Camera: Remember the JPEG quality that fit the last frame

Start from the quality that fit the previous frame instead of the top, so
most frames encode once. Step down while a frame is too big, and step back
up when one comes out under half the limit. Windows and the recompression
fallback share the logic.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

Core/HLE/sceUsbCam.cpp

@@ -16,6 +16,7 @@
 // https://github.com/hrydgard/ppsspp and http://www.ppsspp.org/.
 
 #include <algorithm>
+#include <atomic>
 #include <mutex>
 #include <vector>
 
@@ -436,28 +437,46 @@ int Camera::getMaxFrameSize() {
 	return framesize;
 }
 
-// Re-encodes a frame at lower quality until it fits maxSize, like the PSP camera compresses to the
-// game's framesize. Most platforms' capture code encodes at a fixed quality, so this is the common fallback.
+// The JPEG quality that fit the last frame. Frames of one scene are similar in size, so it's usually
+// right first time. Only the capture thread uses it, but atomic in case a platform has several.
+static std::atomic<int> g_jpegQuality{ 80 };
+
+int Camera::encodeToFit(int maxSize, const std::function<int(int quality)> &encode) {
+	int quality = g_jpegQuality;
+	int size = encode(quality);
+	while ((size < 0 || size > maxSize) && quality > 10) {
+		quality = std::max(10, quality - 10);
+		size = encode(quality);
+	}
+	// Way under the limit: try a better quality next time.
+	if (size >= 0 && size < maxSize / 2 && quality < 90) {
+		quality += 10;
+	}
+	g_jpegQuality = quality;
+	return size;
+}
+
+// Re-encodes a frame until it fits maxSize, like the PSP camera compresses to the game's framesize.
+// Most platforms' capture code encodes at a fixed quality, so this is the common fallback.
 static bool RecompressToFit(const unsigned char *image, long long length, int maxSize, std::vector<uint8_t> *out) {
 	int width = 0, height = 0, comps = 0;
 	unsigned char *rgb = jpgd::decompress_jpeg_image_from_memory(image, (int)length, &width, &height, &comps, 3);
 	if (!rgb) {
 		return false;
 	}
-	bool fits = false;
 	out->resize(width * height * 3 + 1024);
+	int size = Camera::encodeToFit(maxSize, [&](int quality) {
 		jpge::params params;
-	for (int quality = 70; quality >= 10; quality -= 15) {
-		int size = (int)out->size();
 		params.m_quality = quality;
-		if (jpge::compress_image_to_jpeg_file_in_memory(out->data(), size, width, height, 3, rgb, params) && size <= maxSize) {
-			out->resize(size);
-			fits = true;
-			break;
-		}
-	}
+		int outSize = (int)out->size();
+		return jpge::compress_image_to_jpeg_file_in_memory(out->data(), outSize, width, height, 3, rgb, params) ? outSize : -1;
+	});
 	free(rgb);
-	return fits;
+	if (size < 0 || size > maxSize) {
+		return false;
+	}
+	out->resize(size);
+	return true;
 }
 
 void Camera::pushCameraImage(long long length, unsigned char* image) {

Core/HLE/sceUsbCam.h

@@ -18,6 +18,7 @@
 #pragma once
 
 #include <stdint.h>
+#include <functional>
 #include <string>
 #include <vector>
 #include "Core/HLE/FunctionWrappers.h"
@@ -124,4 +125,7 @@ namespace Camera {
 	// The largest JPEG frame the game accepts (framesize in the video setup). The PSP camera compresses
 	// to fit it, so capture code should too.
 	int getMaxFrameSize();
+	// Calls encode(quality), which returns the JPEG size or -1, starting from the quality that fit the
+	// previous frame and going down until the result fits maxSize. Returns the final size (or -1).
+	int encodeToFit(int maxSize, const std::function<int(int quality)> &encode);
 }

Windows/CaptureDevice.cpp

@@ -247,22 +247,17 @@ HRESULT ReaderCallback::OnReadSample(
 						}
 					}
 
-					// Compress image to jpeg from RGB24. Like the PSP camera, lower the quality until the frame
-					// fits the size the game asked for: Go!Edit stores frames in fixed 15KB slots.
-					const int maxFrameSize = Camera::getMaxFrameSize();
+					// Compress image to jpeg from RGB24. Like the PSP camera, fit the size the game asked for:
+					// Go!Edit stores frames in fixed 15KB slots.
+					imgJpegSize = Camera::encodeToFit(Camera::getMaxFrameSize(), [&](int quality) {
 						jpge::params params;
-					for (int quality = 85; ; quality -= 10) {
-						imgJpegSize = device->imgJpegSize;
 						params.m_quality = quality;
-						jpge::compress_image_to_jpeg_file_in_memory(
-							device->imageJpeg, imgJpegSize,
-							dstW,
-							dstH,
-							3,
-							device->imageRGB, params);
-						if (imgJpegSize <= maxFrameSize || quality <= 15) {
-							break;
-						}
+						int size = device->imgJpegSize;
+						return jpge::compress_image_to_jpeg_file_in_memory(
+							device->imageJpeg, size, dstW, dstH, 3, device->imageRGB, params) ? size : -1;
+					});
+					if (imgJpegSize < 0) {
+						imgJpegSize = 0;
 					}
 				}
 #endif