[Android] Color Conversion Codes Not Working: Assertion Error "!_src.empty()" in cvtColor Function

[jesusebzcp]

Issue: Color Conversion Codes Not Working in Android

Description

When trying to use color conversion codes with cvtColor in Android, none of the conversion codes seem to work. The function fails with an assertion error indicating that the source image is empty (!_src.empty()).

Steps to Reproduce

1. Create a Mat object from a base64 image:

const source = OpenCV.base64ToMat(imageUriData);

2. Try to convert the image using any color conversion code:

const dst = OpenCV.createObject(ObjectType.Mat, 0, 0, DataTypes.CV_8UC3);
OpenCV.invoke("cvtColor", source, dst, ColorConversionCodes.COLOR_RGB2BGR);

Expected Behavior

The color conversion should work as expected, converting the image from one color space to another.

Actual Behavior

The conversion fails with an assertion error, suggesting that the source image is empty, even though the image is valid and can be processed in other ways.

Environment

• React Native Fast OpenCV version: ^0.4.5
• Platform: Android
• React Native version: 0.79.1
• Device:TECNO CH6IS

Additional Context

• The same code works on iOS
• The image is being loaded correctly from base64
• The issue occurs with all color conversion codes, including:
  • COLOR_RGB2BGR
  • COLOR_BGR2RGB
  • COLOR_RGBA2BGR
  • COLOR_BGRA2BGR
  • COLOR_BGR2RGBA
  • COLOR_RGB2RGBA
  • COLOR_BGR2GRAY
  • COLOR_RGB2GRAY

Possible Workaround

Currently, we're using COLOR_RGBA2BGR as a workaround, but this might not be the correct conversion for all cases.

Questions

1. Is there a specific way to handle color conversions in Android?
2. Are there any known limitations with color conversions in the Android implementation?
3. Should we be using a different approach for color space conversions in Android?

Thank you for your help in resolving this issue.

[lukaszkurantdev]

@jesusebzcp Hi, it seems that the problem is not related to cvtColor, but to base64 conversion. The !_src.empty() is an error of OpenCV itself.

Can you confirm that source is correct? Does it return Mat data?

const source = OpenCV.base64ToMat(imageUriData);
const dst = OpenCV.toJSValue(dst);
console.log(dst)?

[jesusebzcp]

@lukaszkurantdev Hi, Yes, I confirmed the value of source and it does return a valid Mat object:

LOG  source {"id": "05ff2961-508d-4efa-9e79-e6f7614d4a05", "type": "mat"}

However, the error still occurs when trying to run cvtColor:

ERROR  ❌ error en useSheetScanner: [Error: Exception in HostFunction: Fast OpenCV Error: OpenCV(4.9.0) /home/ci/opencv/modules/imgproc/src/color.cpp:196: error: (-215:Assertion failed) !_src.empty() in function 'cvtColor']

Hi! Yes, here is the output:

LOG  Mat:source {"id": "e6c1ebe3-cc7e-4fee-8efa-a51ece29d2bc", "type": "mat"}
LOG  Mat:bgr {"id": "0511af54-6d3e-4e6c-ba46-7552b51d7b84", "type": "mat"}
ERROR  ❌ error en useSheetScanner: [Error: Exception in HostFunction: Fast OpenCV Error: OpenCV(4.9.0) /home/ci/opencv/modules/imgproc/src/color.cpp:196: error: (-215:Assertion failed) !_src.empty() in function 'cvtColor'

[lukaszkurantdev]

@jesusebzcp

LOG  source {"id": "05ff2961-508d-4efa-9e79-e6f7614d4a05", "type": "mat"}

This only shows id of the mat. Please use toJSValue to get dimensions and number of available channels. Like this:

const source = OpenCV.base64ToMat(imageUriData);
const dst = OpenCV.toJSValue(source);
console.log(dst);

[jesusebzcp]

@lukaszkurantdev Ready, I tried to obtain the value through the console using the method: toJSValue, but I got an error

code:

export const bubbleProcess = async (imageUriData: string, jobName: string) => {
  // Verificamos si existe un imageUri; si no, devolvemos null inmediatamente.
  if (!imageUriData) {
    return null;
  }

  console.log("INIT TEST", imageUriData.slice(0, 100));

  // 🖼️ 1) CONVERTIR imagen base64 → Mat (formato BGR)
  const source = OpenCV.base64ToMat(imageUriData);
  if (source) {
    console.log("source", source);
  } else {
    console.log("empty source");
  }
  const sourceJsValue = OpenCV.toJSValue(source);
  console.log("Mat:source", sourceJsValue);

  // Convertir a BGR primero para asegurar el formato correcto
  const bgr = OpenCV.createObject(ObjectType.Mat, 0, 0, DataTypes.CV_8UC3);
  const bgrJsValue = OpenCV.toJSValue(source);

console.log:

 LOG  INIT TEST /9j/4AAQSkZJRgABAQAAAQABAAD/4gIoSUNDX1BST0ZJTEUAAQEAAAIYAAAAAAIQAABtbnRyUkdC
IFhZWiAAAAAAAAAAAAAAAAB
 LOG  source {"id": "3382c2c2-6398-4343-9a43-b204b67c18bf", "type": "mat"}
 ERROR  Error en useSheetScanner: [Error: Exception in HostFunction: <unknown>]
 LOG  finalizado....

How can you realize the error occurred in this line and there

  const sourceJsValue = OpenCV.toJSValue(source);

[ningrlan]

Hi @jesusebzcp , did you figure anything out? I'm having a similar problem. Following @lukaszkurantdev 's suggestion, the jsvalue of srcMat was printed out with no problems. However, I got an Exception in HostFunction: <unknown> at toJSValue (native) error when printing out the jsvalue of garyMat.

Code:

try {
      console.log("base64ToMat")
      const srcMat = OpenCV.base64ToMat(this.base64);

      if (srcMat) {
        console.log("source", srcMat);
      } else {
        console.log("empty source");
      }
      const srcMatJS = OpenCV.toJSValue(srcMat);  
      console.log("Mat:jsvalue", srcMatJS)
    } catch (error) {
      console.log("error in base64 to mat", error)
    }    

try {
  console.log('grayscaling...');
  let grayMat = OpenCV.createObject(ObjectType.Mat, srcMatJS.cols, srcMatJS.rows, DataTypes.CV_8UC3);
  const grayMatCheck = OpenCV.toJSValue(grayMat);  
  console.log("grayMat:jsvalue", grayMatCheck)
  // const threshMat = OpenCV.createObject(ObjectType.Mat);
  OpenCV.invoke('cvtColor', {
    src: srcMat,
    dst: grayMat,
    code: ColorConversionCodes.COLOR_RGBA2GRAY,
  });
} catch (error) {
  console.log("error in grayscaling", error)
}

console.log

base64ToMat
source {type: 'mat', id: 'a3f5832c-9989-418e-b7a0-5a3be5043238'}
Mat:jsvalue {base64: 'OMITTED', size: true, cols: 100, rows: 50, type: 16}
grayscaling...
grayMat:jsvalue {base64: 'OMITTED', size: true, cols: 50, rows: 100, type: 16}
error in grayscaling Error: Exception in HostFunction: <unknown>
    at invoke (native)
    at ?anon_0_ (http://localhost:8081/index.bundle//&platform=android&dev=true&lazy=true&minify=false&app=com.stellasonosapp&modulesOnly=false&runModule=true&excludeSource=true&sourcePaths=url-server:153328:85)
    at next (native)
    at asyncGeneratorStep (http://localhost:8081/index.bundle//&platform=android&dev=true&lazy=true&minify=false&app=com.stellasonosapp&modulesOnly=false&runModule=true&excludeSource=true&sourcePaths=url-server:22279:19)
    at _next (http://localhost:8081/index.bundle//&platform=android&dev=true&lazy=true&minify=false&app=com.stellasonosapp&modulesOnly=false&runModule=true&excludeSource=true&sourcePaths=url-server:22293:29)
    at tryCallOne (address at InternalBytecode.js:1:1180)
    at anonymous (address at InternalBytecode.js:1:1874)

Would the error be related to creatObject too? I'm confused as what's supposed to be the size of garyMat -- the same as srcMat?