SR).
In a simplified scenario with 2 rollouts and a single dimension, SR alone suffers from “direction
blindness”, failing to recognize the preference direction. With SR + CR, the comparison signal provides
an opposing policy update direction to restrain excessive updates of SR, maintaining score accuracy while
ensuring score differences align with sample quality preferences.
Figure 7 (right). Comparison of reward scores of Qwen2.5-VL-7B and VlogReward. Plan 1 suffers
from chronological disorder (day → night → day), while Plan 2 maintains a coherent temporal flow.
Qwen assigns identical scores to both plans, failing to distinguish the edit quality; VlogReward accurately
identifies the sequence-logic flaws in Plan 1 (assigning pacing score to 1).