effects/zoom: Fix rendering with mixed scale factors

When allocating offscreen texture, we should use screen's scale rather
than current render target's scale.

In addition to that, the cached vbo cannot be used for rendering on
other screens with different scale factors, which can happen.

src/effects/zoom/zoom.cpp

@@ -255,8 +255,8 @@ void ZoomEffect::prePaintScreen(ScreenPrePaintData &data, std::chrono::milliseco
 
 ZoomEffect::OffscreenData *ZoomEffect::ensureOffscreenData(EffectScreen *screen)
 {
-    const QRect rect = effects->renderTargetRect();
-    const qreal devicePixelRatio = effects->renderTargetScale();
+    const QRect rect = screen->geometry();
+    const qreal devicePixelRatio = screen->devicePixelRatio();
     const QSize nativeSize = rect.size() * devicePixelRatio;
 
     OffscreenData &data = m_offscreenData[effects->waylandDisplay() ? screen : nullptr];
@@ -266,30 +266,6 @@ ZoomEffect::OffscreenData *ZoomEffect::ensureOffscreenData(EffectScreen *screen)
         data.texture->setWrapMode(GL_CLAMP_TO_EDGE);
         data.framebuffer = std::make_unique<GLFramebuffer>(data.texture.get());
     }
-    if (!data.vbo || data.viewport != rect) {
-        data.vbo.reset(new GLVertexBuffer(GLVertexBuffer::Static));
-        data.viewport = scaledRect(rect, devicePixelRatio).toRect();
-
-        QVector<float> verts;
-        QVector<float> texcoords;
-
-        // The v-coordinate is flipped because projection matrix is "flipped."
-        texcoords << 1.0 << 1.0;
-        verts << (rect.x() + rect.width()) * devicePixelRatio << rect.y() * devicePixelRatio;
-        texcoords << 0.0 << 1.0;
-        verts << rect.x() * devicePixelRatio << rect.y() * devicePixelRatio;
-        texcoords << 0.0 << 0.0;
-        verts << rect.x() * devicePixelRatio << (rect.y() + rect.height()) * devicePixelRatio;
-
-        texcoords << 1.0 << 0.0;
-        verts << (rect.x() + rect.width()) * devicePixelRatio << (rect.y() + rect.height()) * devicePixelRatio;
-        texcoords << 1.0 << 1.0;
-        verts << (rect.x() + rect.width()) * devicePixelRatio << rect.y() * devicePixelRatio;
-        texcoords << 0.0 << 0.0;
-        verts << rect.x() * devicePixelRatio << (rect.y() + rect.height()) * devicePixelRatio;
-
-        data.vbo->setData(6, 2, verts.constData(), texcoords.constData());
-    }
 
     return &data;
 }
@@ -370,16 +346,20 @@ void ZoomEffect::paintScreen(int mask, const QRegion &region, ScreenPaintData &d
     glClearColor(0.0, 0.0, 0.0, 0.0);
     glClear(GL_COLOR_BUFFER_BIT);
 
-    QMatrix4x4 matrix;
-    matrix.translate(xTranslation * scale, yTranslation * scale);
-    matrix.scale(zoom, zoom);
-
     auto shader = ShaderManager::instance()->pushShader(ShaderTrait::MapTexture);
-    shader->setUniform(GLShader::ModelViewProjectionMatrix, data.projectionMatrix() * matrix);
-    for (auto &[screen, data] : m_offscreenData) {
-        data.texture->bind();
-        data.vbo->render(GL_TRIANGLES);
-        data.texture->unbind();
+    for (auto &[screen, offscreen] : m_offscreenData) {
+        const QRect geometry = screen->geometry();
+
+        QMatrix4x4 matrix;
+        matrix.translate(xTranslation * scale, yTranslation * scale);
+        matrix.scale(zoom, zoom);
+        matrix.translate(geometry.x() * scale, geometry.y() * scale);
+
+        shader->setUniform(GLShader::ModelViewProjectionMatrix, data.projectionMatrix() * matrix);
+
+        offscreen.texture->bind();
+        offscreen.texture->render(geometry.size(), scale);
+        offscreen.texture->unbind();
     }
     ShaderManager::instance()->popShader();
 

src/effects/zoom/zoom.h

@@ -89,7 +89,6 @@ private:
     {
         std::unique_ptr<GLTexture> texture;
         std::unique_ptr<GLFramebuffer> framebuffer;
-        std::unique_ptr<GLVertexBuffer> vbo;
         QRect viewport;
     };