better handling of cast+shift for non-int formats

before, im.cast(uchar, shift = true) where im was float and tagged as
rgb16 would not shift the image, since it's unclear how much to shift a float
type by

now we do two casts: first, we guess the numeric range from the
interpretation, so rgb16 would be ushort, so we cast float->ushort;
second, we cast to the target type and do the shift on the way

see https://github.com/jcupitt/libvips/issues/397

thanks apacheark

ChangeLog

@@ -15,6 +15,8 @@
 - faster hist_find (Lovell Fuller)
 - webpload has a @shrink parameter for shrink-on-load
 - vipsthumbnail knows about webp shrink-on-load
+- more vips_resize() tuning, a bit quicker now
+- better behaviour for vips_cast() shift of non-int types (thanks apacheark)
 
 27/1/16 started 8.2.3
 - fix a crash with SPARC byte-order labq vips images

TODO

@@ -1,33 +1,18 @@
+better handling of cast+shift for non-int formats
 
-- use the incremental webp decoding api to support seq for webp images
-  
-	https://developers.google.com/speed/webp/docs/api#decodingadvancedapi
+before, im.cast(uchar, shift = true) where im was float and tagged as 
+rgb16 would not shift the image, since it's unclear how much to shift a float 
+type by
 
-- try
+now we do two casts: first, we guess the numeric range from the
+interpretation, so rgb16 would be ushort, so we cast float->ushort;
+second, we cast to the target type and do the shift on the way
 
-	$ vips sharpen babe.jpg x.v
 
-  horrible posterization, looks like too few bits
-
-	$ vips colourspace babe.jpg x.v rgb16
-	$ vips sharpen x.v x2.v
-	$ vips colourspace x2.v x3.v srgb
-
-  no better, strange
-
-- we have a buffer reserve list on each image ... could we junk this on
-  "minimise"?
+- python bandrank should work like bandjoin
 
 - try SEQ_UNBUFFERED on jpg source, get out of order error?
 
-- add reduceh > 3 warning for wtc shrink to 1280x1280
-
-  or was it 1703?
-
-- colour ops are doing cast + extract_band before every op ... for something
-  like colourspace, which can potentially do many ops in a row, this is a LOT
-  of casting and extracting
-
 - could load pdf thumbnails?
 
 - new vips_reduce:
@@ -69,6 +54,12 @@
 
 
 
+- use the incremental webp decoding api to support seq for webp images
+
+	https://developers.google.com/speed/webp/docs/api#decodingadvancedapi
+
+  doesn't seem to be possible
+
 - does ruby need to unpack RefString as well? what about C++?
 
 - are the mosaic functions calling vips_fastcor()? it must be very slow

libvips/conversion/cast.c

@@ -49,6 +49,8 @@
  * 	- cast to uint now removes <0 values
  * 11/2/15
  * 	- add @shift option
+ * 1/3/16
+ * 	- better behaviour for shift of non-int types (thanks apacheark)
  */
 
 /*
@@ -375,10 +377,8 @@ vips_cast_gen( VipsRegion *or, void *vseq, void *a, void *b,
 	VipsCast *cast = (VipsCast *) b;
 	VipsConversion *conversion = (VipsConversion *) b;
 	VipsRect *r = &or->valid;
-	int le = r->left;
-	int to = r->top;
-	int bo = VIPS_RECT_BOTTOM( r );
 	int sz = VIPS_REGION_N_ELEMENTS( or );
+
 	int x, y;
 
 	if( vips_region_prepare( ir, r ) )
@@ -386,11 +386,11 @@ vips_cast_gen( VipsRegion *or, void *vseq, void *a, void *b,
 
 	VIPS_GATE_START( "vips_cast_gen: work" );
 
-	for( y = to; y < bo; y++ ) {
-		VipsPel *in = VIPS_REGION_ADDR( ir, le, y ); 
-		VipsPel *out = VIPS_REGION_ADDR( or, le, y ); 
+	for( y = 0; y < r->height; y++ ) {
+		VipsPel *in = VIPS_REGION_ADDR( ir, r->left, r->top + y ); 
+		VipsPel *out = VIPS_REGION_ADDR( or, r->left, r->top + y ); 
 
-		switch( cast->in->BandFmt ) { 
+		switch( ir->im->BandFmt ) { 
 		case VIPS_FORMAT_UCHAR: 
 			BAND_SWITCH_INNER( unsigned char,
 				VIPS_INT_INT, 
@@ -495,6 +495,20 @@ vips_cast_build( VipsObject *object )
 		return( -1 );
 	in = t[0]; 
 
+	/* If @shift is on but we're not in an int format and we're going to
+	 * an int format, we need to cast to int first. For example, what 
+	 * about a float image tagged as rgb16 being cast to uint8? We need 
+	 * to cast to ushort before we do the final cast to uint8.
+	 */
+	if( cast->shift && 
+		!vips_band_format_isint( in->BandFmt ) &&
+		vips_band_format_isint( cast->format ) ) {
+		if( vips_cast( in, &t[1], 
+			vips_image_guess_format( in ), NULL ) )
+			return( -1 );
+		in = t[1];
+	}
+
 	if( vips_image_pipelinev( conversion->out, 
 		VIPS_DEMAND_STYLE_THINSTRIP, in, NULL ) )
 		return( -1 );

libvips/include/vips/header.h

@@ -114,6 +114,7 @@ int vips_image_get_width( const VipsImage *image );
 int vips_image_get_height( const VipsImage *image );
 int vips_image_get_bands( const VipsImage *image );
 VipsBandFormat vips_image_get_format( const VipsImage *image );
+VipsBandFormat vips_image_guess_format( const VipsImage *image );
 VipsCoding vips_image_get_coding( const VipsImage *image );
 VipsInterpretation vips_image_get_interpretation( const VipsImage *image );
 VipsInterpretation vips_image_guess_interpretation( const VipsImage *image );

libvips/iofuncs/header.c

@@ -386,6 +386,86 @@ vips_image_get_format( const VipsImage *image )
 	return( image->BandFmt );
 }
 
+/**
+ * vips_image_guess_format:
+ * @image: image to guess for
+ *
+ * Return the #VipsBandFormat for an image, guessing a sane value if
+ * the set value looks crazy.
+ *
+ * For example, for a float image tagged as rgb16, we'd return ushort. 
+ *
+ * Returns: a sensible #VipsBandFormat for the image.
+ */
+VipsBandFormat
+vips_image_guess_format( const VipsImage *image )
+{
+	VipsBandFormat format;
+
+	switch( image->Type ) {
+	case VIPS_INTERPRETATION_B_W: 
+	case VIPS_INTERPRETATION_HISTOGRAM: 
+	case VIPS_INTERPRETATION_MULTIBAND: 
+		format = image->BandFmt;
+		break;
+
+	case VIPS_INTERPRETATION_FOURIER: 
+		if( image->BandFmt == VIPS_FORMAT_DOUBLE ||
+			image->BandFmt == VIPS_FORMAT_DPCOMPLEX )
+			format = VIPS_FORMAT_DPCOMPLEX;
+		else
+			format = VIPS_FORMAT_COMPLEX;
+		break;
+
+	case VIPS_INTERPRETATION_sRGB: 
+		format = VIPS_FORMAT_UCHAR;
+		break;
+
+	case VIPS_INTERPRETATION_XYZ: 
+	case VIPS_INTERPRETATION_LAB: 
+	case VIPS_INTERPRETATION_RGB: 
+	case VIPS_INTERPRETATION_CMC: 
+	case VIPS_INTERPRETATION_LCH: 
+	case VIPS_INTERPRETATION_HSV: 
+	case VIPS_INTERPRETATION_scRGB: 
+	case VIPS_INTERPRETATION_YXY: 
+		format = VIPS_FORMAT_FLOAT;
+		break;
+
+	case VIPS_INTERPRETATION_CMYK: 
+		if( image->BandFmt != VIPS_FORMAT_USHORT )
+			format = VIPS_FORMAT_UCHAR;
+		else
+			format = image->BandFmt;
+		break;
+
+	case  VIPS_INTERPRETATION_LABQ:
+		format = VIPS_FORMAT_UCHAR;
+		break;
+
+	case  VIPS_INTERPRETATION_LABS:
+		format = VIPS_FORMAT_SHORT;
+		break;
+
+	case  VIPS_INTERPRETATION_GREY16:
+	case  VIPS_INTERPRETATION_RGB16:
+		format = VIPS_FORMAT_USHORT;
+		break;
+
+	case  VIPS_INTERPRETATION_MATRIX:
+		if( image->BandFmt != VIPS_FORMAT_DOUBLE )
+			format = VIPS_FORMAT_FLOAT;
+		else
+			format = image->BandFmt;
+		break;
+
+	default:
+		g_assert_not_reached();
+	}
+
+	return( format );
+}
+
 /**
  * vips_image_get_coding:
  * @image: image to get from