获取从 blob URL 生成的黑色图像

Question

I am using a library react-avatar-editor for creating profile picture of the user. Below is the code which I am using the send the cropped image to the server

const canvasScaled = this.editor.getImageScaledToCanvas(); //a canvas object
canvasScaled.toBlob((blob) => {
  this.setState({
      preview: blob
    },
    () => {
      let data = new FormData();
      const file = new File([blob], "my-image", {
        type: "image/png"
      });
      data.append("file", file);
      axios({
        method: "POST",
        data: data,
        url: "/media",
        headers: {
          'Content-Type': 'multipart/form-data'
        }
      }).then().catch()
    }
  );
})

But the image generated at the server is a black image. Tested with many files but result is same - a black image.

My backend service is written in JAVA so I tested if the backend service handling the image correctly or not, I wrote a simple file selected

<input type="file" accept={"image/png"} onChange={onSelect}/>

<button onClick={() => {
        const data = new FormData();
        data.append("file", this.state.file);
        axios({
            method: "POST",
            data: data,
            url: "/media",
            headers: {
              'Content-Type': 'multipart/form-data'
            }
        }).then().catch()
}}>Send<button>

And yes, the service side code is working fine with above POST request. Which means, I have definitely something wrong written at the react-avatar-editor handling code.

Is it the blob causing the issue? How the image is sent in the case of a file-selector? Base64 or blob?

Update: I noticed a strange behavior with an image. If I upload this file This gets converted to which is not a complete black image. Something fishy is going on with colors.

To check if the library itself is creating the black image, I check it using canvasScaled.toDataURL("image/png") and used this into an image, and yes there is no issue with the created canvas, image is getting rendered.

Answer 1

Try data.append("file", blob); without calling new File . According to MDN,

A File object is a specific kind of a Blob, and can be used in any context that a Blob can. In particular, FileReader, URL.createObjectURL(), createImageBitmap(), and XMLHttpRequest.send() accept both Blobs and Files.

Answer 2

add this in second line after defining canvasScaled

var ctx = canvasScaled.getContext("2d");
ctx.fillStyle = "white";
ctx.fillRect(0, 0, canvas.width, canvas.height);