Class Activation Map (Zhou 2016) — visualization helpers.

The heavy lifts (CAM compute, bilinear upsample, logit recomputation) are in F32Array.lean via FFI. This file provides:

• viridis : Float → (UInt8 × UInt8 × UInt8) — perceptually uniform 256-entry colormap (also legible in greyscale).
• overlayHeatmap : ... — α-blend a heatmap over an RGB image.
• writePPM : ... — binary P6 PPM serializer (zero deps; convert to PNG via convert outside if final asset size matters).

No autodiff is needed — for any network ending in ... → globalAvgPool → dense ic oc act GradCAM collapses to the closed form Zhou et al. computed three years before Selvaraju et al. generalized it. See planning/archive/gradcam.md.

def Cam.viridis (t : Float) : UInt8 × UInt8 × UInt8

Map t ∈ [0, 1] to an RGB triple using a lerped viridis palette. Out-of-range t is clamped to [0, 1].

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def Cam.overlayHeatmap (imgRGB heat : ByteArray) (H W : Nat) (alpha : Float) : ByteArray

α-blend a heatmap over an RGB image and emit the result as a contiguous [H, W, 3] UInt8 buffer (PPM-ready).

• imgRGB : already-denormalized [H, W, 3] UInt8 (R, G, B interleaved row-major). Caller is responsible for the ImageNet-mean/std de-normalization for whatever dataset.
• heat : f32 [H, W] with values in [0, 1].

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def Cam.writePPM (path : String) (H W : Nat) (pixels : ByteArray) : IO Unit

Write a binary P6 PPM file. pixels is [H, W, 3] UInt8 row-major.

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• Cam.writePPM path H W pixels = IO.FS.writeBinFile { toString := path } ((toString "P6\n" ++ toString W ++ toString " " ++ toString H ++ toString "\n255\n").toUTF8.append pixels)

def Cam.writeHeatmapPPM (path : String) (heat : ByteArray) (H W : Nat) : IO Unit

Render a heatmap directly as a colored PPM (no overlay) — useful for quick "did this work at all" checks.

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