[Bugfix][Frontend] Honor output_compression on the image generations route (#7447)

Signed-off-by: hsliu_ustc <hsliu_ustc@noreply.gitcode.com>
Signed-off-by: amy-why-3459 <wuhaiyan17@huawei.com>
Co-authored-by: amy-why-3459 <wuhaiyan17@huawei.com>

tests/entrypoints/openai_api/test_image_api_utils.py

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM-Omni project
+
+"""L1 tests for the image response encoder's compression contract."""
+
+import base64
+import io
+
+import pytest
+from PIL import Image
+
+from vllm_omni.entrypoints.openai.image_api_utils import (
+    encode_image_base64_with_compression,
+)
+
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
+
+@pytest.fixture(scope="module")
+def image():
+    # Photographic-content proxy: noise-plus-gradient content compresses very
+    # differently across PNG compress levels, which is what the mapping under
+    # test controls.
+    import numpy as np
+
+    rng = np.random.default_rng(0)
+    grad = np.linspace(0, 255, 256, dtype=np.float32)
+    arr = np.clip(grad[None, :, None] + rng.integers(-24, 25, (256, 256, 3)), 0, 255).astype(np.uint8)
+    return Image.fromarray(arr, mode="RGB")
+
+
+def test_png_output_compression_maps_to_compress_level(image):
+    """output_compression 100 -> compress_level 0, 1 -> compress_level 9."""
+
+    def encode(compression):
+        buffer = io.BytesIO()
+        image.save(buffer, format="PNG", compress_level=max(0, min(9, 9 - compression // 11)))
+        return len(base64.b64encode(buffer.getvalue()))
+
+    fast = encode_image_base64_with_compression(image, format="png", output_compression=100)
+    small = encode_image_base64_with_compression(image, format="png", output_compression=1)
+
+    # Level-0 PNG barely compresses; the mapped level-9 encode is smaller.
+    assert len(small) < len(fast)
+    # The mapping must produce the same sizes as the equivalent compress_level.
+    assert len(fast) == encode(100)
+    assert len(small) == encode(1)
+
+
+def test_jpeg_output_compression_maps_to_quality(image):
+    high = base64.b64decode(encode_image_base64_with_compression(image, format="jpeg", output_compression=100))
+    low = base64.b64decode(encode_image_base64_with_compression(image, format="jpeg", output_compression=10))
+
+    assert len(high) > len(low)