From e6441ec4f4a68518c3f01c753ef179226f56ebfc Mon Sep 17 00:00:00 2001
From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Date: Mon, 9 Sep 2024 16:13:20 +0300
Subject: qcam: viewfinder_gl: Render image centered in letterbox

Mimic the letterbox behaviour of the Qt viewfinder by rendering the
image centered. This is done by adding a projection matrix to the vertex
shader to scale the rendered rectangle.

Another option would have been to keep using glViewport() (which would
have needed to be moved to paintGL(), as Qt resets the viewport to span
the full widget before calling). Hidpi displays would then need special
handling of the device pixel ratio, which is done automatically by Qt
when it sets the default viewport. Using a projection matrix avoids this
complication.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
---
 src/apps/qcam/viewfinder_gl.cpp | 31 ++++++++++++++++++++-----------
 1 file changed, 20 insertions(+), 11 deletions(-)

(limited to 'src/apps/qcam/viewfinder_gl.cpp')

diff --git a/src/apps/qcam/viewfinder_gl.cpp b/src/apps/qcam/viewfinder_gl.cpp
index b2096faf..f31956ff 100644
--- a/src/apps/qcam/viewfinder_gl.cpp
+++ b/src/apps/qcam/viewfinder_gl.cpp
@@ -12,6 +12,7 @@
 #include <QByteArray>
 #include <QFile>
 #include <QImage>
+#include <QMatrix4x4>
 #include <QStringList>
 
 #include <libcamera/formats.h>
@@ -464,6 +465,7 @@ bool ViewFinderGL::createFragmentShader()
 
 	vertexBuffer_.release();
 
+	projMatrixUniform_ = shaderProgram_.uniformLocation("proj_matrix");
 	textureUniformY_ = shaderProgram_.uniformLocation("tex_y");
 	textureUniformU_ = shaderProgram_.uniformLocation("tex_u");
 	textureUniformV_ = shaderProgram_.uniformLocation("tex_v");
@@ -509,7 +511,7 @@ void ViewFinderGL::initializeGL()
 	glEnable(GL_TEXTURE_2D);
 	glDisable(GL_DEPTH_TEST);
 
-	static const GLfloat coordinates[2][4][2]{
+	const GLfloat coordinates[2][4][2]{
 		{
 			/* Vertex coordinates */
 			{ -1.0f, -1.0f },
@@ -801,6 +803,17 @@ void ViewFinderGL::doRender()
 	shaderProgram_.setUniformValue(textureUniformStrideFactor_,
 				       static_cast<float>(size_.width() - 1) /
 				       (stridePixels - 1));
+
+	/*
+	 * Place the viewfinder in the centre of the widget, preserving the
+	 * aspect ratio of the image.
+	 */
+	QMatrix4x4 projMatrix;
+	QSizeF scaledSize = size_.scaled(size(), Qt::KeepAspectRatio);
+	projMatrix.scale(scaledSize.width() / size().width(),
+			 scaledSize.height() / size().height());
+
+	shaderProgram_.setUniformValue(projMatrixUniform_, projMatrix);
 }
 
 void ViewFinderGL::paintGL()
@@ -818,18 +831,14 @@ void ViewFinderGL::paintGL()
 		close();
 	}
 
-	if (image_) {
-		glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
-		glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
+	if (!image_)
+		return;
 
-		doRender();
-		glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
-	}
-}
+	glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
+	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
-void ViewFinderGL::resizeGL(int w, int h)
-{
-	glViewport(0, 0, w, h);
+	doRender();
+	glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
 }
 
 QSize ViewFinderGL::sizeHint() const
-- 
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