From 8321974fc7eb20f91ca4437bdd4ef94576e36098 Mon Sep 17 00:00:00 2001
From: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
Date: Thu, 13 Jul 2023 17:00:38 +0200
Subject: [PATCH] Make it possible to zoom the VideoWidget.

---
 video_widget.cpp | 106 +++++++++++++++++++++++++++++++++++++++++++++--
 video_widget.h   |   8 ++++
 2 files changed, 110 insertions(+), 4 deletions(-)

diff --git a/video_widget.cpp b/video_widget.cpp
index ba8cf51..b25b938 100644
--- a/video_widget.cpp
+++ b/video_widget.cpp
@@ -31,6 +31,7 @@ extern "C" {
 #include <unordered_set>
 
 #include <QOpenGLFunctions>
+#include <QWheelEvent>
 
 using namespace std;
 using namespace std::chrono;
@@ -425,21 +426,118 @@ void VideoWidget::paintGL()
 
 	glBegin(GL_QUADS);
 
+	// (0,0)
 	glVertexAttrib2f(1, tx1, ty1);
-	glVertex2f(0.0f, 0.0f);
+	glVertex2f(zoom_matrix[2 * 3 + 0], zoom_matrix[2 * 3 + 1]);
 
+	// (0,1)
 	glVertexAttrib2f(1, tx1, ty2);
-	glVertex2f(0.0f, 1.0f);
+	glVertex2f(zoom_matrix[1 * 3 + 0] + zoom_matrix[2 * 3 + 0], zoom_matrix[1 * 3 + 1] + zoom_matrix[2 * 3 + 1]);
 
+	// (1,1)
 	glVertexAttrib2f(1, tx2, ty2);
-	glVertex2f(1.0f, 1.0f);
+	glVertex2f(zoom_matrix[0 * 3 + 0] + zoom_matrix[1 * 3 + 0] + zoom_matrix[2 * 3 + 0],
+	           zoom_matrix[1 * 3 + 0] + zoom_matrix[1 * 3 + 1] + zoom_matrix[2 * 3 + 1]);
 
+	// (1,0)
 	glVertexAttrib2f(1, tx2, ty1);
-	glVertex2f(1.0f, 0.0f);
+	glVertex2f(zoom_matrix[0 * 3 + 0] + zoom_matrix[2 * 3 + 0],
+	           zoom_matrix[1 * 3 + 0] + zoom_matrix[2 * 3 + 1]);
 
 	glEnd();
 }
 
+void matmul3x3(const double a[9], const double b[9], double res[9])
+{
+	for (int i = 0; i < 3; ++i) {
+		for (int j = 0; j < 3; ++j) {
+			double sum = 0.0;
+			for (int k = 0; k < 3; ++k) {
+				sum += a[i * 3 + k] * b[k * 3 + j];
+			}
+			res[i * 3 + j] = sum;
+		}
+	}
+}
+
+void VideoWidget::wheelEvent(QWheelEvent *event)
+{
+	int delta = event->angleDelta().y();
+	if (delta == 0) {
+		return;
+	}
+	double x = event->position().x() / width();
+	double y = 1.0 - event->position().y() / height();
+	double zoom = delta > 0 ? pow(1.01, delta) : pow(1/1.01, -delta);
+
+	const double inv_translation_matrix[9] = {
+		1.0, 0.0, 0.0,
+		0.0, 1.0, 0.0,
+		-x, -y, 1.0
+	};
+	const double scale_matrix[9] = {
+		zoom, 0.0, 0.0,
+		0.0, zoom, 0.0,
+		0.0, 0.0, 1.0
+	};
+	const double translation_matrix[9] = {
+		1.0, 0.0, 0.0,
+		0.0, 1.0, 0.0,
+		x, y, 1.0
+	};
+	double tmp1[9], tmp2[9];
+	matmul3x3(zoom_matrix, inv_translation_matrix, tmp1);
+	matmul3x3(tmp1, scale_matrix, tmp2);
+	matmul3x3(tmp2, translation_matrix, zoom_matrix);
+
+	fixup_zoom_matrix();
+}
+
+// Normalize the matrix so that we never get skew or similar,
+// and also never can zoom or pan too far out.
+void VideoWidget::fixup_zoom_matrix()
+{
+	// Correct for any numerical errors (we know the matrix must be orthogonal
+	// and have zero rotation).
+	zoom_matrix[4] = zoom_matrix[0];
+	zoom_matrix[1] = zoom_matrix[2] = zoom_matrix[3] = zoom_matrix[5] = 0.0;
+	zoom_matrix[8] = 1.0;
+
+	// We can't zoom further out than 1:1. (Perhaps it would be nice to
+	// reuse the last zoom-in point to do this, but the center will have to do
+	// for now.)
+	if (zoom_matrix[0] < 1.0) {
+		const double zoom = 1.0 / zoom_matrix[0];
+		const double inv_translation_matrix[9] = {
+			1.0, 0.0, 0.0,
+			0.0, 1.0, 0.0,
+			-0.5, -0.5, 1.0
+		};
+		const double scale_matrix[9] = {
+			zoom, 0.0, 0.0,
+			0.0, zoom, 0.0,
+			0.0, 0.0, 1.0
+		};
+		const double translation_matrix[9] = {
+			1.0, 0.0, 0.0,
+			0.0, 1.0, 0.0,
+			0.5, 0.5, 1.0
+		};
+		double tmp1[9], tmp2[9];
+		matmul3x3(zoom_matrix, inv_translation_matrix, tmp1);
+		matmul3x3(tmp1, scale_matrix, tmp2);
+		matmul3x3(tmp2, translation_matrix, zoom_matrix);
+	}
+
+	// Looking at the points we'll draw with glVertex2f(), make sure none of them are
+	// inside the square (which would generally mean we've panned ourselves out-of-bounds).
+	// We simply adjust the translation, which is possible because we fixed scaling above.
+	zoom_matrix[6] = min(zoom_matrix[6], 0.0);  // Left side (x=0).
+	zoom_matrix[7] = min(zoom_matrix[7], 0.0);  // Bottom side (y=0).
+	zoom_matrix[6] = std::max(zoom_matrix[6], 1.0 - zoom_matrix[0]);  // Right side (x=1).
+	zoom_matrix[7] = std::max(zoom_matrix[7], 1.0 - zoom_matrix[4]);  // Top side (y=1).
+}
+
 void VideoWidget::open(const string &filename)
 {
 	stop();
diff --git a/video_widget.h b/video_widget.h
index 293c968..801d5cd 100644
--- a/video_widget.h
+++ b/video_widget.h
@@ -37,6 +37,7 @@ public:
 	void initializeGL() override;
 	void resizeGL(int w, int h) override;
 	void paintGL() override;
+	void wheelEvent(QWheelEvent *event) override;
 
 signals:
 	void position_changed(uint64_t pos);
@@ -59,6 +60,11 @@ private:
 	GLuint last_chroma_width = 0, last_chroma_height = 0;
 	GLfloat cbcr_offset[2];
 	double display_aspect = 1.0;
+	double zoom_matrix[9] = {  // Column-major, to match with OpenGL.
+		1.0, 0.0, 0.0,
+		0.0, 1.0, 0.0,
+		0.0, 0.0, 1.0,
+	};
 
 	int64_t pts_origin;
 	int64_t last_pts;
@@ -95,6 +101,8 @@ private:
 	Frame make_video_frame(const AVFrame *frame);
 	bool process_queued_commands(AVFormatContext *format_ctx, AVCodecContext *video_codec_ctx, int video_stream_index, bool *seeked);
 	void store_pts(int64_t pts);
+
+	void fixup_zoom_matrix();
 };
 
 #endif  // !defined(_VIDEO_WIDGET_H)
-- 
2.39.2