rhi: gl: Add a feature flag for reading back non-zero mip levels

The joys of "level - Specifies the mipmap level of the texture
image to be attached, which must be 0." for glFramebufferTexture2D
in OpenGL ES 2.0.

Change-Id: Iaf19502f48d7ba73b26abb72535bfa6696a1e182
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

7 files changed, 41 insertions, 11 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index bea8890234..57c466678e 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -572,7 +572,13 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
     supported for QRhiBuffer instances with a usage different than
     UniformBuffer. While this is supported in the majority of cases, it will be
     unsupported with OpenGL ES older than 3.0.
-*/
+
+    \value ReadBackNonBaseMipLevel Indicates that specifying a mip level other
+    than 0 is supported when reading back texture contents. When not supported,
+    specifying a non-zero level in QRhiReadbackDescription leads to returning
+    an all-zero image. In practice this feature will be unsupported with OpenGL
+    ES 2.0, while it will likely be supported everywhere else.
+ */
 
 /*!
     \enum QRhi::BeginFrameFlag
diff --git a/src/gui/rhi/qrhi_p.h b/src/gui/rhi/qrhi_p.h
index 993fc44b58..bfcc1d39f5 100644
--- a/src/gui/rhi/qrhi_p.h
+++ b/src/gui/rhi/qrhi_p.h
@@ -1436,7 +1436,8 @@ public:
         BaseVertex,
         BaseInstance,
         TriangleFanTopology,
-        ReadBackNonUniformBuffer
+        ReadBackNonUniformBuffer,
+        ReadBackNonBaseMipLevel
     };
 
     enum BeginFrameFlag {
diff --git a/src/gui/rhi/qrhid3d11.cpp b/src/gui/rhi/qrhid3d11.cpp
index 7c39fe0b11..096a8c2756 100644
--- a/src/gui/rhi/qrhid3d11.cpp
+++ b/src/gui/rhi/qrhid3d11.cpp
@@ -473,6 +473,8 @@ bool QRhiD3D11::isFeatureSupported(QRhi::Feature feature) const
         return false;
     case QRhi::ReadBackNonUniformBuffer:
         return true;
+    case QRhi::ReadBackNonBaseMipLevel:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;
diff --git a/src/gui/rhi/qrhigles2.cpp b/src/gui/rhi/qrhigles2.cpp
index fa99ee5e25..ca1c91499d 100644
--- a/src/gui/rhi/qrhigles2.cpp
+++ b/src/gui/rhi/qrhigles2.cpp
@@ -500,6 +500,11 @@ bool QRhiGles2::create(QRhi::Flags flags)
     // extension(s) (which is not in ES 3.0...messy)
     caps.properMapBuffer = f->hasOpenGLExtension(QOpenGLExtensions::MapBufferRange);
 
+    if (caps.gles)
+        caps.nonBaseLevelFramebufferTexture = caps.ctxMajor >= 3; // ES 3.0
+    else
+        caps.nonBaseLevelFramebufferTexture = true;
+
     if (!caps.gles) {
         f->glEnable(GL_VERTEX_PROGRAM_POINT_SIZE);
         f->glEnable(GL_POINT_SPRITE);
@@ -744,6 +749,8 @@ bool QRhiGles2::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::ReadBackNonUniformBuffer:
         return !caps.gles || caps.properMapBuffer;
+    case QRhi::ReadBackNonBaseMipLevel:
+        return caps.nonBaseLevelFramebufferTexture;
     default:
         Q_UNREACHABLE();
         return false;
@@ -2156,23 +2163,31 @@ void QRhiGles2::executeCommandBuffer(QRhiCommandBuffer *cb)
             QRhiReadbackResult *result = cmd.args.readPixels.result;
             GLuint tex = cmd.args.readPixels.texture;
             GLuint fbo = 0;
+            int mipLevel = 0;
             if (tex) {
                 result->pixelSize = QSize(cmd.args.readPixels.w, cmd.args.readPixels.h);
                 result->format = cmd.args.readPixels.format;
-                f->glGenFramebuffers(1, &fbo);
-                f->glBindFramebuffer(GL_FRAMEBUFFER, fbo);
-                f->glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
-                                          cmd.args.readPixels.readTarget, cmd.args.readPixels.texture, cmd.args.readPixels.level);
+                mipLevel = cmd.args.readPixels.level;
+                if (mipLevel == 0 || caps.nonBaseLevelFramebufferTexture) {
+                    f->glGenFramebuffers(1, &fbo);
+                    f->glBindFramebuffer(GL_FRAMEBUFFER, fbo);
+                    f->glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
+                                              cmd.args.readPixels.readTarget, cmd.args.readPixels.texture, mipLevel);
+                }
             } else {
                 result->pixelSize = currentSwapChain->pixelSize;
                 result->format = QRhiTexture::RGBA8;
                 // readPixels handles multisample resolving implicitly
             }
             result->data.resize(result->pixelSize.width() * result->pixelSize.height() * 4);
-            // With GLES (2.0?) GL_RGBA is the only mandated readback format, so stick with it.
-            f->glReadPixels(0, 0, result->pixelSize.width(), result->pixelSize.height(),
-                            GL_RGBA, GL_UNSIGNED_BYTE,
-                            result->data.data());
+            if (mipLevel == 0 || caps.nonBaseLevelFramebufferTexture) {
+                // With GLES (2.0?) GL_RGBA is the only mandated readback format, so stick with it.
+                f->glReadPixels(0, 0, result->pixelSize.width(), result->pixelSize.height(),
+                                GL_RGBA, GL_UNSIGNED_BYTE,
+                                result->data.data());
+            } else {
+                result->data.fill('\0');
+            }
             if (fbo) {
                 f->glBindFramebuffer(GL_FRAMEBUFFER, ctx->defaultFramebufferObject());
                 f->glDeleteFramebuffers(1, &fbo);
diff --git a/src/gui/rhi/qrhigles2_p_p.h b/src/gui/rhi/qrhigles2_p_p.h
index 29cccb0c9c..8814d9c19d 100644
--- a/src/gui/rhi/qrhigles2_p_p.h
+++ b/src/gui/rhi/qrhigles2_p_p.h
@@ -753,7 +753,8 @@ public:
               baseVertex(false),
               compute(false),
               textureCompareMode(false),
-              properMapBuffer(false)
+              properMapBuffer(false),
+              nonBaseLevelFramebufferTexture(false)
         { }
         int ctxMajor;
         int ctxMinor;
@@ -785,6 +786,7 @@ public:
         uint compute : 1;
         uint textureCompareMode : 1;
         uint properMapBuffer : 1;
+        uint nonBaseLevelFramebufferTexture : 1;
     } caps;
     QGles2SwapChain *currentSwapChain = nullptr;
     QVector<GLint> supportedCompressedFormats;
diff --git a/src/gui/rhi/qrhimetal.mm b/src/gui/rhi/qrhimetal.mm
index 9c579ebfe1..694882ab93 100644
--- a/src/gui/rhi/qrhimetal.mm
+++ b/src/gui/rhi/qrhimetal.mm
@@ -559,6 +559,8 @@ bool QRhiMetal::isFeatureSupported(QRhi::Feature feature) const
         return false;
     case QRhi::ReadBackNonUniformBuffer:
         return true;
+    case QRhi::ReadBackNonBaseMipLevel:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;
diff --git a/src/gui/rhi/qrhivulkan.cpp b/src/gui/rhi/qrhivulkan.cpp
index 154908b333..0d364dd9fc 100644
--- a/src/gui/rhi/qrhivulkan.cpp
+++ b/src/gui/rhi/qrhivulkan.cpp
@@ -3823,6 +3823,8 @@ bool QRhiVulkan::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::ReadBackNonUniformBuffer:
         return true;
+    case QRhi::ReadBackNonBaseMipLevel:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;