[Mesa-dev] [PATCH 2/2] meta: Handle bitmaps with alpha test enabled.

[Francisco Jerez]

Brian Paul <brianp at vmware.com> writes:

>> +static GLboolean
>> +alpha_test_bitmap(struct gl_context *ctx)
>> +{
>> +   struct gl_colorbuffer_attrib *color = &ctx->Color;
>> +   GLenum func = (color->AlphaEnabled ? color->AlphaFunc : GL_ALWAYS);
>> +   GLubyte ref = FLOAT_TO_UBYTE(color->AlphaRef);
>> +   GLubyte alpha = FLOAT_TO_UBYTE(ctx->Current.RasterColor[ACOMP]);
>
> I don't think we should convert the alpha values to GLubyte here.  If
> the hardware does the alpha comparison with floats, the outcome could
> be different than comparing with GLubytes.  More on that below.
>
Doing a floating point comparison on a fixed point color buffer would be
incorrect. Fixed point comparison on a floating point color buffer seems
to be fine, the spec is very relaxed in that regard.

From ARB_color_buffer_float:

|  28. At what precision should alpha test be carried out?  At the
|      precision of the framebuffer?  Or some other unspecified
|      precision?  What happens if you have a framebuffer with no
|      alpha?
|
|      RESOLVED: No specific precision requirements are added, except
|      that the reference value used in the alpha test should be
|      converted to the same precision and in the same manner as the
|      fragment's alpha. This requirement is intended to avoid cases
|      where the act of converting the alpha value of a fragment to
|      fixed-point (or lower-precision floating-point) might change the
|      result of the test.
|
|[...]
|
|  37. Is it expected that a floating-point color read from a
|      floating-point color buffer exactly match a floating-point
|      color in a fragment?  Will the alpha test of GL_EQUAL
|      be expected to work?
| 
|      RESOLVED: This behavior is not required by this extension.
|      Floating-point data may have different precision at different
|      parts of the pipeline.
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