Add takeTexture() to QOpenGLFramebufferObject

Add an API that allows to retrieve and detach the texture from the
framebuffer object. The next bind() call will then create and attach a
new texture.

[ChangeLog][QtGui][QOpenGLFramebufferObject] Added takeTexture() for
retrieving and detaching the texture from the framebuffer object.

Task-number: QTBUG-35881
Change-Id: I2cca37f5872c1685b1238047f8b912e6534ab781
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>

3 files changed, 97 insertions, 41 deletions

diff --git a/src/gui/opengl/qopenglframebufferobject.cpp b/src/gui/opengl/qopenglframebufferobject.cpp
index c06ba40b47..d5de8b009a 100644
--- a/src/gui/opengl/qopenglframebufferobject.cpp
+++ b/src/gui/opengl/qopenglframebufferobject.cpp
@@ -446,42 +446,12 @@ void QOpenGLFramebufferObjectPrivate::init(QOpenGLFramebufferObject *, const QSi
     funcs.glGenFramebuffers(1, &fbo);
     funcs.glBindFramebuffer(GL_FRAMEBUFFER, fbo);
 
-    GLuint texture = 0;
     GLuint color_buffer = 0;
 
     QT_CHECK_GLERROR();
     // init texture
     if (samples == 0) {
-        glGenTextures(1, &texture);
-        glBindTexture(target, texture);
-
-        glTexParameteri(target, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
-        glTexParameteri(target, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
-        glTexParameteri(target, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
-        glTexParameteri(target, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
-
-        glTexImage2D(target, 0, internal_format, size.width(), size.height(), 0,
-                GL_RGBA, GL_UNSIGNED_BYTE, NULL);
-        if (mipmap) {
-            int width = size.width();
-            int height = size.height();
-            int level = 0;
-            while (width > 1 || height > 1) {
-                width = qMax(1, width >> 1);
-                height = qMax(1, height >> 1);
-                ++level;
-                glTexImage2D(target, level, internal_format, width, height, 0,
-                        GL_RGBA, GL_UNSIGNED_BYTE, NULL);
-            }
-        }
-        funcs.glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
-                target, texture, 0);
-
-        QT_CHECK_GLERROR();
-        valid = checkFramebufferStatus(ctx);
-        glBindTexture(target, 0);
-
-        color_buffer = 0;
+        initTexture(texture_target, internal_format, size, mipmap);
     } else {
         mipmap = false;
         funcs.glGenRenderbuffers(1, &color_buffer);
@@ -492,8 +462,10 @@ void QOpenGLFramebufferObjectPrivate::init(QOpenGLFramebufferObject *, const QSi
         QT_CHECK_GLERROR();
         valid = checkFramebufferStatus(ctx);
 
-        if (valid)
+        if (valid) {
             funcs.glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_SAMPLES, &samples);
+            color_buffer_guard = new QOpenGLSharedResourceGuard(ctx, color_buffer, freeRenderbufferFunc);
+        }
     }
 
     format.setTextureTarget(target);
@@ -506,20 +478,59 @@ void QOpenGLFramebufferObjectPrivate::init(QOpenGLFramebufferObject *, const QSi
     funcs.glBindFramebuffer(GL_FRAMEBUFFER, ctx->d_func()->current_fbo);
     if (valid) {
         fbo_guard = new QOpenGLSharedResourceGuard(ctx, fbo, freeFramebufferFunc);
-        if (color_buffer)
-            color_buffer_guard = new QOpenGLSharedResourceGuard(ctx, color_buffer, freeRenderbufferFunc);
-        else
-            texture_guard = new QOpenGLSharedResourceGuard(ctx, texture, freeTextureFunc);
     } else {
-        if (color_buffer)
-            funcs.glDeleteRenderbuffers(1, &color_buffer);
-        else
-            glDeleteTextures(1, &texture);
+        if (color_buffer_guard) {
+            color_buffer_guard->free();
+            color_buffer_guard = 0;
+        } else if (texture_guard) {
+            texture_guard->free();
+            texture_guard = 0;
+        }
         funcs.glDeleteFramebuffers(1, &fbo);
     }
     QT_CHECK_GLERROR();
 }
 
+void QOpenGLFramebufferObjectPrivate::initTexture(GLenum target, GLenum internal_format,
+                                                  const QSize &size, bool mipmap)
+{
+    QOpenGLContext *ctx = QOpenGLContext::currentContext();
+    GLuint texture = 0;
+
+    glGenTextures(1, &texture);
+    glBindTexture(target, texture);
+
+    glTexParameteri(target, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
+    glTexParameteri(target, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
+    glTexParameteri(target, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
+    glTexParameteri(target, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
+
+    glTexImage2D(target, 0, internal_format, size.width(), size.height(), 0,
+                 GL_RGBA, GL_UNSIGNED_BYTE, NULL);
+    if (mipmap) {
+        int width = size.width();
+        int height = size.height();
+        int level = 0;
+        while (width > 1 || height > 1) {
+            width = qMax(1, width >> 1);
+            height = qMax(1, height >> 1);
+            ++level;
+            glTexImage2D(target, level, internal_format, width, height, 0,
+                         GL_RGBA, GL_UNSIGNED_BYTE, NULL);
+        }
+    }
+    funcs.glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
+                                 target, texture, 0);
+
+    QT_CHECK_GLERROR();
+    glBindTexture(target, 0);
+    valid = checkFramebufferStatus(ctx);
+    if (valid)
+        texture_guard = new QOpenGLSharedResourceGuard(ctx, texture, freeTextureFunc);
+    else
+        glDeleteTextures(1, &texture);
+}
+
 void QOpenGLFramebufferObjectPrivate::initAttachments(QOpenGLContext *ctx, QOpenGLFramebufferObject::Attachment attachment)
 {
     int samples = format.samples();
@@ -905,6 +916,10 @@ bool QOpenGLFramebufferObject::isValid() const
     framebuffer to this framebuffer object.
     Returns \c true upon success, false otherwise.
 
+    \note If takeTexture() was called, a new texture is created and associated
+    with the framebuffer object. This is potentially expensive and changes the
+    context state (the currently bound texture).
+
     \sa release()
 */
 bool QOpenGLFramebufferObject::bind()
@@ -920,7 +935,10 @@ bool QOpenGLFramebufferObject::bind()
         qWarning("QOpenGLFramebufferObject::bind() called from incompatible context");
 #endif
     d->funcs.glBindFramebuffer(GL_FRAMEBUFFER, d->fbo());
-    d->valid = d->checkFramebufferStatus(current);
+    if (d->texture_guard || d->format.samples() != 0)
+        d->valid = d->checkFramebufferStatus(current);
+    else
+        d->initTexture(d->format.textureTarget(), d->format.internalTextureFormat(), d->size, d->format.mipmap());
     if (d->valid && current)
         current->d_func()->current_fbo = d->fbo();
     return d->valid;
@@ -967,6 +985,8 @@ bool QOpenGLFramebufferObject::release()
 
     If a multisample framebuffer object is used then the value returned
     from this function will be invalid.
+
+    \sa takeTexture()
 */
 GLuint QOpenGLFramebufferObject::texture() const
 {
@@ -975,6 +995,40 @@ GLuint QOpenGLFramebufferObject::texture() const
 }
 
 /*!
+   \fn GLuint QOpenGLFramebufferObject::takeTexture()
+
+   Returns the texture id for the texture attached to this framebuffer
+   object. The ownership of the texture is transferred to the caller.
+
+   If the framebuffer object is currently bound, an implicit release()
+   will be done. During the next call to bind() a new texture will be
+   created.
+
+   If a multisample framebuffer object is used, then there is no
+   texture and the return value from this function will be invalid.
+   Similarly, incomplete framebuffer objects will also return 0.
+
+   \since 5.3
+
+   \sa texture(), bind(), release()
+ */
+GLuint QOpenGLFramebufferObject::takeTexture()
+{
+    Q_D(QOpenGLFramebufferObject);
+    GLuint id = 0;
+    if (isValid() && d->texture_guard) {
+        QOpenGLContext *current = QOpenGLContext::currentContext();
+        if (current && current->shareGroup() == d->fbo_guard->group() && current->d_func()->current_fbo == d->fbo())
+            release();
+        id = d->texture_guard->id();
+        // Do not call free() on texture_guard, just null it out.
+        // This way the texture will not be deleted when the guard is destroyed.
+        d->texture_guard = 0;
+    }
+    return id;
+}
+
+/*!
     \fn QSize QOpenGLFramebufferObject::size() const
 
     Returns the size of the texture attached to this framebuffer
diff --git a/src/gui/opengl/qopenglframebufferobject.h b/src/gui/opengl/qopenglframebufferobject.h
index 215d3701ca..a431618f6d 100644
--- a/src/gui/opengl/qopenglframebufferobject.h
+++ b/src/gui/opengl/qopenglframebufferobject.h
@@ -97,6 +97,7 @@ public:
     int height() const { return size().height(); }
 
     GLuint texture() const;
+    GLuint takeTexture();
     QSize size() const;
     QImage toImage() const;
     Attachment attachment() const;
diff --git a/src/gui/opengl/qopenglframebufferobject_p.h b/src/gui/opengl/qopenglframebufferobject_p.h
index c8c69c4b3e..1c7d2e1e5d 100644
--- a/src/gui/opengl/qopenglframebufferobject_p.h
+++ b/src/gui/opengl/qopenglframebufferobject_p.h
@@ -116,6 +116,7 @@ public:
               QOpenGLFramebufferObject::Attachment attachment,
               GLenum internal_format, GLenum texture_target,
               GLint samples = 0, bool mipmap = false);
+    void initTexture(GLenum target, GLenum internal_format, const QSize &size, bool mipmap);
     void initAttachments(QOpenGLContext *ctx, QOpenGLFramebufferObject::Attachment attachment);
 
     bool checkFramebufferStatus(QOpenGLContext *ctx) const;