gl: add support for multiple color attachments

...when running on gles 3.0+ or gl.

src/rhi/qrhigles2.cpp

@@ -1461,10 +1461,22 @@ void QRhiGles2::executeCommandBuffer(QRhiCommandBuffer *cb)
                                cmd.args.bindShaderResources.dynamicOffsetCount);
             break;
         case QGles2CommandBuffer::Command::BindFramebuffer:
-            if (cmd.args.bindFramebuffer.fbo)
+            if (cmd.args.bindFramebuffer.fbo) {
                 f->glBindFramebuffer(GL_FRAMEBUFFER, cmd.args.bindFramebuffer.fbo);
-            else
+                if (caps.maxDrawBuffers > 1) {
+                    const int colorAttCount = cmd.args.bindFramebuffer.colorAttCount;
+                    QVarLengthArray<GLenum, 8> bufs;
+                    for (int i = 0; i < colorAttCount; ++i)
+                        bufs.append(GL_COLOR_ATTACHMENT0 + i);
+                    f->glDrawBuffers(colorAttCount, bufs.constData());
+                }
+            } else {
                 f->glBindFramebuffer(GL_FRAMEBUFFER, ctx->defaultFramebufferObject());
+                if (caps.maxDrawBuffers > 1) {
+                    GLenum bufs = GL_BACK;
+                    f->glDrawBuffers(1, &bufs);
+                }
+            }
             if (caps.srgbCapableDefaultFramebuffer) {
                 if (cmd.args.bindFramebuffer.srgb)
                     f->glEnable(GL_FRAMEBUFFER_SRGB);
@@ -1795,6 +1807,7 @@ void QRhiGles2::beginPass(QRhiCommandBuffer *cb,
     case QRhiRenderTarget::RtRef:
         rtD = &QRHI_RES(QGles2ReferenceRenderTarget, rt)->d;
         fbCmd.args.bindFramebuffer.fbo = 0;
+        fbCmd.args.bindFramebuffer.colorAttCount = 1;
         break;
     case QRhiRenderTarget::RtTexture:
     {
@@ -1803,6 +1816,7 @@ void QRhiGles2::beginPass(QRhiCommandBuffer *cb,
         wantsColorClear = !rtTex->m_flags.testFlag(QRhiTextureRenderTarget::PreserveColorContents);
         wantsDsClear = !rtTex->m_flags.testFlag(QRhiTextureRenderTarget::PreserveDepthStencilContents);
         fbCmd.args.bindFramebuffer.fbo = rtTex->framebuffer;
+        fbCmd.args.bindFramebuffer.colorAttCount = rtD->colorAttCount;
     }
         break;
     default:
@@ -2318,8 +2332,9 @@ bool QGles2TextureRenderTarget::build()
     Q_ASSERT(!m_desc.depthStencilBuffer() || !m_desc.depthTexture());
     const bool hasDepthStencil = m_desc.depthStencilBuffer() || m_desc.depthTexture();
 
-    if (colorAttachments.count() > 1)
-        qWarning("QGles2TextureRenderTarget: Multiple color attachments are not supported");
+    if (colorAttachments.count() > rhiD->caps.maxDrawBuffers)
+        qWarning("QGles2TextureRenderTarget: Too many color attachments (%d, max is %d)",
+                 colorAttachments.count(), rhiD->caps.maxDrawBuffers);
     if (m_desc.depthTexture() && !rhiD->caps.depthTexture)
         qWarning("QGles2TextureRenderTarget: Depth texture is not supported and will be ignored");
 
@@ -2329,8 +2344,9 @@ bool QGles2TextureRenderTarget::build()
     rhiD->f->glGenFramebuffers(1, &framebuffer);
     rhiD->f->glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
 
-    if (!colorAttachments.isEmpty()) {
-        const QRhiColorAttachment &colorAtt(colorAttachments.constFirst());
+    d.colorAttCount = colorAttachments.count();
+    for (int i = 0; i < d.colorAttCount; ++i) {
+        const QRhiColorAttachment &colorAtt(colorAttachments[i]);
         QRhiTexture *texture = colorAtt.texture();
         QRhiRenderBuffer *renderBuffer = colorAtt.renderBuffer();
         Q_ASSERT(texture || renderBuffer);
@@ -2338,18 +2354,19 @@ bool QGles2TextureRenderTarget::build()
             QGles2Texture *texD = QRHI_RES(QGles2Texture, texture);
             Q_ASSERT(texD->texture && texD->specified);
             const GLenum faceTargetBase = texD->flags().testFlag(QRhiTexture::CubeMap) ? GL_TEXTURE_CUBE_MAP_POSITIVE_X : texD->target;
-            rhiD->f->glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, faceTargetBase + colorAtt.layer(), texD->texture, colorAtt.level());
-            d.pixelSize = texD->pixelSize();
-            d.sampleCount = 1;
+            rhiD->f->glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0 + i, faceTargetBase + colorAtt.layer(), texD->texture, colorAtt.level());
+            if (i == 0) {
+                d.pixelSize = texD->pixelSize();
+                d.sampleCount = 1;
+            }
         } else if (renderBuffer) {
             QGles2RenderBuffer *rbD = QRHI_RES(QGles2RenderBuffer, renderBuffer);
-            rhiD->f->glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, rbD->renderbuffer);
-            d.pixelSize = rbD->pixelSize();
-            d.sampleCount = rbD->samples;
+            rhiD->f->glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0 + i, GL_RENDERBUFFER, rbD->renderbuffer);
+            if (i == 0) {
+                d.pixelSize = rbD->pixelSize();
+                d.sampleCount = rbD->samples;
+            }
         }
-        d.colorAttCount = 1;
-    } else {
-        d.colorAttCount = 0;
     }
 
     if (hasDepthStencil) {
@@ -2357,9 +2374,17 @@ bool QGles2TextureRenderTarget::build()
             QGles2RenderBuffer *depthRbD = QRHI_RES(QGles2RenderBuffer, m_desc.depthStencilBuffer());
             rhiD->f->glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthRbD->renderbuffer);
             rhiD->f->glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_STENCIL_ATTACHMENT, GL_RENDERBUFFER, depthRbD->renderbuffer);
+            if (d.colorAttCount == 0) {
+                d.pixelSize = depthRbD->pixelSize();
+                d.sampleCount = depthRbD->samples;
+            }
         } else {
             QGles2Texture *depthTexD = QRHI_RES(QGles2Texture, m_desc.depthTexture());
             rhiD->f->glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, depthTexD->texture, 0);
+            if (d.colorAttCount == 0) {
+                d.pixelSize = depthTexD->pixelSize();
+                d.sampleCount = 1;
+            }
         }
         d.dsAttCount = 1;
     } else {

src/rhi/qrhigles2_p.h

@@ -315,6 +315,7 @@ struct QGles2CommandBuffer : public QRhiCommandBuffer
             struct {
                 GLuint fbo;
                 bool srgb;
+                int colorAttCount;
             } bindFramebuffer;
             struct {
                 GLenum target;