Lesson 52: Open-Vocabulary Detection¶
Lesson 40's detector predicts one of a small, fixed set of classes baked in at training time — exactly Lesson 37's classification limitation, just applied to boxes instead of whole images. Open-vocabulary detection (Grounding DINO, OWL-ViT) removes that limit by splitting detection into two separable skills: finding objects (a class-agnostic skill, surprisingly transferable across object categories) and naming them (delegated to a CLIP-style text-embedding match, Lesson 49, which can recognize any category describable in words). This lesson builds both halves and tests the combination on a category the detector's box regressor never received a single labeled box for.
import numpy as np
import torch
import torch.nn as nn
import torch.nn.functional as F
import matplotlib.pyplot as plt
import matplotlib.patches as patches
Base classes vs. a novel class¶
Three shapes: plus, circle, square. The box regressor will only ever see labeled boxes for two of them — the base classes. Square is the novel class: the detector never sees a single box-labeled square during training, only plus and circle.
SIZE = 32
SHAPES = ['plus', 'circle', 'square']
BASE_SHAPES = ['plus', 'circle']
NOVEL_SHAPE = 'square'
def make_image(shape_type, cx, cy, size=SIZE, r=6):
img = np.zeros((size, size), dtype=np.float32)
if shape_type == 'plus':
img[cy-1:cy+2, cx-r:cx+r+1] = 1.0
img[cy-r:cy+r+1, cx-1:cx+2] = 1.0
elif shape_type == 'circle':
yy, xx = np.mgrid[0:size, 0:size]
img[((xx-cx)**2 + (yy-cy)**2) <= r**2] = 1.0
else: # square
img[cy-r:cy+r+1, cx-r:cx+r+1] = 1.0
return img
def make_detection_example(rng, shapes, size=SIZE, r=6):
shape_type = rng.choice(shapes)
cx, cy = rng.integers(r + 2, size - r - 2), rng.integers(r + 2, size - r - 2)
img = make_image(shape_type, cx, cy)
img = np.clip(img + rng.normal(0, 0.05, img.shape), 0, 1).astype(np.float32)
box = np.array([cx - r, cy - r, 2 * r, 2 * r], dtype=np.float32) / size
return img, box, shape_type
def crop_and_resize(img, box, out_size=SIZE):
x0, y0, w, h = (box * SIZE).astype(int)
x0, y0 = max(0, x0), max(0, y0)
x1, y1 = min(SIZE, x0 + max(w, 1)), min(SIZE, y0 + max(h, 1))
crop = img[y0:y1, x0:x1]
if crop.size == 0:
return np.zeros((out_size, out_size), dtype=np.float32)
resized = F.interpolate(torch.tensor(crop[None, None]), size=(out_size, out_size),
mode='bilinear', align_corners=False)
return resized[0, 0].numpy()
fig, axes = plt.subplots(1, 3, figsize=(7, 2.5))
demo_rng = np.random.default_rng(42)
for ax, shape in zip(axes, SHAPES):
im, box, _ = make_detection_example(demo_rng, [shape])
ax.imshow(im, cmap='gray')
x0, y0, w, h = box * SIZE
ax.add_patch(patches.Rectangle((x0, y0), w, h, edgecolor='lime', facecolor='none', linewidth=2))
ax.set_title(f'{shape} (base={shape in BASE_SHAPES})', fontsize=9)
ax.axis('off')
plt.show()
Half 1: a class-agnostic box regressor¶
Lesson 40's box regression, unchanged, except the target is never a class label — only "where is the object," trained exclusively on plus and circle scenes.
class BoxRegressor(nn.Module):
def __init__(self):
super().__init__()
self.conv = nn.Sequential(
nn.Conv2d(1, 16, 5, padding=2), nn.ReLU(), nn.MaxPool2d(2),
nn.Conv2d(16, 32, 5, padding=2), nn.ReLU(), nn.AdaptiveMaxPool2d(1))
self.fc = nn.Linear(32, 4)
def forward(self, x):
return torch.sigmoid(self.fc(self.conv(x).flatten(1)))
def iou_batch(pred, target):
px0, py0, pw, ph = pred[:, 0], pred[:, 1], pred[:, 2], pred[:, 3]
tx0, ty0, tw, th = target[:, 0], target[:, 1], target[:, 2], target[:, 3]
px1, py1, tx1, ty1 = px0 + pw, py0 + ph, tx0 + tw, ty0 + th
ix0, iy0 = torch.maximum(px0, tx0), torch.maximum(py0, ty0)
ix1, iy1 = torch.minimum(px1, tx1), torch.minimum(py1, ty1)
inter = (ix1 - ix0).clamp(min=0) * (iy1 - iy0).clamp(min=0)
union = pw * ph + tw * th - inter
return inter / union.clamp(min=1e-8)
rng = np.random.default_rng(1)
N = 400
Xtr, Btr = [], []
for _ in range(N):
img, box, _ = make_detection_example(rng, BASE_SHAPES) # base classes only
Xtr.append(img); Btr.append(box)
Xtr, Btr = np.array(Xtr, dtype=np.float32), np.array(Btr, dtype=np.float32)
torch.manual_seed(0)
box_model = BoxRegressor()
opt = torch.optim.Adam(box_model.parameters(), lr=0.005)
Xt, Bt = torch.tensor(Xtr).unsqueeze(1), torch.tensor(Btr)
for _ in range(400):
opt.zero_grad()
loss = F.mse_loss(box_model(Xt), Bt)
loss.backward()
opt.step()
# test localization on ALL THREE shapes, including the never-boxed novel square
test_rng = np.random.default_rng(2)
Xte, Bte, labels_te = [], [], []
for _ in range(150):
img, box, lbl = make_detection_example(test_rng, SHAPES)
Xte.append(img); Bte.append(box); labels_te.append(lbl)
Xte, Bte = np.array(Xte, dtype=np.float32), np.array(Bte, dtype=np.float32)
with torch.no_grad():
pred_boxes = box_model(torch.tensor(Xte).unsqueeze(1))
ious = iou_batch(pred_boxes, torch.tensor(Bte)).numpy()
for shape in SHAPES:
mask = np.array([l == shape for l in labels_te])
print(f'{shape:>8} (base={shape in BASE_SHAPES}): mean box IoU = {ious[mask].mean():.3f}')
Localization transfers to the novel shape reasonably well — never having seen a labeled square box, the regressor still finds squares with a respectable IoU, though noticeably less precisely than the base classes it was actually trained on. Class-agnostic "is there an object here" is a more generic, more transferable skill than exact class identity.
Half 2: a CLIP-style classifier, trained on all three classes¶
This is the "broad pretraining" stand-in — a dual image/text encoder (Lesson 49), contrastively trained on labeled crops of all three shapes, plus and circle and square. Real open-vocabulary detectors rely on exactly this asymmetry: the image-text encoder was pretrained on a vastly broader vocabulary (hundreds of millions of image-caption pairs) than any detection dataset's box annotations ever cover, so it already "knows" what a square looks like even though the detector's box-training data never had one.
VOCAB = ['a', 'photo', 'of', 'plus', 'circle', 'square']
W2ID = {w: i for i, w in enumerate(VOCAB)}
def caption_for(shape):
return [W2ID[w] for w in ['a', 'photo', 'of', shape]]
class ImageEncoder(nn.Module):
def __init__(self, out_dim=16):
super().__init__()
self.conv = nn.Sequential(
nn.Conv2d(1, 16, 5, padding=2), nn.ReLU(), nn.MaxPool2d(2),
nn.Conv2d(16, 32, 5, padding=2), nn.ReLU(), nn.AdaptiveMaxPool2d(1))
self.proj = nn.Linear(32, out_dim)
def forward(self, x):
return self.proj(self.conv(x).flatten(1))
class TextEncoder(nn.Module):
def __init__(self, vocab_size, out_dim=16, embed_dim=8):
super().__init__()
self.embed = nn.Embedding(vocab_size, embed_dim)
self.proj = nn.Linear(embed_dim, out_dim)
def forward(self, ids):
return self.proj(self.embed(ids).mean(dim=1))
def clip_loss(img_emb, txt_emb, temperature=0.1):
img_emb = F.normalize(img_emb, dim=1)
txt_emb = F.normalize(txt_emb, dim=1)
logits = img_emb @ txt_emb.T / temperature
targets = torch.arange(img_emb.shape[0])
return (F.cross_entropy(logits, targets) + F.cross_entropy(logits.T, targets)) / 2
# train on CROPS, matching how the detector will feed it patches at inference time --
# training on full uncropped images instead would create a train/test mismatch in object scale
clip_rng = np.random.default_rng(3)
N_CLIP = 400
X_clip, cap_clip = [], []
for _ in range(N_CLIP):
shape = clip_rng.choice(SHAPES)
cx, cy = clip_rng.integers(8, SIZE - 8), clip_rng.integers(8, SIZE - 8)
img = make_image(shape, cx, cy, r=6)
img = np.clip(img + clip_rng.normal(0, 0.05, img.shape), 0, 1).astype(np.float32)
box = np.array([cx - 6, cy - 6, 12, 12], dtype=np.float32) / SIZE
X_clip.append(crop_and_resize(img, box)); cap_clip.append(caption_for(shape))
X_clip = np.array(X_clip, dtype=np.float32)
cap_clip = np.array(cap_clip, dtype=np.int64)
torch.manual_seed(1)
img_enc = ImageEncoder()
txt_enc = TextEncoder(len(VOCAB))
opt2 = torch.optim.Adam(list(img_enc.parameters()) + list(txt_enc.parameters()), lr=0.01)
caps_tensor = torch.tensor(cap_clip)
n_clip = len(X_clip)
for epoch in range(300):
idx = np.random.default_rng(epoch).permutation(n_clip)[:64]
loss = clip_loss(img_enc(torch.tensor(X_clip[idx]).unsqueeze(1)), txt_enc(caps_tensor[idx]))
opt2.zero_grad()
loss.backward()
opt2.step()
print(f'CLIP training final loss: {loss.item():.3f}')
The full pipeline: detect, crop, classify by text match¶
For each test image: run the class-agnostic box regressor, crop the predicted region, embed it, and compare against text-prompt embeddings for all three shape names — including "square," which the box regressor never trained on. To isolate why any errors happen, also evaluate the same classifier using the ground-truth box instead of the predicted one, which removes localization error from the picture entirely.
with torch.no_grad():
prompt_embs = {s: F.normalize(txt_enc(torch.tensor([caption_for(s)])), dim=1) for s in SHAPES}
def classify_crop(img, box):
with torch.no_grad():
crop = crop_and_resize(img, box)
emb = F.normalize(img_enc(torch.tensor(crop[None, None])), dim=1)
sims = {s: (emb @ prompt_embs[s].T).item() for s in SHAPES}
return max(sims, key=sims.get)
eval_rng = np.random.default_rng(2)
n_correct_pred, n_correct_gt, n_total = {s: 0 for s in SHAPES}, {s: 0 for s in SHAPES}, {s: 0 for s in SHAPES}
for _ in range(150):
img, gt_box, true_shape = make_detection_example(eval_rng, SHAPES)
with torch.no_grad():
pred_box = box_model(torch.tensor(img[None, None])).squeeze(0).numpy()
pred_using_predicted_box = classify_crop(img, pred_box)
pred_using_gt_box = classify_crop(img, gt_box)
n_total[true_shape] += 1
n_correct_pred[true_shape] += (pred_using_predicted_box == true_shape)
n_correct_gt[true_shape] += (pred_using_gt_box == true_shape)
print(f'{"class":>8} {"base?":>7} {"end-to-end (predicted box)":>28} {"classification only (true box)":>32}')
for s in SHAPES:
acc_pred = n_correct_pred[s] / n_total[s]
acc_gt = n_correct_gt[s] / n_total[s]
print(f'{s:>8} {str(s in BASE_SHAPES):>7} {acc_pred:>27.1%} {acc_gt:>31.1%}')
With the ground-truth box, classification hits 100% on all three classes — the CLIP-style classifier genuinely recognizes "square" by text prompt alone, despite the detector never once training on a square's box. That confirms the core open-vocabulary claim cleanly. But end-to-end, using the detector's own predicted box, square accuracy drops well behind the base classes — not because the classifier forgot what a square looks like, but because the class-agnostic box regressor's boxes for the novel shape are measurably less precise (recall the IoU gap from Half 1), and a distorted crop is harder to classify correctly even for a perfectly capable classifier. Novel-category classification and novel-category localization are genuinely separate problems with separate failure modes, and this experiment isolates exactly which one is responsible for any given end-to-end error — precisely the diagnostic real open-vocabulary detection papers report (base-class vs. novel-class AP, broken out separately) when describing where their systems still fall short.
Exercise¶
- Increase the training set size for
box_modelfrom 400 to 2000 base-class examples. Does more base-class training data improve novel-class (square) localization IoU, even though square itself is still never in the training labels? - Add a fourth shape (e.g. a small triangle, following Lesson 37's pattern) that is novel to both the box regressor and the CLIP classifier. Does the pipeline fail at the localization stage, the classification stage, or both — and how would you tell them apart using this lesson's ground-truth-box diagnostic?
crop_and_resizealways resizes to the same output size regardless of the predicted box's size. Real open-vocabulary detectors are sensitive to box-size errors for exactly this reason — a box that's the wrong aspect ratio distorts the crop before classification ever sees it. Measure the correlation between predicted-box IoU and classification correctness across the square test cases: do the worst-localized squares also tend to be the misclassified ones?