3 files changed, 59 insertions, 1 deletions

diff --git a/mesalib/src/mesa/main/clear.c b/mesalib/src/mesa/main/clear.c
index 077c5fca3..9df1f5e09 100644
--- a/mesalib/src/mesa/main/clear.c
+++ b/mesalib/src/mesa/main/clear.c
@@ -107,6 +107,31 @@ _mesa_ClearColorIuiEXT(GLuint r, GLuint g, GLuint b, GLuint a)
 
 
 /**
+ * Returns true if color writes are enabled for the given color attachment.
+ *
+ * Beyond checking ColorMask, this uses _mesa_format_has_color_component to
+ * ignore components that don't actually exist in the format (such as X in
+ * XRGB).
+ */
+static bool
+color_buffer_writes_enabled(const struct gl_context *ctx, unsigned idx)
+{
+   struct gl_renderbuffer *rb = ctx->DrawBuffer->_ColorDrawBuffers[idx];
+   GLuint c;
+   GLubyte colorMask = 0;
+
+   if (rb) {
+      for (c = 0; c < 4; c++) {
+         if (_mesa_format_has_color_component(rb->Format, c))
+            colorMask |= ctx->Color.ColorMask[idx][c];
+      }
+   }
+
+   return colorMask != 0;
+}
+
+
+/**
  * Clear buffers.
  * 
  * \param mask bit-mask indicating the buffers to be cleared.
@@ -181,7 +206,7 @@ _mesa_Clear( GLbitfield mask )
          for (i = 0; i < ctx->DrawBuffer->_NumColorDrawBuffers; i++) {
             GLint buf = ctx->DrawBuffer->_ColorDrawBufferIndexes[i];
 
-            if (buf >= 0) {
+            if (buf >= 0 && color_buffer_writes_enabled(ctx, i)) {
                bufferMask |= 1 << buf;
             }
          }
diff --git a/mesalib/src/mesa/main/formats.c b/mesalib/src/mesa/main/formats.c
index e74625f23..fb2501c69 100644
--- a/mesalib/src/mesa/main/formats.c
+++ b/mesalib/src/mesa/main/formats.c
@@ -2207,6 +2207,35 @@ _mesa_format_num_components(mesa_format format)
 
 
 /**
+ * Returns true if a color format has data stored in the R/G/B/A channels,
+ * given an index from 0 to 3.
+ */
+bool
+_mesa_format_has_color_component(mesa_format format, int component)
+{
+   const struct gl_format_info *info = _mesa_get_format_info(format);
+
+   assert(info->BaseFormat != GL_DEPTH_COMPONENT &&
+          info->BaseFormat != GL_DEPTH_STENCIL &&
+          info->BaseFormat != GL_STENCIL_INDEX);
+
+   switch (component) {
+   case 0:
+      return (info->RedBits + info->IntensityBits + info->LuminanceBits) > 0;
+   case 1:
+      return (info->GreenBits + info->IntensityBits + info->LuminanceBits) > 0;
+   case 2:
+      return (info->BlueBits + info->IntensityBits + info->LuminanceBits) > 0;
+   case 3:
+      return (info->AlphaBits + info->IntensityBits) > 0;
+   default:
+      assert(!"Invalid color component: must be 0..3");
+      return false;
+   }
+}
+
+
+/**
  * Return number of bytes needed to store an image of the given size
  * in the given format.
  */
diff --git a/mesalib/src/mesa/main/formats.h b/mesalib/src/mesa/main/formats.h
index 3079f0356..89bd0219e 100644
--- a/mesalib/src/mesa/main/formats.h
+++ b/mesalib/src/mesa/main/formats.h
@@ -34,6 +34,7 @@
 
 
 #include <GL/gl.h>
+#include <stdbool.h>
 
 
 #ifdef __cplusplus
@@ -474,6 +475,9 @@ _mesa_get_uncompressed_format(mesa_format format);
 extern GLuint
 _mesa_format_num_components(mesa_format format);
 
+extern bool
+_mesa_format_has_color_component(mesa_format format, int component);
+
 GLboolean
 _mesa_format_matches_format_and_type(mesa_format mesa_format,
 				     GLenum format, GLenum type,