From 3f53ced44ea112a39eec70216c8c72fa6867726e Mon Sep 17 00:00:00 2001 From: Luke Hoersten Date: Mon, 15 Jun 2026 07:50:23 -0500 Subject: firmware: double-buffer the panel + zero-copy decode → ~22 fps ceiling MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Per-frame paint cost drops from ~24 ms to ~45 µs (≈500× faster) by enabling num_fbs=2 on the DPI panel and decoding straight into the back framebuffer. Measured on the bench: before: lock=7us ttfb=370us body=40ms dec=6ms paint=24ms post=35us = ~70ms / ~14fps after : lock=7us ttfb=330us body=38ms dec=6ms paint=42us post=25us = ~45ms / ~22fps How the win actually lands: - num_fbs=2 in the esp_lcd_dpi_panel_config_t makes the IDF driver allocate two framebuffers and stream from one while we fill the other. - display_back_buffer() returns the inactive fb pointer + its size. - jpeg_decoder_decode() now accepts a caller-provided destination buffer instead of owning its own scratch. http_api passes the panel back-fb so the hardware JPEG decoder writes BGR888 pixels straight into where the DSI will eventually scan from. Zero memcpy in the hot path. - display_flip_back_buffer() calls esp_lcd_panel_draw_bitmap with the fb pointer. Because the buffer is inside the panel's own fb range, the IDF driver skips its memcpy and just does a cache writeback + swaps cur_fb_index. The actual flip happens on the next vsync, asynchronously — the call returns in microseconds. The remaining ceiling is network body time (~38 ms for ~210 KB JPEGs) and the hardware decoder (~6 ms). Per-viewport JPEG quality (smaller files = shorter body) is the next lever; everything firmware-side is already at or near floor. Also drop the old static jpeg output scratch + JPEG_DECODER_MAX_OUTPUT_BYTES constant — nothing references them anymore. --- main/jpeg_decoder.h | 19 ++++++++++++------- 1 file changed, 12 insertions(+), 7 deletions(-) (limited to 'main/jpeg_decoder.h') diff --git a/main/jpeg_decoder.h b/main/jpeg_decoder.h index 2ac436d..e8a8504 100644 --- a/main/jpeg_decoder.h +++ b/main/jpeg_decoder.h @@ -23,12 +23,17 @@ void jpeg_decoder_unlock(void); // JPEG_DECODER_MAX_INPUT_BYTES. Caller fills before calling decode. void *jpeg_decoder_input_buffer(void); -// Decode the JPEG sitting in the input buffer. Fills out_bgr888 with a -// pointer to the decoded 24-bit image (3 bytes/pixel, BGR memory order so -// the DSI engine + TC358762 + Pi panel render channels correctly) and -// reports the image's width/height in pixels. The output buffer is owned -// by the decoder — valid only until the next jpeg_decoder_unlock(). -esp_err_t jpeg_decoder_decode(size_t jpeg_len, - void **out_bgr888, +// Decode the JPEG sitting in the input buffer into the caller-provided +// out_buf (must be at least out_cap bytes, ≥ 800*480*3 for a full +// panel-sized frame). Bytes land in BGR memory order so the DSI engine +// + TC358762 + Pi panel render channels correctly. Reports image +// width/height in pixels. +// +// In the /frame hot path the caller passes display_back_buffer() so +// the hardware decoder writes pixels straight into the panel's back +// framebuffer with zero intermediate copies. +esp_err_t jpeg_decoder_decode(size_t jpeg_len, + void *out_buf, + size_t out_cap, uint16_t *out_width, uint16_t *out_height); -- cgit v1.2.3