2026-10-04 — libjpeg-turbo, libpng and zlib-ng against stb_image¶

Question. What does each vendored decoder buy over the stb_image fallback, in time, memory and size?

Answer. - libjpeg-turbo decodes JPEG to gray 1.4–6.4× faster than stb_image, with 30% less peak memory. Decoding at PH_DECODE_SCALE_EIGHTH is a further 1.1–1.6× faster to gray (1.1–2.3× to RGB; least on the progressive file, 1.02–1.07×) and cuts peak memory about ninefold (87 → 10 MB on 20 MP). - libpng + zlib-ng decodes photographic PNG to gray 1.25–1.7× faster than stb_image on Linux and needs 83 MB of peak memory against stb_image's 141 MB (309 MB on macOS) on a 20 MP RGB file. On small screenshots stb_image is as fast. - zlib-ng carries the Linux PNG gain: libpng on the system zlib is only 1.0–1.17× faster than stb_image on x86-64. On macOS, libpng on the system zlib decodes photographs 10–35% faster than on zlib-ng. - Size: the native build's test_benchmark is 0.9–1.3 MB stripped, the stb_image-only one 0.3–0.4 MB.

What was measured¶


Library code	`33c252e`
Benchmark code	tag `bench/2026-10-04-decoders-vs-stb` (`4c2cca6`: `scripts/gen_bench_corpus.py`, `scripts/bench_decoders.sh`, `test_benchmark` taking the decode scale from `PH_BENCH_DECODE_SCALE`, workflow `bench-decoders.yml`)
CI run	37172361010, 2026-10-04 02:52 UTC, 7 rounds
Local run	2026-10-04, 5 rounds
Raw data	`data/2026-10-04-decoders-vs-stb/` (Linux only)

Platform	CPU	OS	Compiler	System zlib
Linux x86-64	AMD EPYC 7763 (GitHub `ubuntu-latest`)	Ubuntu 24.04, image 20260927.320.1	GCC 13.3.0	1.3 (`1:1.3.dfsg-3.1ubuntu2.2`)
Linux arm64	Neoverse-N2 (GitHub `ubuntu-24.04-arm`)	Ubuntu 24.04, image 20260927.135.1	GCC 13.3.0	1.3 (same package)
macOS arm64	Apple M3 Pro, 36 GB	macOS 26.6.2	Apple clang 21.0.0	Apple 1.2.12

The x86-64 runner is a different CPU from the one in the libpng-against-spng record of the same day, so absolute times of the two records do not compare; ratios within a record do.

Decoders: libjpeg-turbo 3.2.0 (libjpeg API), libpng 1.6.58, zlib-ng 2.3.3, stb_image 2.30. Three builds of test_benchmark, Release, WebP off: - native — libjpeg-turbo + libpng + zlib-ng; - syszlib — libpng + the system zlib, JPEG through stb_image (PNG rows only); - stb — every native decoder off.

Corpus: the eight PNGs of the libpng-against-spng record, and six JPEGs from the same photograph (Pillow 12.3.0): quality 90 at 20 MP as 4:2:0, 4:4:4, progressive 4:2:0 and gray; quality 85 4:2:0 at 9 MP and at 2 MP.

Method: as in the libpng-against-spng record — test_benchmark --json load, gray and RGB, rotated order, median of min_ms. × is stb_image's time over the native one. JPEG also runs on the native build at PH_DECODE_SCALE_EIGHTH (libjpeg's DCT-domain downscale, which stb_image does not have). Footprint: stripped test_benchmark, and peak RSS (/usr/bin/time) of one process decoding the largest file of each format once.

Linux x86-64¶

PNG, load to gray¶

file	libpng+zlib-ng	libpng+zlib	stb	zlib-ng vs stb	zlib vs stb
photo_gray16_9mp.png	65.94	79.28	83.82	1.27×	1.06×
photo_gray8_20mp.png	85.44	109.91	128.95	1.51×	1.17×
photo_rgb16_9mp.png	217.09	268.89	271.50	1.25×	1.01×
photo_rgb8_20mp.png	190.10	237.16	259.88	1.37×	1.10×
photo_rgb8_2mp.png	19.17	26.58	27.89	1.46×	1.05×
photo_rgba8_9mp.png	99.45	135.85	141.74	1.43×	1.04×
screenshot_pal8_4mp.png	9.84	18.01	7.86	0.80×	0.44×
screenshot_rgb8_4mp.png	8.72	34.94	12.45	1.43×	0.36×

JPEG, load to gray¶

file	libjpeg-turbo	libjpeg-turbo ⅛	stb	turbo vs stb	turbo ⅛ vs stb
q85_420_2mp.jpg	3.39	2.49	14.26	4.21×	5.73×
q85_420_9mp.jpg	19.48	15.63	64.79	3.33×	4.14×
q90_420_20mp.jpg	72.32	61.83	184.15	2.55×	2.98×
q90_420_prog_20mp.jpg	236.93	229.96	413.51	1.75×	1.80×
q90_444_20mp.jpg	77.71	67.53	206.37	2.66×	3.06×
q90_gray_20mp.jpg	72.62	62.92	104.19	1.43×	1.66×

PNG, load to rgb¶

file	libpng+zlib-ng	libpng+zlib	stb	zlib-ng vs stb	zlib vs stb
photo_gray16_9mp.png	85.23	98.47	89.52	1.05×	0.91×
photo_gray8_20mp.png	102.63	127.47	145.33	1.42×	1.14×
photo_rgb16_9mp.png	206.99	258.92	262.04	1.27×	1.01×
photo_rgb8_20mp.png	166.65	213.74	238.98	1.43×	1.12×
photo_rgb8_2mp.png	16.64	24.02	25.58	1.54×	1.06×
photo_rgba8_9mp.png	96.90	129.87	136.09	1.40×	1.05×
screenshot_pal8_4mp.png	5.14	13.32	3.59	0.70×	0.27×
screenshot_rgb8_4mp.png	4.01	30.25	8.16	2.04×	0.27×

JPEG, load to rgb¶

file	libjpeg-turbo	libjpeg-turbo ⅛	stb	turbo vs stb	turbo ⅛ vs stb
q85_420_2mp.jpg	4.86	2.71	11.91	2.45×	4.40×
q85_420_9mp.jpg	25.20	16.49	54.93	2.18×	3.33×
q90_420_20mp.jpg	88.31	63.76	163.35	1.85×	2.56×
q90_420_prog_20mp.jpg	249.27	231.46	392.45	1.57×	1.70×
q90_444_20mp.jpg	98.82	71.84	185.16	1.87×	2.58×
q90_gray_20mp.jpg	87.75	63.12	120.77	1.38×	1.91×

Footprint¶

build	stripped test_benchmark	peak RSS, largest PNG	peak RSS, largest JPEG
native	1307 KB	83764 KB	87744 KB (⅛: 10508 KB)
syszlib	647 KB	83680 KB	—
stb	407 KB	141780 KB	125764 KB

Linux arm64¶

PNG, load to gray¶

file	libpng+zlib-ng	libpng+zlib	stb	zlib-ng vs stb	zlib vs stb
photo_gray16_9mp.png	56.62	63.89	75.29	1.33×	1.18×
photo_gray8_20mp.png	81.48	98.52	136.46	1.67×	1.39×
photo_rgb16_9mp.png	149.94	186.18	228.30	1.52×	1.23×
photo_rgb8_20mp.png	174.62	202.83	243.08	1.39×	1.20×
photo_rgb8_2mp.png	16.12	20.59	23.66	1.47×	1.15×
photo_rgba8_9mp.png	90.93	112.67	126.97	1.40×	1.13×
screenshot_pal8_4mp.png	7.22	13.22	7.69	1.07×	0.58×
screenshot_rgb8_4mp.png	8.66	28.83	13.20	1.52×	0.46×

JPEG, load to gray¶

file	libjpeg-turbo	libjpeg-turbo ⅛	stb	turbo vs stb	turbo ⅛ vs stb
q85_420_2mp.jpg	3.40	2.14	21.74	6.39×	10.14×
q85_420_9mp.jpg	19.11	13.52	94.67	4.95×	7.00×
q90_420_20mp.jpg	69.59	53.80	266.59	3.83×	4.95×
q90_420_prog_20mp.jpg	197.47	183.83	430.24	2.18×	2.34×
q90_444_20mp.jpg	75.21	59.48	331.74	4.41×	5.58×
q90_gray_20mp.jpg	70.88	55.13	144.82	2.04×	2.63×

PNG, load to rgb¶

file	libpng+zlib-ng	libpng+zlib	stb	zlib-ng vs stb	zlib vs stb
photo_gray16_9mp.png	74.08	81.21	78.85	1.06×	0.97×
photo_gray8_20mp.png	107.09	124.18	160.00	1.49×	1.29×
photo_rgb16_9mp.png	142.66	178.90	221.33	1.55×	1.24×
photo_rgb8_20mp.png	157.88	186.59	228.42	1.45×	1.22×
photo_rgb8_2mp.png	14.29	18.77	21.96	1.54×	1.17×
photo_rgba8_9mp.png	87.92	110.11	124.56	1.42×	1.13×
screenshot_pal8_4mp.png	3.85	9.86	4.64	1.21×	0.47×
screenshot_rgb8_4mp.png	5.30	25.48	10.12	1.91×	0.40×

JPEG, load to rgb¶

file	libjpeg-turbo	libjpeg-turbo ⅛	stb	turbo vs stb	turbo ⅛ vs stb
q85_420_2mp.jpg	5.60	2.42	20.10	3.59×	8.32×
q85_420_9mp.jpg	27.85	14.57	88.05	3.16×	6.04×
q90_420_20mp.jpg	103.32	56.13	251.77	2.44×	4.49×
q90_420_prog_20mp.jpg	222.77	186.41	416.18	1.87×	2.23×
q90_444_20mp.jpg	113.49	62.95	316.41	2.79×	5.03×
q90_gray_20mp.jpg	86.57	55.17	170.14	1.97×	3.08×

Footprint¶

build	stripped test_benchmark	peak RSS, largest PNG	peak RSS, largest JPEG
native	1027 KB	83104 KB	86888 KB (⅛: 9980 KB)
syszlib	579 KB	83016 KB	—
stb	323 KB	141260 KB	125268 KB

macOS arm64¶

PNG, load to gray¶

file	libpng+zlib-ng	libpng+zlib	stb	zlib-ng vs stb	zlib vs stb
photo_gray16_9mp.png	41.16	30.66	56.07	1.36×	1.83×
photo_gray8_20mp.png	76.70	63.22	118.62	1.55×	1.88×
photo_rgb16_9mp.png	136.74	124.28	152.31	1.11×	1.23×
photo_rgb8_20mp.png	122.45	90.89	193.35	1.58×	2.13×
photo_rgb8_2mp.png	10.80	8.41	16.20	1.50×	1.93×
photo_rgba8_9mp.png	65.73	51.96	102.35	1.56×	1.97×
screenshot_pal8_4mp.png	4.37	5.17	4.18	0.96×	0.81×
screenshot_rgb8_4mp.png	6.46	8.32	6.36	0.99×	0.76×

JPEG, load to gray¶

file	libjpeg-turbo	libjpeg-turbo ⅛	stb	turbo vs stb	turbo ⅛ vs stb
q85_420_2mp.jpg	2.08	1.55	9.69	4.66×	6.26×
q85_420_9mp.jpg	12.62	10.14	44.83	3.55×	4.42×
q90_420_20mp.jpg	47.74	41.37	128.32	2.69×	3.10×
q90_420_prog_20mp.jpg	141.63	139.28	263.60	1.86×	1.89×
q90_444_20mp.jpg	50.88	46.27	147.78	2.90×	3.19×
q90_gray_20mp.jpg	47.63	42.23	82.55	1.73×	1.95×

PNG, load to rgb¶

file	libpng+zlib-ng	libpng+zlib	stb	zlib-ng vs stb	zlib vs stb
photo_gray16_9mp.png	49.53	38.44	58.85	1.19×	1.53×
photo_gray8_20mp.png	84.06	70.48	128.40	1.53×	1.82×
photo_rgb16_9mp.png	131.05	119.89	146.80	1.12×	1.22×
photo_rgb8_20mp.png	111.42	79.83	184.69	1.66×	2.31×
photo_rgb8_2mp.png	9.59	6.90	15.19	1.58×	2.20×
photo_rgba8_9mp.png	63.47	49.74	101.45	1.60×	2.04×
screenshot_pal8_4mp.png	1.98	2.68	2.14	1.08×	0.80×
screenshot_rgb8_4mp.png	4.06	5.90	4.33	1.07×	0.73×

JPEG, load to rgb¶

file	libjpeg-turbo	libjpeg-turbo ⅛	stb	turbo vs stb	turbo ⅛ vs stb
q85_420_2mp.jpg	2.68	1.72	8.47	3.16×	4.93×
q85_420_9mp.jpg	15.52	10.89	40.11	2.58×	3.68×
q90_420_20mp.jpg	53.83	42.93	117.72	2.19×	2.74×
q90_420_prog_20mp.jpg	147.44	140.60	252.33	1.71×	1.79×
q90_444_20mp.jpg	59.38	47.42	137.54	2.32×	2.90×
q90_gray_20mp.jpg	48.32	42.48	90.60	1.88×	2.13×

Footprint¶

build	stripped test_benchmark	peak RSS, largest PNG	peak RSS, largest JPEG
native	936 KB	81408 KB	86704 KB (⅛: 9440 KB)
syszlib	513 KB	81264 KB	—
stb	327 KB	309072 KB	125280 KB