gallium: more elaborate tracking of front color buffer state

This fixes the case where the app calls SwapBuffers then calls glReadPixels
to read the front color buffer.  We now keep track of when the front buffer
is a _logically_ copy of the back buffer (after SwapBuffers) and read from
the back color buffer instead of the front.

1 files changed, 31 insertions, 2 deletions

diff --git a/src/mesa/state_tracker/st_cb_readpixels.c b/src/mesa/state_tracker/st_cb_readpixels.c
index 2bcc8c99fb..29a06ffaa9 100644
--- a/src/mesa/state_tracker/st_cb_readpixels.c
+++ b/src/mesa/state_tracker/st_cb_readpixels.c
@@ -129,6 +129,34 @@ st_read_stencil_pixels(GLcontext *ctx, GLint x, GLint y,
 
 
 /**
+ * Return renderbuffer to use for reading color pixels for glRead/CopyPixel
+ * commands.
+ * Special care is needed for the front buffer.
+ */
+struct st_renderbuffer *
+st_get_color_read_renderbuffer(GLcontext *ctx)
+{
+   struct gl_framebuffer *fb = ctx->ReadBuffer;
+   struct st_renderbuffer *strb =
+      st_renderbuffer(fb->_ColorReadBuffer);
+   struct st_renderbuffer *front = 
+      st_renderbuffer(fb->Attachment[BUFFER_FRONT_LEFT].Renderbuffer);
+
+   if (strb == front
+       && ctx->st->frontbuffer_status == FRONT_STATUS_COPY_OF_BACK) {
+      /* reading from front color buffer, which is a logical copy of the
+       * back color buffer.
+       */
+      struct st_renderbuffer *back = 
+         st_renderbuffer(fb->Attachment[BUFFER_BACK_LEFT].Renderbuffer);
+      strb = back;
+   }
+
+   return strb;
+}
+
+
+/**
  * Do glReadPixels by getting rows from the framebuffer surface with
  * get_tile().  Convert to requested format/type with Mesa image routines.
  * Image transfer ops are done in software too.
@@ -173,12 +201,13 @@ st_readpixels(GLcontext *ctx, GLint x, GLint y, GLsizei width, GLsizei height,
       strb = st_renderbuffer(ctx->ReadBuffer->_DepthBuffer);
    }
    else {
-      strb = st_renderbuffer(ctx->ReadBuffer->_ColorReadBuffer);
+      /* Read color buffer */
+      strb = st_get_color_read_renderbuffer(ctx);
    }
+
    if (!strb)
       return;
 
-
    if (format == GL_RGBA && type == GL_FLOAT) {
       /* write tile(row) directly into user's buffer */
       df = (GLfloat *) _mesa_image_address2d(&clippedPacking, dest, width,