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∣θbridge∣|\theta_{\text{bridge}}|

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This is the formal shape of “freeze the backbones”: the gradient update is masked to zero everywhere except the bridge parameters θbridge\theta_{\text{bridge}} , so |θbridge\theta_{\text{bridge}}| — a few hundred million parameters in BLIP-2’s case, dramatically fewer than either tower — is the entire quantity being optimised, and θvision\theta_{\text{vision}} and θLM\theta_{\text{LM}} contribute a forward pass but never a gradient.

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θbridge\theta_{\text{bridge}}

Symbol theta_bridge

thetaba_bridge is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

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∣θbridge∣|\theta_{\text{bridge}}|

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This mathematical expression combines the displayed quantities; its precise role follows from the surrounding article text.

This is the formal shape of “freeze the backbones”: the gradient update is masked to zero everywhere except the bridge parameters θbridge\theta_{\text{bridge}} , so |θbridge\theta_{\text{bridge}}| — a few hundred million parameters in BLIP-2’s case, dramatically fewer than either tower — is the entire quantity being optimised, and θvision\theta_{\text{vision}} and θLM\theta_{\text{LM}} contribute a forward pass but never a gradient.

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