Lws includes a rewrite of picojpeg that performs stateful, line-at-a-time decoding.

The heap memory requirement is 2.1KB plus an internally-allocated either 8 or 16-line pixel buffer, the width of the image, and with either Y (for grayscale jpeg) or RGB bytes per pixel. Eg for a 600px wide image

Type	Heap requirement
Grayscale	6.5KB
RGB 4:4:4	16.4KB
RGB 4:2:2v	16.4KB
RGB 4:4:2h	31KB
RGB 4:4:0	31KB

No other allocations occur during decode.

In particular the input JPEG data is stream parsed into the JPEG MCU buffer, so there is no requirement for it all to be in memory at the same time, and there is no framebuffer required, only a line of pixels is processed in isolation at a time.

The results in an extremely tight decoder suitable for microcontroller type platforms that lack enough memory to hold a framebuffer, but can stream the rendered data out over SPI or i2c to a display device that does have its own (usually write-only) framebuffer memory.

Creating and destroying the decoding context

The apis to create and destroy a decoding context are very simple...

LWS_VISIBLE LWS_EXTERN lws_jpeg_t *
lws_jpeg_new(void);
 
LWS_VISIBLE LWS_EXTERN void
lws_jpeg_free(lws_jpeg_t **jpeg);

Performing the decoding

The only decoding API provides input PNG data which may or may not be partly or wholly consumed, to produce a line of output pixels that can be found at *ppix.

LWS_VISIBLE LWS_EXTERN lws_stateful_ret_t
lws_jpeg_emit_next_line(lws_jpeg_t *jpeg, const uint8_t **ppix,
                        const uint8_t **buf, size_t *size);

If input data is consumed, *buf and *size are adjusted accordingly. This api returns a bitfield consisting of:

Return value bit	Meaning
`LWS_SRET_OK` (0, no bits set)	Completed
`LWS_SRET_WANT_INPUT`	Decoder needs to be called again with more PNG input before it can produce a line of pixels
`LWS_SRET_WANT_OUTPUT`	Decoder has paused to emit a line of pixels, and can resume
`LWS_SRET_FATAL`	Decoder has encountered a fatal error, any return greater than `LWS_SRET_FATAL` indicates the type of error
`LWS_SRET_NO_FURTHER_IN`	Indicate no further new input will be used
`LWS_SRET_NO_FURTHER_OUT`	Indicate no further output is forthcoming

To get early information about the dimensions and colourspace of the JPEG, you can call this api initially with restricted chunk size (eg, 128 bytes) until lws_jpeg_get_components() returns nonzero. You can continue where you left off later when you want to receive the result pixels.

Output format

To minimize the internal buffer, the provided line of pixels is either just a Y grayscale byte per pixel if a grayscale JPEG, or 3 RGB bytes per pixel. You can query which by using lws_jpeg_get_components() to find out how many bytes per pixel.

Although 4:4:4, 4:2:2 of both orientations, and 4:2:0 are handled differently internally, they all present 3-byte RGB output of the full width at *ppix.