Re: [PATCH RFC] libv4lconvert: SDR conversion from U8 to FLOAT

[Antti Palosaari]

On 11.11.2013 16:38, Hans Verkuil wrote:
On 11/11/2013 03:19 PM, Andy Walls wrote:
On Sun, 2013-11-10 at 19:16 +0200, Antti Palosaari wrote:
Convert unsigned 8 to float 32 [-1 to +1], which is commonly
used format for baseband signals.

Hi Annti,

I don't think this a good idea.  Floating point representations are
inherently non-portable.  Even though most everything now uses IEEE-754
representation, things like denormaliazed numbers may be treated
differently by different machines.  If someone saves the data to a file,
endianess issues aside, there are no guarantees that a different machine
reading is going to interpret all the floating point data from that file
properly.

I really would recommend staying with scaled integer representations or
explicit integer mantissa, exponent representations.

For what it's worth: ALSA does support float format as well (both LE and BE).

I want use existing data formats and that [-1 to +1] scaled 32-bit 
IEEE-754 floating point is de facto format for SDR application (actually 
pair of floats as a complex).

Doing conversion inside libv4lconvert makes it very easy for write 
application. Currently I have implemented GNU Radio and SDRsharp plugins 
that feeds data from device via libv4l2 using mmap and conversion.

Thanks to pointing endianess issue, I didn't though it all. I suspect 
those apps just relies to local endianess. So do I have to implement 
float format conversion with both endianess?



Regards,

        Hans


Two more comments below...

Signed-off-by: Antti Palosaari <cr...@iki.fi>
---
  contrib/freebsd/include/linux/videodev2.h |  4 ++++
  include/linux/videodev2.h                 |  4 ++++
  lib/libv4lconvert/libv4lconvert.c         | 29 ++++++++++++++++++++++++++++-
  3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/contrib/freebsd/include/linux/videodev2.h 
b/contrib/freebsd/include/linux/videodev2.h
index 1fcfaeb..8829400 100644
--- a/contrib/freebsd/include/linux/videodev2.h
+++ b/contrib/freebsd/include/linux/videodev2.h
@@ -465,6 +465,10 @@ struct v4l2_pix_format {
  #define V4L2_PIX_FMT_SE401      v4l2_fourcc('S', '4', '0', '1') /* se401 
janggu compressed rgb */
  #define V4L2_PIX_FMT_S5C_UYVY_JPG v4l2_fourcc('S', '5', 'C', 'I') /* S5C73M3 
interleaved UYVY/JPEG */

+/* SDR */
+#define V4L2_PIX_FMT_FLOAT    v4l2_fourcc('D', 'F', '3', '2') /* float 32-bit 
*/
+#define V4L2_PIX_FMT_U8       v4l2_fourcc('D', 'U', '0', '8') /* unsigned 
8-bit */
+
  /*
   *    F O R M A T   E N U M E R A T I O N
   */
diff --git a/include/linux/videodev2.h b/include/linux/videodev2.h
index 437f1b0..14299a6 100644
--- a/include/linux/videodev2.h
+++ b/include/linux/videodev2.h
@@ -431,6 +431,10 @@ struct v4l2_pix_format {
  #define V4L2_PIX_FMT_SE401      v4l2_fourcc('S', '4', '0', '1') /* se401 
janggu compressed rgb */
  #define V4L2_PIX_FMT_S5C_UYVY_JPG v4l2_fourcc('S', '5', 'C', 'I') /* S5C73M3 
interleaved UYVY/JPEG */

+/* SDR */
+#define V4L2_PIX_FMT_FLOAT    v4l2_fourcc('D', 'F', '3', '2') /* float 32-bit 
*/
+#define V4L2_PIX_FMT_U8       v4l2_fourcc('D', 'U', '0', '8') /* unsigned 
8-bit */
+
  /*
   *    F O R M A T   E N U M E R A T I O N
   */
diff --git a/lib/libv4lconvert/libv4lconvert.c 
b/lib/libv4lconvert/libv4lconvert.c
index e2afc27..38c9125 100644
--- a/lib/libv4lconvert/libv4lconvert.c
+++ b/lib/libv4lconvert/libv4lconvert.c
@@ -78,7 +78,8 @@ static void v4lconvert_get_framesizes(struct v4lconvert_data 
*data,
        { V4L2_PIX_FMT_RGB24,           24,      1,      5,     0 }, \
        { V4L2_PIX_FMT_BGR24,           24,      1,      5,     0 }, \
        { V4L2_PIX_FMT_YUV420,          12,      6,      1,     0 }, \
-       { V4L2_PIX_FMT_YVU420,          12,      6,      1,     0 }
+       { V4L2_PIX_FMT_YVU420,          12,      6,      1,     0 }, \
+       { V4L2_PIX_FMT_FLOAT,            0,      0,      0,     0 }

  static const struct v4lconvert_pixfmt supported_src_pixfmts[] = {
        SUPPORTED_DST_PIXFMTS,
@@ -131,6 +132,8 @@ static const struct v4lconvert_pixfmt 
supported_src_pixfmts[] = {
        { V4L2_PIX_FMT_Y6,               8,     20,     20,     0 },
        { V4L2_PIX_FMT_Y10BPACK,        10,     20,     20,     0 },
        { V4L2_PIX_FMT_Y16,             16,     20,     20,     0 },
+       /* SDR formats */
+       { V4L2_PIX_FMT_U8,              0,      0,      0,      0 },
  };

  static const struct v4lconvert_pixfmt supported_dst_pixfmts[] = {
@@ -1281,6 +1284,25 @@ static int v4lconvert_convert_pixfmt(struct 
v4lconvert_data *data,
                }
                break;

+       /* SDR */
+       case V4L2_PIX_FMT_U8:
+               switch (dest_pix_fmt) {
+               case V4L2_PIX_FMT_FLOAT:
+                       {
+                               /* 8-bit unsigned to 32-bit float */
+                               unsigned int i;
+                               float ftmp;
+                               for (i = 0; i < src_size; i++) {
+                                       ftmp = *src++;
+                                       ftmp -= 127.5;
+                                       ftmp /= 127.5;
+                                       memcpy(dest, &ftmp, 4);
+                                       dest += 4;

Replace the 4's with sizeof(float).

You have no guarantees that sizeof(float) == 4, but it is usally a safe
assumption for 'float' on Unix.

sizeof(long double) is certainly different for IA32 machines (80 bits)
vs. other 32 bit platforms.  I was burned by this many years ago on a
RedHat 9.0 machine (the GNU Ada Translator's libm bindings made some bad
assumptions about the size of float types).

Will do.




+                               }
+                       }
+               }
+               break;
+
        default:
                V4LCONVERT_ERR("Unknown src format in conversion\n");
                errno = EINVAL;
@@ -1349,6 +1371,11 @@ int v4lconvert_convert(struct v4lconvert_data *data,
                temp_needed =
                        my_src_fmt.fmt.pix.width * my_src_fmt.fmt.pix.height * 
3 / 2;
                break;
+       /* SDR */
+       case V4L2_PIX_FMT_FLOAT:
+               dest_needed = src_size * 4; /* 8-bit to 32-bit */

Change the 4 to sizeof(float).

+               temp_needed = dest_needed;
+               break;
        default:
                V4LCONVERT_ERR("Unknown dest format in conversion\n");
                errno = EINVAL;

Regards,
Andy

regards
Antti


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