litert-community/RT-DETRv2-S-LiteRT

RT-DETRv2-S — LiteRT (CompiledModel GPU)

RT-DETRv2 (Baidu, 2024 — PekingU/rtdetr_v2_r18vd) object detection, converted to LiteRT and running 100% on the CompiledModel GPU (ML Drift) on a phone, with no CPU/ONNX fallback.

RT-DETRv2 is a transformer detector (ResNet18-vd backbone + a hybrid AIFI/CCFM encoder + a plain deformable-attention DETR decoder). Off-the-shelf it is GPU-incompatible (deformable grid_sample → GATHER_ND, two-stage query selection → TOPK/GATHER). Here it is converted with litert-torch and split into two GPU graphs with a host step between them, so both transformer graphs run on the GPU.

Files

File	What it is	Size (fp16)
rtdetr_graphA_fp16.tflite	ResNet18-vd backbone + hybrid encoder + score head → enc_class[1,8400,80], memory_raw[1,8400,256]	33.8 MB
rtdetr_graphB_fp16.tflite	two-stage combine + plain decoder + heads → boxes[1,300,4] (cxcywh), logits[1,300,80]	7.7 MB
host_params.bin	host per-token tail weights (enc_output + enc_bbox_head), valid mask, anchors (fp32)	0.9 MB
coco_labels.txt	80 contiguous COCO class names (id 0–79)	—

How it runs (two-graph split)

image[1,3,640,640]
  →[GPU Graph A]→ enc_class, memory_raw
  →[host: top-300 by max class score; per-token tail on the 300 selected (fp32):
          target = enc_output(valid·memory_raw)   (Linear + LayerNorm)
          ref    = enc_bbox_head(target) + anchors (3-layer MLP)]
  →[GPU Graph B  (memory_raw, target, ref)]→ boxes[1,300,4], logits[1,300,80]
  →[host: sigmoid + threshold + cxcywh→xyxy + light NMS]→ detections

The two-stage query selection (TOPK/GATHER) has no GPU op, but the proposal grid is image-independent, so the model splits there. The per-token tail (enc_output + enc_bbox_head) runs on the host over the 300 selected tokens (exact, since per-token ops commute with the gather).

Why the per-token tail is on the host — a Mali 3D-sequence fan-out bug

Both graphs convert GPU-clean, but a naïve Graph A (emitting enc_class/enc_coord/output_memory/memory_raw together) silently produced wrong boxes on device — large objects vanished while small ones stayed perfect. A 3-D token tensor [1,N,256] (from conv.flatten(2).transpose(1,2)) that is both a graph output and consumed by another node — or that fans out to several consumers — gets clobbered on the longer branch (4-D conv-map outputs are fine). output_memory fed both heads; the 3-layer box head lost, so its reference-box deltas collapsed to ~0. Fix: Graph A emits only the two fp16-clean leaves (enc_class + memory_raw×2) and the per-token tail moves to the host.

Minimal usage

Android (Kotlin, CompiledModel GPU)

val ga = CompiledModel.create(context.assets, "rtdetr_graphA_fp16.tflite",
    CompiledModel.Options(Accelerator.GPU), null)
val gb = CompiledModel.create(context.assets, "rtdetr_graphB_fp16.tflite",
    CompiledModel.Options(Accelerator.GPU), null)
val aIn = ga.createInputBuffers(); val aOut = ga.createOutputBuffers()
val bIn = gb.createInputBuffers(); val bOut = gb.createOutputBuffers()
aIn[0].writeFloat(chw)                   // [1,3,640,640] RGB in [0,1], NCHW
ga.run(aIn, aOut)                        // -> enc_class[1,8400,80], memory_raw*2[1,8400,256]
// host step: /2 -> top-300 -> per-token tail (host_params.bin) -> target[1,300,256], ref[1,300,4]
// (resolve buffer slots by float size; full math in the Python below / litert-samples object_detection)
bIn[0].writeFloat(memory); bIn[1].writeFloat(target); bIn[2].writeFloat(ref)
gb.run(bIn, bOut)
val boxes = bOut[0].readFloat()          // [1,300,4] cxcywh in [0,1]
val logits = bOut[1].readFloat()         // [1,300,80] -> sigmoid + threshold + light NMS

Python (desktop verification)

import numpy as np
from PIL import Image
from ai_edge_litert.interpreter import Interpreter

NP_, NQ, NC, H = 8400, 300, 80, 256
img = Image.open("photo.jpg").convert("RGB").resize((640, 640))
x = (np.asarray(img, np.float32) / 255.0).transpose(2, 0, 1)[None]  # [1,3,640,640], [0,1] only

# host_params.bin (fp32 LE): enc_output W[256,256],b,gamma,beta · bbox-MLP W0,b0,W1,b1,W2[4,256],b2 · valid[8400] · anchors[8400,4]
p = np.fromfile("host_params.bin", np.float32); o = 0
def take(*s):
    global o; n = int(np.prod(s)); v = p[o:o+n].reshape(s); o += n; return v
eoW, eoB, eoG, eoBe = take(H, H), take(H), take(H), take(H)
W0, b0, W1, b1, W2, b2 = take(H, H), take(H), take(H, H), take(H), take(4, H), take(4)
valid, anchors = take(NP_), take(NP_, 4)

def run(path, feeds):  # feed/fetch tensors by shape (converter slot order is arbitrary)
    it = Interpreter(model_path=path); it.allocate_tensors()
    for d in it.get_input_details(): it.set_tensor(d["index"], feeds[tuple(d["shape"][1:])])
    it.invoke(); return {tuple(d["shape"][1:]): it.get_tensor(d["index"]) for d in it.get_output_details()}

a = run("rtdetr_graphA_fp16.tflite", {(3, 640, 640): x})
enc_cls, mem = a[(NP_, NC)][0], a[(NP_, H)][0] / 2.0                # Graph A emits memory_raw*2 — undo

top = np.argsort(-enc_cls.max(-1))[:NQ]                             # top-300 by max class logit
t = (valid[top, None] * mem[top]) @ eoW.T + eoB                     # per-token tail: enc_output Linear...
t = (t - t.mean(-1, keepdims=True)) / np.sqrt(t.var(-1, keepdims=True) + 1e-5) * eoG + eoBe  # ...+ LayerNorm
h = np.maximum(t @ W0.T + b0, 0); h = np.maximum(h @ W1.T + b1, 0)
ref = h @ W2.T + b2 + anchors[top]                                  # enc_bbox_head MLP + anchors

b = run("rtdetr_graphB_fp16.tflite",
        {(NP_, H): mem[None], (NQ, H): t[None].astype(np.float32), (NQ, 4): ref[None].astype(np.float32)})
boxes, logits = b[(NQ, 4)][0], b[(NQ, NC)][0]                       # cxcywh in [0,1] / 80-way logits
labels = open("coco_labels.txt").read().splitlines()
score = 1 / (1 + np.exp(-logits.max(-1))); cls = logits.argmax(-1)
for q in np.where(score > 0.4)[0]:                                  # + light NMS (IoU 0.7) in a real app
    cx, cy, w, hh = boxes[q]
    print(f"{labels[cls[q]]:12s} {score[q]:.2f}  xyxy=({cx-w/2:.3f},{cy-hh/2:.3f},{cx+w/2:.3f},{cy+hh/2:.3f})")

On-device (Pixel 8a, Tensor G3 — verified)

Both graphs run 100% GPU-resident (LITERT_CL): Graph A fully delegated, Graph B 704/704. The device chain reproduces the PyTorch detections exactly — COCO val giraffe image 7/7, cats image (000000039769) 6/6, every box at IoU 0.98–1.00 with matching class and score.

End-to-end ~615 ms/frame on a Pixel 8a: Graph B's deformable decoder over RT-DETR's 8400 tokens / 80×80 levels is ~350 ms of GPU compute (the GATHER-free tent-matmul grid_sample turns an O(points) gather into an O(H·W) matmul). So this model is accurate and fully-GPU but not real-time on this device; it suits still-image / snapshot detection. (A real-time camera demo of the same family is RF-DETR Nano, whose single small deformable level runs at ~9 fps.)

Preprocessing / outputs

• Input: square resize to 640×640, RGB, [0,1] rescale only (no ImageNet normalization), NCHW.
• Output: Graph B boxes are cxcywh normalized to [0,1]; logits are 80-way (contiguous COCO id 0–79). Host applies sigmoid + score threshold + cxcywh→xyxy + light NMS.

Conversion notes

Converted with litert-torch (NCHW preserved — onnx2tf destroys ViT attention). Re-authoring (per-graph tflite-vs-torch correlation 1.0): deformable grid_sample → a GATHER/CAST-free tent-matmul, MSDeformAttn ≤4D, baked AIFI sine pos-embed, ResNet18-vd stem zero-pad maxpool (the -inf-pad maxpool lowers to a Mali-rejected PADV2), a down-scaled fp16-safe LayerNorm, and the 3D-fan-out fix above (emit clean leaves + host-side per-token tail).

A runnable Android sample (CompiledModel GPU) and the conversion scripts are in the official ai-edge-litert/litert-samples object_detection example.

Performance

Measured on a Pixel 8a (Tensor G3, Android 16) with the standard TFLite benchmark_model tool — 10 warm-up runs then 50 timed runs, reported as the tool's mean.

Runtime	Backend	Graph on GPU	Latency
TFLite benchmark_model (TfLiteGpuDelegateV2) — rtdetr_graphA_fp16.tflite	GPU (OpenCL)	184 / 365	925.2 ms
TFLite benchmark_model (TfLiteGpuDelegateV2) — rtdetr_graphB_fp16.tflite	GPU (OpenCL)	95 / 704	did not run
TFLite benchmark_model — rtdetr_graphA_fp16.tflite	CPU (XNNPACK, 4 threads)	—	548.0 ms
TFLite benchmark_model — rtdetr_graphB_fp16.tflite	CPU (XNNPACK, 4 threads)	—	663.4 ms

Any on-device figure recorded when this model shipped came from a different runtime. It was taken through LiteRT's own CompiledModel accelerator (logcat reports it as LITERT_CL), which is the path the Kotlin sample app and the LiteRT API use, and it appears elsewhere on this card. The rows above are the classic TFLite OpenCL delegate, measured with a tool anyone can download and re-run. The two are not comparable, so read the rows above as a reproducible floor rather than as this model's speed on LiteRT.

On this delegate the CPU is the faster choice for rtdetr_graphA_fp16.tflite (548.0 ms on CPU against 925.2 ms on GPU) — worth knowing before you reach for the GPU on a mid-range phone.

Note that the GPU does not take the whole graph here (184 / 365 in rtdetr_graphA_fp16.tflite, 95 / 704 in rtdetr_graphB_fp16.tflite); the remainder runs on the CPU and the split costs a per-partition round trip.

License

Apache-2.0, inherited from lyuwenyu/RT-DETR.