diff --git a/tools/cmd/icat/native.go b/tools/cmd/icat/native.go
index 489ce0ffe..fdd193e6b 100644
--- a/tools/cmd/icat/native.go
+++ b/tools/cmd/icat/native.go
@@ -34,7 +34,7 @@ func add_frame(imgd *image_data, img image.Image, is_opaque bool) {
 			rgb = &images.NRGB{Pix: m.Slice(), Stride: 3 * f.width, Rect: dest_rect}
 			f.shm = m
 		}
-		imgd.format_uppercase = "RGB"
+		f.transmission_format_uppercase = "RGB"
 		f.in_memory_bytes = rgb.Pix
 		final_img = rgb
 	} else {
@@ -46,7 +46,7 @@ func add_frame(imgd *image_data, img image.Image, is_opaque bool) {
 			rgba = &image.NRGBA{Pix: m.Slice(), Stride: 4 * f.width, Rect: dest_rect}
 			f.shm = m
 		}
-		imgd.format_uppercase = "RGBA"
+		f.transmission_format_uppercase = "RGBA"
 		f.in_memory_bytes = rgba.Pix
 		final_img = rgba
 	}
diff --git a/tools/cmd/icat/process_images.go b/tools/cmd/icat/process_images.go
index 4403f2120..63386a013 100644
--- a/tools/cmd/icat/process_images.go
+++ b/tools/cmd/icat/process_images.go
@@ -137,11 +137,12 @@ func (self *opened_input) Release() {
 }
 
 type image_frame struct {
-	filename              string
-	shm                   shm.MMap
-	in_memory_bytes       []byte
-	filename_is_temporary bool
-	width, height         int
+	filename                      string
+	shm                           shm.MMap
+	in_memory_bytes               []byte
+	filename_is_temporary         bool
+	width, height                 int
+	transmission_format_uppercase string
 }
 
 type image_data struct {
@@ -190,6 +191,7 @@ func make_output_from_input(imgd *image_data, f *opened_input) {
 	imgd.frames = append(imgd.frames, &frame)
 	frame.width = imgd.canvas_width
 	frame.height = imgd.canvas_height
+	frame.transmission_format_uppercase = imgd.format_uppercase
 	if ok {
 		frame.in_memory_bytes = bb.data
 	} else {
diff --git a/tools/cmd/icat/transmit.go b/tools/cmd/icat/transmit.go
index 93e4d58a9..47b94faea 100644
--- a/tools/cmd/icat/transmit.go
+++ b/tools/cmd/icat/transmit.go
@@ -32,13 +32,15 @@ func gc_for_image(imgd *image_data, frame_num int, frame *image_frame) *graphics
 	if imgd.image_number != 0 {
 		gc.SetImageNumber(imgd.image_number)
 	}
-	switch imgd.format_uppercase {
+	switch frame.transmission_format_uppercase {
 	case "PNG":
 		gc.SetFormat(graphics.GRT_format_png)
-	default:
+	case "RGB":
 		gc.SetFormat(graphics.GRT_format_rgb)
 	case "RGBA":
 		gc.SetFormat(graphics.GRT_format_rgba)
+	default:
+		panic(fmt.Sprintf("Unknown transmission format: %s", frame.transmission_format_uppercase))
 	}
 
 	return &gc