display images: show as image each expression result that implements Go interface 'image.Image'

Author: cosmos72

README.md

@@ -188,16 +188,10 @@ $ docker run -it -p 8888:8888 -v /path/to/local/notebooks:/path/to/notebooks/in/
 gophernotes uses [gomacro](https://github.com/cosmos72/gomacro) under the hood to evaluate Go code interactively. You can evaluate most any Go code with gomacro, but there are some limitation, which are discussed in further detail [here](https://github.com/cosmos72/gomacro#current-status).  Most noteably, gophernotes does NOT support:
 
 - third party packages when running natively on Mac and Windows - This is a current limitation of the Go `plugin` package.
-- unexported struct fields
-- interfaces - They can be declared, but nothing more: there is no way to implement them or call their methods
-- extracting methods from types - For example time.Duration.String should return a func(time.Duration) string but currently gives an error. Instead extracting methods from objects is supported: time.Duration(1s).String correctly returns a func() string
-- goto
-- named return values
-- named imports like:
-
-    ```
-    import tf "github.com/tensorflow/tensorflow/tensorflow/go"
-    ```
+- unexported struct fields are emulated
+- interpreted interfaces - They can be declared, but nothing more: there is no way to implement them or call their methods
+  Note: interfaces imported from standard library or third party packages are compiled, and fully functional.
+- goto is only partially implemented
 
 ## Troubleshooting
 

image.go

@@ -0,0 +1,61 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"image"
+	"image/png"
+	"log"
+)
+
+// publishImages sends a "display_data" broadcast message for each image.Image found in vals,
+// then replaces it with a placeholder string equal to reflect.TypeOf(val).String()
+// to avoid overloading the front-end with huge amounts of output text:
+// fmt.Sprint(val) is often very large for an image
+func publishImages(vals []interface{}, receipt msgReceipt) []interface{} {
+	for i, val := range vals {
+		if img, ok := val.(image.Image); ok {
+			err := publishImage(img, receipt)
+			if err != nil {
+				log.Printf("Error publishing image: %v\n", err)
+			} else {
+				vals[i] = fmt.Sprintf("%T", val)
+			}
+		}
+	}
+	return vals
+}
+
+// publishImages sends a "display_data" broadcast message for given image.
+func publishImage(img image.Image, receipt msgReceipt) error {
+	bytes, mime, err := encodePng(img)
+	if err != nil {
+		return err
+	}
+	data := bundledMIMEData{
+		mime: bytes,
+	}
+	metadata := bundledMIMEData{
+		mime: imageMetadata(img),
+	}
+	return receipt.PublishDisplayData(data, metadata)
+}
+
+// encodePng converts an image.Image to PNG []byte
+func encodePng(img image.Image) (data []byte, mime string, err error) {
+	var buf bytes.Buffer
+	err = png.Encode(&buf, img)
+	if err != nil {
+		return nil, "", err
+	}
+	return buf.Bytes(), "image/png", nil
+}
+
+// imageMetadata returns image size, represented as bundledMIMEData{"width": width, "height": height}
+func imageMetadata(img image.Image) bundledMIMEData {
+	rect := img.Bounds()
+	return bundledMIMEData{
+		"width":  rect.Dx(),
+		"height": rect.Dy(),
+	}
+}

kernel.go

@@ -347,8 +347,6 @@ func handleExecuteRequest(ir *interp.Interp, receipt msgReceipt) error {
 
 	vals, executionErr := doEval(ir, code)
 
-	//TODO if value is a certain type like image then display it instead
-
 	// Close and restore the streams.
 	wOut.Close()
 	os.Stdout = oldStdout
@@ -360,6 +358,9 @@ func handleExecuteRequest(ir *interp.Interp, receipt msgReceipt) error {
 	writersWG.Wait()
 
 	if executionErr == nil {
+		// if one or more value is image.Image, display it instead
+		vals = publishImages(vals, receipt)
+
 		content["status"] = "ok"
 		content["user_expressions"] = make(map[string]string)
 

messages.go

@@ -269,6 +269,21 @@ func (receipt *msgReceipt) PublishExecutionError(err string, trace []string) err
 	)
 }
 
+// PublishDisplayData publishes a single image.
+func (receipt *msgReceipt) PublishDisplayData(data, metadata bundledMIMEData) error {
+	return receipt.Publish("display_data",
+		struct {
+			Data      bundledMIMEData `json:"data"`
+			Metadata  bundledMIMEData `json:"metadata"`
+			Transient bundledMIMEData `json:"transient"`
+		}{
+			Data:      data,
+			Metadata:  metadata,
+			Transient: make(bundledMIMEData),
+		},
+	)
+}
+
 const (
 	// StreamStdout defines the stream name for standard out on the front-end. It
 	// is used in `PublishWriteStream` to specify the stream to write to.