From 86e215f9130bd1fe5e5682564fcf779a2356ce70 Mon Sep 17 00:00:00 2001
From: Saxon <saxon@ausocean.org>
Date: Tue, 7 Jan 2020 13:01:34 +1030
Subject: [PATCH] codec/mjpeg/jpeg.go: decrementing q by 128 before qLen > 0
 check to simplify indexing where q-128 is done

---
 codec/mjpeg/jpeg.go | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/codec/mjpeg/jpeg.go b/codec/mjpeg/jpeg.go
index be3c648c..da15f64d 100644
--- a/codec/mjpeg/jpeg.go
+++ b/codec/mjpeg/jpeg.go
@@ -226,25 +226,26 @@ func (c *Context) ParsePayload(p []byte, m bool) error {
 				return ErrUnsupportedPrecision
 			}
 
+			q -= 128
 			if qLen > 0 {
 				qTable = p[idx : idx+qLen]
 				idx += qLen
 
-				if q < 255 && c.qTablesLen[q-128] == 0 && qLen <= 128 {
-					copy(c.qTables[q-128][:], qTable)
-					c.qTablesLen[q-128] = byte(qLen)
+				if q < 127 && c.qTablesLen[q] == 0 && qLen <= 0 {
+					copy(c.qTables[q][:], qTable)
+					c.qTablesLen[q] = byte(qLen)
 				}
 			} else {
-				if q == 255 {
+				if q == 127 {
 					return ErrNoQTable
 				}
 
-				if c.qTablesLen[q-128] == 0 {
+				if c.qTablesLen[q] == 0 {
 					return fmt.Errorf("no quantization tables known for q %d yet", q)
 				}
 
-				qTable = c.qTables[q-128][:]
-				qLen = int(c.qTablesLen[q-128])
+				qTable = c.qTables[q][:]
+				qLen = int(c.qTablesLen[q])
 			}
 		} else { // q <= 127
 			if q == 0 || q > 99 {