Propagate parts-list rendering failures with lcResult

A null framebuffer readback was treated as empty geometry, so a parts
list could retain a blank entry or silently omit it after rendering failed.
The QImage return type also conflated an empty inventory with an error.

Return lcResult<QImage> and abort on failed thumbnail readback after
framebuffer and renderer cleanup. Report framebuffer creation and image
allocation failures explicitly while preserving successful empty inventory
and unavailable-geometry behavior. HTML export propagates the error text;
the instructions UI clears its stale image and displays a warning parented
to the scene's view.

Validation: full debug build and whitespace checks passed. A temporary
OpenGL fault-injection harness covered first and later readback failures
with both missing-parts policies, framebuffer creation failure, successful
empty results, and restoration of caller context, surface, and framebuffer
bindings. The warning dialog was not exercised interactively.

common/lc_instructionsdialog.cpp

@@ -65,8 +65,19 @@ void lcInstructionsPartsListItem::Update()
 	QFont Font = mInstructions->GetFontProperty(lcInstructionsPropertyType::PLIFont, mModel, mStep);
 	QColor TextColor = mInstructions->GetColorProperty(lcInstructionsPropertyType::PLITextColor, mModel, mStep);
 
-	QImage PartsImage = mModel->GetPartsListImage(300, mStep, lcRGBAFromQColor(BackgroundColor), Font, TextColor, false);
-	setPixmap(QPixmap::fromImage(PartsImage));
+	const lcResult<QImage> PartsImage = mModel->GetPartsListImage(300, mStep, lcRGBAFromQColor(BackgroundColor), Font, TextColor, false);
+
+	if (!PartsImage)
+	{
+		setPixmap(QPixmap());
+
+		QWidget* Parent = scene() && !scene()->views().isEmpty() ? scene()->views().front() : nullptr;
+		QMessageBox::warning(Parent, QObject::tr("LeoCAD"), PartsImage.error());
+
+		return;
+	}
+
+	setPixmap(QPixmap::fromImage(PartsImage.value()));
 }
 
 lcInstructionsPageWidget::lcInstructionsPageWidget(QWidget* Parent, lcInstructions* Instructions, lcInstructionsPropertiesWidget* PropertiesWidget)

common/lc_model.cpp

@@ -1764,7 +1764,7 @@ lcResult<QImage> lcModel::RenderStepImageWithReadyAssets(bool Zoom, int Width, i
 	return Image;
 }
 
-QImage lcModel::GetPartsListImage(int MaxWidth, lcStep Step, quint32 BackgroundColor, QFont Font, QColor TextColor, bool KeepMissingParts) const
+lcResult<QImage> lcModel::GetPartsListImage(int MaxWidth, lcStep Step, quint32 BackgroundColor, QFont Font, QColor TextColor, bool KeepMissingParts) const
 {
 	lcScopedGLContextState GLState;
 
@@ -1812,6 +1812,10 @@ QImage lcModel::GetPartsListImage(int MaxWidth, lcStep Step, quint32 BackgroundC
 	if (Images.empty())
 	{
 		QImage Image(1, 1, QImage::Format_ARGB32);
+
+		if (Image.isNull())
+			return lcUnexpected(tr("Could not allocate the pieces list image."));
+
 		Image.fill(lcQColorFromRGBA(BackgroundColor));
 		return Image;
 	}
@@ -1835,12 +1839,7 @@ QImage lcModel::GetPartsListImage(int MaxWidth, lcStep Step, quint32 BackgroundC
 	View.SetSize(ThumbnailSize, ThumbnailSize);
 
 	if (!View.BeginRenderToImage(ThumbnailSize, ThumbnailSize))
-	{
-		if (gMainWindow)
-			QMessageBox::warning(gMainWindow, tr("LeoCAD"), tr("Error creating images."));
-
-		return QImage();
-	}
+		return lcUnexpected(tr("Could not create the pieces list framebuffer."));
 
 	float OrthoSize = 200.0f;
 
@@ -1879,6 +1878,8 @@ QImage lcModel::GetPartsListImage(int MaxWidth, lcStep Step, quint32 BackgroundC
 	Context->SetDefaultState();
 	Context->SetProjectionMatrix(ProjectionMatrix);
 
+	bool RenderFailed = false;
+
 	for (lcPartsListImage& Image : Images)
 	{
 		View.BindRenderFramebuffer();
@@ -1904,11 +1905,20 @@ QImage lcModel::GetPartsListImage(int MaxWidth, lcStep Step, quint32 BackgroundC
 
 		View.UnbindRenderFramebuffer();
 		Image.Thumbnail = View.GetRenderFramebufferImage().convertToFormat(QImage::Format_ARGB32);
+
+		if (Image.Thumbnail.isNull())
+		{
+			RenderFailed = true;
+			break;
+		}
 	}
 
 	View.EndRenderToImage();
 	Context->ClearResources();
 
+	if (RenderFailed)
+		return lcUnexpected(tr("Could not read the pieces list image."));
+
 	auto CalculateImageBounds = [](lcPartsListImage& Image)
 	{
 		const QImage& Thumbnail = Image.Thumbnail;
@@ -1956,11 +1966,19 @@ QImage lcModel::GetPartsListImage(int MaxWidth, lcStep Step, quint32 BackgroundC
 	if (Images.empty())
 	{
 		QImage Image(1, 1, QImage::Format_ARGB32);
+
+		if (Image.isNull())
+			return lcUnexpected(tr("Could not allocate the pieces list image."));
+
 		Image.fill(lcQColorFromRGBA(BackgroundColor));
 		return Image;
 	}
 
 	QImage DummyImage(16, 16, QImage::Format_ARGB32);
+
+	if (DummyImage.isNull())
+		return lcUnexpected(tr("Could not allocate the pieces list image."));
+
 	QPainter DummyPainter(&DummyImage);
 
 	DummyPainter.setFont(Font);
@@ -2010,6 +2028,10 @@ QImage lcModel::GetPartsListImage(int MaxWidth, lcStep Step, quint32 BackgroundC
 	}
 
 	QImage PainterImage(ImageWidth + 40, CurrentHeight + 40, QImage::Format_ARGB32);
+
+	if (PainterImage.isNull())
+		return lcUnexpected(tr("Could not allocate the pieces list image."));
+
 	PainterImage.fill(lcQColorFromRGBA(BackgroundColor));
 
 	QPainter Painter(&PainterImage);

common/lc_model.h

@@ -332,7 +332,8 @@ public:
 	void GetScene(lcScene* Scene, const lcCamera* ViewCamera, bool AllowHighlight, bool AllowFade) const;
 	void AddSubModelRenderMeshes(lcScene* Scene, const lcMatrix44& WorldMatrix, int DefaultColorIndex, lcRenderMeshState RenderMeshState, bool ParentActive) const;
 	QImage GetStepImage(bool Zoom, int Width, int Height, lcStep Step);
-	QImage GetPartsListImage(int MaxWidth, lcStep Step, quint32 BackgroundColor, QFont Font, QColor TextColor, bool KeepMissingParts) const;
+	// Empty inventory succeeds with a null image; rendering failures return an error.
+	lcResult<QImage> GetPartsListImage(int MaxWidth, lcStep Step, quint32 BackgroundColor, QFont Font, QColor TextColor, bool KeepMissingParts) const;
 	lcResult<void> SaveStepImages(const QString& BaseName, bool AddStepSuffix, bool Zoom, int Width, int Height, lcStep Start, lcStep End);
 
 	void RayTest(lcObjectRayTest& ObjectRayTest) const;

common/project.cpp

@@ -2257,16 +2257,16 @@ lcResult<void> Project::ExportHTML(const lcHTMLExportOptions& Options)
 		if (PartsList.empty())
 			return lcResult<void>();
 
-		QImage Image = Model->GetPartsListImage(1024, Step, LC_RGBA(255, 255, 255, 0), QFont("Arial", 16, QFont::Bold), Qt::black, true);
+		const lcResult<QImage> Image = Model->GetPartsListImage(1024, Step, LC_RGBA(255, 255, 255, 0), QFont("Arial", 16, QFont::Bold), Qt::black, true);
 
-		if (Image.isNull())
-			return lcUnexpected(tr("Could not render the pieces list image."));
+		if (!Image)
+			return lcUnexpected(Image.error());
 
 		QString ImageName = BaseName + QLatin1String("-parts.png");
 		QString FileName = QFileInfo(Dir, ImageName).absoluteFilePath();
 		QImageWriter Writer(FileName);
 
-		if (!Writer.write(Image))
+		if (!Writer.write(Image.value()))
 			return lcUnexpected(tr("Error writing to file '%1':\n%2").arg(FileName, Writer.errorString()));
 
 		Stream << QString::fromLatin1("<p><IMG SRC=\"%1\" /></p><br><br>\r\n").arg(ImageName.toHtmlEscaped());