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jpg: ignore sampling factors for grayscale as colormap1() doesnt handle it

--- a/sys/src/cmd/jpg/readjpg.c

+++ b/sys/src/cmd/jpg/readjpg.c

@@ -7,7 +7,7 @@

 enum {

 	/* Constants, all preceded by byte 0xFF */

 	SOF	=0xC0,	/* Start of Frame */

-	SOF2=0xC2,	/* Start of Frame; progressive Huffman */

+	SOF2	=0xC2,	/* Start of Frame; progressive Huffman */

 	JPG	=0xC8,	/* Reserved for JPEG extensions */

 	DHT	=0xC4,	/* Define Huffman Tables */

 	DAC	=0xCC,	/* Arithmetic coding conditioning */

@@ -322,12 +322,15 @@

 		case SOF2:

 			header->Y = int2(b, 1);

 			header->X = int2(b, 3);

-			header->Nf =b[5];

+			header->Nf = b[5];

 			for(i=0; i<header->Nf; i++){

 				header->comp[i].C = b[6+3*i+0];

 				nibbles(b[6+3*i+1], &H, &V);

 				if(H<=0 || V<=0)

 					jpgerror(header, "non-positive sampling factor (Hsamp or Vsamp)");

+				/* hack: colormap1() doesnt handle resampling */

+				if(header->Nf == 1)

+					H = V = 1;

 				header->comp[i].H = H;

 				header->comp[i].V = V;

 				header->comp[i].Tq = b[6+3*i+2];

-- 

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