rhi: gl: Attempt reading back floating point formats

...in the hope that they may work. If not, that's it, but at least we
tried.

Task-number: QTBUG-76970
Change-Id: I134c5cc4cfb5ad1e6f9edbfcf506df20022e127a
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

2 files changed, 27 insertions, 9 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index 4b07f4c346..aface30606 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -618,13 +618,16 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
     dFdy(), and fwidth() are supported in shaders.
 
     \value ReadBackAnyTextureFormat Indicates that reading back texture
-    contents can be expected to work for any QRhiTexture::Format. When reported
-    as false, which will typically happen with OpenGL, only the formats
-    QRhiTexture::RGBA8 and QRhiTexture::BGRA8 are guaranteed to be supported
-    for readbacks. In addition, with OpenGL, but not OpenGL ES, reading back
-    the 1 byte per component formats QRhiTexture::R8 and
-    QRhiTexture::RED_OR_ALPHA8 are supported as well. Backends other than
-    OpenGL can be expected to return true for this feature.
+    contents can be expected to work for any QRhiTexture::Format. Backends
+    other than OpenGL can be expected to return true for this feature. When
+    reported as false, which will typically happen with OpenGL, only the
+    formats QRhiTexture::RGBA8 and QRhiTexture::BGRA8 are guaranteed to be
+    supported for readbacks. In addition, with OpenGL, but not OpenGL ES,
+    reading back the 1 byte per component formats QRhiTexture::R8 and
+    QRhiTexture::RED_OR_ALPHA8 are supported as well. Reading back floating
+    point formats QRhiTexture::RGBA16F and RGBA32F may work too with OpenGL, as
+    long as the implementation provides support for these, but QRhi can give no
+    guarantees, as indicated by this flag.
 
     \value PipelineCacheDataLoadSave Indicates that the pipelineCacheData() and
     setPipelineCacheData() functions are functional. When not supported, the
diff --git a/src/gui/rhi/qrhigles2.cpp b/src/gui/rhi/qrhigles2.cpp
index fac05f4947..cd1e4ec944 100644
--- a/src/gui/rhi/qrhigles2.cpp
+++ b/src/gui/rhi/qrhigles2.cpp
@@ -2740,8 +2740,23 @@ void QRhiGles2::executeCommandBuffer(QRhiCommandBuffer *cb)
                         }
                     }
                 } else {
-                    result->data.resize(w * h * 4);
-                    f->glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, result->data.data());
+                    switch (result->format) {
+                    // For floating point formats try it because this can be
+                    // relevant for some use cases; if it works, then fine, if
+                    // not, there's nothing we can do.
+                    case QRhiTexture::RGBA16F:
+                        result->data.resize(w * h * 8);
+                        f->glReadPixels(0, 0, w, h, GL_RGBA, GL_HALF_FLOAT, result->data.data());
+                        break;
+                    case QRhiTexture::RGBA32F:
+                        result->data.resize(w * h * 16);
+                        f->glReadPixels(0, 0, w, h, GL_RGBA, GL_FLOAT, result->data.data());
+                        break;
+                    default:
+                        result->data.resize(w * h * 4);
+                        f->glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, result->data.data());
+                        break;
+                    }
                 }
             } else {
                 result->data.resize(w * h * 4);