From 8f1d8139c40d8b050ac606c3d62eee7f1f37410f Mon Sep 17 00:00:00 2001
From: Laszlo Agocs <laszlo.agocs@qt.io>
Date: Wed, 8 Feb 2023 09:05:44 +0100
Subject: [PATCH] Avoid glDrawBuffers on ES 2.0 / WebGL 1
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Fixes: QTBUG-111007
Pick-to: 6.5
Change-Id: If1d84df56ad47ac89ea43ad5091392416fe9fc5f
Reviewed-by: Christian Strømme <christian.stromme@qt.io>
---
 src/gui/rhi/qrhigles2.cpp   | 14 +++++++++++---
 src/gui/rhi/qrhigles2_p_p.h |  4 +++-
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/gui/rhi/qrhigles2.cpp b/src/gui/rhi/qrhigles2.cpp
index 11b1b965b8..77448a8b21 100644
--- a/src/gui/rhi/qrhigles2.cpp
+++ b/src/gui/rhi/qrhigles2.cpp
@@ -766,12 +766,20 @@ bool QRhiGles2::create(QRhi::Flags flags)
 
     f->glGetIntegerv(GL_MAX_TEXTURE_SIZE, &caps.maxTextureSize);
 
-    if (caps.ctxMajor >= 3 || actualFormat.renderableType() == QSurfaceFormat::OpenGL) {
+    caps.gles = actualFormat.renderableType() == QSurfaceFormat::OpenGLES;
+
+    if (!caps.gles || caps.ctxMajor >= 3) {
+        // non-ES or ES 3.0+
         f->glGetIntegerv(GL_MAX_DRAW_BUFFERS, &caps.maxDrawBuffers);
+        caps.hasDrawBuffersFunc = true;
         f->glGetIntegerv(GL_MAX_SAMPLES, &caps.maxSamples);
         caps.maxSamples = qMax(1, caps.maxSamples);
     } else {
+        // ES 2.0 / WebGL 1
         caps.maxDrawBuffers = 1;
+        caps.hasDrawBuffersFunc = false;
+        // This does not mean MSAA is not supported, just that we cannot query
+        // the supported sample counts.
         caps.maxSamples = 1;
     }
 
@@ -781,7 +789,6 @@ bool QRhiGles2::create(QRhi::Flags flags)
     caps.npotTextureFull = f->hasOpenGLFeature(QOpenGLFunctions::NPOTTextures)
             && f->hasOpenGLFeature(QOpenGLFunctions::NPOTTextureRepeat);
 
-    caps.gles = actualFormat.renderableType() == QSurfaceFormat::OpenGLES;
     if (caps.gles)
         caps.fixedIndexPrimitiveRestart = caps.ctxMajor >= 3; // ES 3.0
     else
@@ -3073,7 +3080,8 @@ void QRhiGles2::executeCommandBuffer(QRhiCommandBuffer *cb)
                 else
                     bufs.append(caps.gles ? GL_BACK : GL_BACK_LEFT);
             }
-            f->glDrawBuffers(bufs.count(), bufs.constData());
+            if (caps.hasDrawBuffersFunc)
+                f->glDrawBuffers(bufs.count(), bufs.constData());
             if (caps.srgbCapableDefaultFramebuffer) {
                 if (cmd.args.bindFramebuffer.srgb)
                     f->glEnable(GL_FRAMEBUFFER_SRGB);
diff --git a/src/gui/rhi/qrhigles2_p_p.h b/src/gui/rhi/qrhigles2_p_p.h
index f27320cab5..7caec5f7d5 100644
--- a/src/gui/rhi/qrhigles2_p_p.h
+++ b/src/gui/rhi/qrhigles2_p_p.h
@@ -960,7 +960,8 @@ public:
               texture3D(false),
               tessellation(false),
               geometryShader(false),
-              texture1D(false)
+              texture1D(false),
+              hasDrawBuffersFunc(false)
         { }
         int ctxMajor;
         int ctxMinor;
@@ -1012,6 +1013,7 @@ public:
         uint tessellation : 1;
         uint geometryShader : 1;
         uint texture1D : 1;
+        uint hasDrawBuffersFunc : 1;
     } caps;
     QGles2SwapChain *currentSwapChain = nullptr;
     QSet<GLint> supportedCompressedFormats;