Am 20.09.2013 13:59, schrieb jfons...@vmware.com:
> From: José Fonseca <jfons...@vmware.com>
>
> We must take rounding in consideration when re-scaling to narrow
> normalized channels, such as 2-bit normalized alpha.
> ---
> src/gallium/drivers/llvmpipe/lp_state_fs.c | 84
> +++++++++++++++++++++++++++---
> 1 file changed, 78 insertions(+), 6 deletions(-)
>
> diff --git a/src/gallium/drivers/llvmpipe/lp_state_fs.c
> b/src/gallium/drivers/llvmpipe/lp_state_fs.c
> index 07e6685..875a3cf 100644
> --- a/src/gallium/drivers/llvmpipe/lp_state_fs.c
> +++ b/src/gallium/drivers/llvmpipe/lp_state_fs.c
> @@ -876,7 +876,22 @@ lp_blend_type_from_format_desc(const struct
> util_format_description *format_desc
>
>
> /**
> - * Scale a normalized value from src_bits to dst_bits
> + * Scale a normalized value from src_bits to dst_bits.
> + *
> + * The exact calculation is
> + *
> + * dst = iround(src * dst_mask / src_mask)
> + *
> + * or with integer rounding
> + *
> + * dst = src * (2*dst_mask + sign(src)*src_mask) / (2*src_mask)
> + *
> + * where
> + *
> + * src_mask = (1 << src_bits) - 1
> + * dst_mask = (1 << dst_bits) - 1
> + *
> + * but we try to avoid division and multiplication through shifts.
> */
> static INLINE LLVMValueRef
> scale_bits(struct gallivm_state *gallivm,
> @@ -889,11 +904,68 @@ scale_bits(struct gallivm_state *gallivm,
> LLVMValueRef result = src;
>
> if (dst_bits < src_bits) {
> - /* Scale down by LShr */
> - result = LLVMBuildLShr(builder,
> - src,
> - lp_build_const_int_vec(gallivm, src_type,
> src_bits - dst_bits),
> - "");
> + int delta_bits = src_bits - dst_bits;
> +
> + if (delta_bits <= dst_bits) {
> + /*
> + * Approximate the rescaling with a single shift.
> + *
> + * This gives the wrong rounding.
> + */
> +
> + result = LLVMBuildLShr(builder,
> + src,
> + lp_build_const_int_vec(gallivm, src_type,
> delta_bits),
> + "");
> +
> + } else {
> + /*
> + * Try more accurate rescaling.
> + */
> +
> + /*
> + * Drop the least significant bits to make space for the
> multiplication.
> + *
> + * XXX: A better approach would be to use a wider integer type as
> intermediate. But
> + * this is enough to convert alpha from 16bits -> 2 when rendering
> to
> + * PIPE_FORMAT_R10G10B10A2_UNORM.
> + */
> + result = LLVMBuildLShr(builder,
> + src,
> + lp_build_const_int_vec(gallivm, src_type,
> dst_bits),
> + "");
> +
> +
> + result = LLVMBuildMul(builder,
> + result,
> + lp_build_const_int_vec(gallivm, src_type,
> (1LL << dst_bits) - 1),
> + "");
> +
> + /*
> + * Add a rounding term before the division.
> + *
> + * TODO: Handle signed integers too.
> + */
> + if (!src_type.sign) {
> + result = LLVMBuildAdd(builder,
> + result,
> + lp_build_const_int_vec(gallivm, src_type,
> (1LL << (delta_bits - 1))),
> + "");
> + }
> +
> + /*
> + * Approximate the division by src_mask with a src_bits shift.
> + *
> + * Given the src has already been shifted by dst_bits, all we need
> + * to do is to shift by the difference.
> + */
> +
> + result = LLVMBuildLShr(builder,
> + result,
> + lp_build_const_int_vec(gallivm, src_type,
> delta_bits),
> + "");
> + }
> +
> } else if (dst_bits > src_bits) {
> /* Scale up bits */
> int db = dst_bits - src_bits;
>
Looks good to me, though that up/downscaling for A2 looks a bit
complicated but we can't avoid it.
Roland
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