mesa: fix interpretation of glClearBuffer(drawbuffer)

This corresponding piglit tests supported this incorrect behavior instead of
pointing at it.

Reviewed-by: Ian Romanick <[email protected]>
Cc: 10.0 9.2 9.1 <[email protected]>
(cherry picked from commit 03d848ea1003abefd8fe51a5b4a780527cd852af)

1 files changed, 25 insertions, 6 deletions

diff --git a/src/mesa/main/clear.c b/src/mesa/main/clear.c
index 304d135d14e..f0b525fa017 100644
--- a/src/mesa/main/clear.c
+++ b/src/mesa/main/clear.c
@@ -219,7 +219,25 @@ make_color_buffer_mask(struct gl_context *ctx, GLint drawbuffer)
    const struct gl_renderbuffer_attachment *att = ctx->DrawBuffer->Attachment;
    GLbitfield mask = 0x0;
 
-   switch (drawbuffer) {
+   /* From the GL 4.0 specification:
+    *	If buffer is COLOR, a particular draw buffer DRAW_BUFFERi is
+    *	specified by passing i as the parameter drawbuffer, and value
+    *	points to a four-element vector specifying the R, G, B, and A
+    *	color to clear that draw buffer to. If the draw buffer is one
+    *	of FRONT, BACK, LEFT, RIGHT, or FRONT_AND_BACK, identifying
+    *	multiple buffers, each selected buffer is cleared to the same
+    *	value.
+    *
+    * Note that "drawbuffer" and "draw buffer" have different meaning.
+    * "drawbuffer" specifies DRAW_BUFFERi, while "draw buffer" is what's
+    * assigned to DRAW_BUFFERi. It could be COLOR_ATTACHMENT0, FRONT, BACK,
+    * etc.
+    */
+   if (drawbuffer < 0 || drawbuffer >= (GLint)ctx->Const.MaxDrawBuffers) {
+      return INVALID_MASK;
+   }
+
+   switch (ctx->DrawBuffer->ColorDrawBuffer[drawbuffer]) {
    case GL_FRONT:
       if (att[BUFFER_FRONT_LEFT].Renderbuffer)
          mask |= BUFFER_BIT_FRONT_LEFT;
@@ -255,11 +273,12 @@ make_color_buffer_mask(struct gl_context *ctx, GLint drawbuffer)
          mask |= BUFFER_BIT_BACK_RIGHT;
       break;
    default:
-      if (drawbuffer < 0 || drawbuffer >= (GLint)ctx->Const.MaxDrawBuffers) {
-         mask = INVALID_MASK;
-      }
-      else if (att[BUFFER_COLOR0 + drawbuffer].Renderbuffer) {
-         mask |= (BUFFER_BIT_COLOR0 << drawbuffer);
+      {
+         GLuint buf = ctx->DrawBuffer->_ColorDrawBufferIndexes[drawbuffer];
+
+         if (buf >= 0 && att[buf].Renderbuffer) {
+            mask |= 1 << buf;
+         }
       }
    }