for ReadXXXX() image-readers, use the value of pic->use_argb

This is to infer the needed conversion to YUV(A) or RGB(A).

This is useful to avoid some conversion steps between ARGB and YUVA.
For instance, if the input file is a JPEG, we decode to RGB and
convert to YUV right away, without the intermediate step to ARGB.

The only caveat is that cropping/scaling might give slightly different result,
because of YUV420 downsampling. Therefore, we omit this feature
at cwebp level, when -crop or -rescale is used.

Change-Id: I5a3abe5108982f2a4570e841e3d9baffc73f5bee

examples/cwebp.c

@@ -967,7 +967,11 @@ int main(int argc, const char *argv[]) {
     goto Error;
   }
 
-  // Read the input
+  // Read the input. We need to decide if we prefer ARGB or YUVA
+  // samples, depending on the expected compression mode (this saves
+  // some conversion steps).
+  picture.use_argb = (config.lossless || config.preprocessing > 0 ||
+                      crop || (resize_w | resize_h) > 0);
   if (verbose) {
     StopwatchReset(&stop_watch);
   }

examples/jpegdec.c

@@ -272,7 +272,6 @@ int ReadJPEG(FILE* in_file, WebPPicture* const pic, Metadata* const metadata) {
   // WebP conversion.
   pic->width = width;
   pic->height = height;
-  pic->use_argb = 1;      // store raw RGB samples
   ok = WebPPictureImportRGB(pic, rgb, stride);
   if (!ok) goto Error;
 

examples/jpegdec.h

@@ -23,7 +23,7 @@ struct Metadata;
 struct WebPPicture;
 
 // Reads a JPEG from 'in_file', returning the decoded output in 'pic'.
-// The output is RGB.
+// The output is RGB or YUV depending on pic->use_argb value.
 // Returns true on success.
 int ReadJPEG(FILE* in_file, struct WebPPicture* const pic,
              struct Metadata* const metadata);

examples/pngdec.c

@@ -270,7 +270,6 @@ int ReadPNG(FILE* in_file, WebPPicture* const pic, int keep_alpha,
 
   pic->width = (int)width;
   pic->height = (int)height;
-  pic->use_argb = 1;
   ok = has_alpha ? WebPPictureImportRGBA(pic, rgb, (int)stride)
                  : WebPPictureImportRGB(pic, rgb, (int)stride);
 

examples/pngdec.h

@@ -22,8 +22,9 @@ struct Metadata;
 struct WebPPicture;
 
 // Reads a PNG from 'in_file', returning the decoded output in 'pic'.
+// Output is RGBA or YUVA, depending on pic->use_argb value.
 // If 'keep_alpha' is true and the PNG has an alpha channel, the output is RGBA
-// otherwise it will be RGB.
+// or YUVA. Otherwise, alpha channel is dropped and output is RGB or YUV.
 // Returns true on success.
 int ReadPNG(FILE* in_file, struct WebPPicture* const pic, int keep_alpha,
             struct Metadata* const metadata);

examples/tiffdec.c

@@ -100,7 +100,6 @@ int ReadTIFF(const char* const filename,
 #ifdef WORDS_BIGENDIAN
       TIFFSwabArrayOfLong(raster, width * height);
 #endif
-      pic->use_argb = 1;
       ok = keep_alpha
          ? WebPPictureImportRGBA(pic, (const uint8_t*)raster, stride)
          : WebPPictureImportRGBX(pic, (const uint8_t*)raster, stride);

examples/tiffdec.h

@@ -20,8 +20,9 @@ struct Metadata;
 struct WebPPicture;
 
 // Reads a TIFF from 'filename', returning the decoded output in 'pic'.
+// Output is RGBA or YUVA, depending on pic->use_argb value.
 // If 'keep_alpha' is true and the TIFF has an alpha channel, the output is RGBA
-// otherwise it will be RGB.
+// or YUVA. Otherwise, alpha channel is dropped and output is RGB or YUV.
 // Returns true on success.
 int ReadTIFF(const char* const filename,
              struct WebPPicture* const pic, int keep_alpha,

examples/webpdec.c

@@ -41,6 +41,8 @@ int ReadWebP(const char* const in_file, WebPPicture* const pic,
 
   if (ExUtilLoadWebP(in_file, &data, &data_size, bitstream)) {
     const int has_alpha = keep_alpha && bitstream->has_alpha;
+    // TODO(skal): use MODE_YUV(A), depending on the expected
+    // input pic->use_argb. This would save some conversion steps.
     output_buffer->colorspace = has_alpha ? MODE_RGBA : MODE_RGB;
 
     status = ExUtilDecodeWebP(data, data_size, 0, &config);
@@ -49,7 +51,6 @@ int ReadWebP(const char* const in_file, WebPPicture* const pic,
       const int stride = output_buffer->u.RGBA.stride;
       pic->width = output_buffer->width;
       pic->height = output_buffer->height;
-      pic->use_argb = 1;
       ok = has_alpha ? WebPPictureImportRGBA(pic, rgba, stride)
                      : WebPPictureImportRGB(pic, rgba, stride);
     }

examples/webpdec.h

@@ -20,8 +20,9 @@ struct Metadata;
 struct WebPPicture;
 
 // Reads a WebP from 'in_file', returning the decoded output in 'pic'.
-// If 'keep_alpha' is true and the WebP has an alpha channel, the output is
-// RGBA otherwise it will be RGB.
+// Output is RGBA or YUVA, depending on pic->use_argb value.
+// If 'keep_alpha' is true and the TIFF has an alpha channel, the output is RGBA
+// or YUVA. Otherwise, alpha channel is dropped and output is RGB or YUV.
 // Returns true on success.
 int ReadWebP(const char* const in_file, struct WebPPicture* const pic,
              int keep_alpha, struct Metadata* const metadata);

examples/wicdec.c

@@ -320,7 +320,7 @@ int ReadPictureWithWIC(const char* const filename,
     int ok;
     pic->width = width;
     pic->height = height;
-    pic->use_argb = 1;
+    pic->use_argb = 1;    // For WIC, we always force to argb
     ok = importer->import(pic, rgb, stride);
     if (!ok) hr = E_FAIL;
   }

examples/wicdec.h

@@ -21,7 +21,7 @@ struct WebPPicture;
 
 // Reads an image from 'filename', returning the decoded output in 'pic'.
 // If 'keep_alpha' is true and the image has an alpha channel, the output is
-// RGBA otherwise it will be RGB.
+// RGBA otherwise it will be RGB. pic->use_argb is always forced to true.
 // Returns true on success.
 int ReadPictureWithWIC(const char* const filename,
                        struct WebPPicture* const pic, int keep_alpha,