Crop GLES texture before drawing

Question

I'm using a SurfaceView to render the Camera preview to the screen via GLES. Is it possible to crop the texture in the method below before it is rendered? I'd like to crop a 16:9 texture to appear as 4:3 on the screen.

public void drawFrame(int width, int height, float sourceAspectRatio, float targetAspectRatio, boolean flipHorizontally, boolean flipVertically) {

    //Aspect ratio correction. Source Aspect ratio is taken from Camera.Parameters.getPictureSize()
    //assuming that value is the native resolution of the camera. Sometimes the native camera aspect ratio
    //doesn't match the display's aspect ratio.
    float scaleX, scaleY;
    if (sourceAspectRatio >= targetAspectRatio) {
        scaleX = sourceAspectRatio/targetAspectRatio;
        scaleY = 1;
    } else {
        scaleX = 1;
        scaleY = targetAspectRatio/sourceAspectRatio;
    }

    if (flipHorizontally) scaleX = -scaleX;
    if (flipVertically) scaleY = -scaleY;

    checkGlError("onDrawFrame start");
    mSurfaceTexture.getTransformMatrix(mSTMatrix);

    GLES20.glViewport(0,0,width,height);

    GLES20.glClearColor(0.0f, 1.0f, 0.0f, 1.0f);
    GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT);

    GLES20.glUseProgram(mProgram);
    checkGlError("glUseProgram");

    /**
     * Explanation of double binding: http://stackoverflow.com/questions/20386515/glsurfaceview-framerate-issues-on-nexus-5
     *
     * I was able to replicate the behavior, and my GL wizard office-mate figured out the problem.
     * Basically, one of the EGL contexts isn't noticing that the texture contents have changed, so it keeps rendering older data.
     * We think it's getting occasional updates because it has a set of buffers that it cycles through,
     * so eventually it re-uses the buffer you're looking at.
     * I was able to fix the problem in my code by updating the texture renderer class, changing this:
     *
     * GLES20.glBindTexture(GLES11Ext.GL_TEXTURE_EXTERNAL_OES, mTextureID)
     *
     * to this
     *
     * GLES20.glBindTexture(GLES11Ext.GL_TEXTURE_EXTERNAL_OES, 0);
     * GLES20.glBindTexture(GLES11Ext.GL_TEXTURE_EXTERNAL_OES, mTextureID);
     *
     * The un-bind and re-bind causes the driver to pick up the right buffer.
     */
    GLES20.glActiveTexture(GLES20.GL_TEXTURE0);
    GLES20.glBindTexture(GLES11Ext.GL_TEXTURE_EXTERNAL_OES, 0);
    GLES20.glBindTexture(GLES11Ext.GL_TEXTURE_EXTERNAL_OES, mTextureID);

    mTriangleVertices.position(TRIANGLE_VERTICES_DATA_POS_OFFSET);
    GLES20.glVertexAttribPointer(maPositionHandle, 3, GLES20.GL_FLOAT, false, TRIANGLE_VERTICES_DATA_STRIDE_BYTES, mTriangleVertices);
    checkGlError("glVertexAttribPointer maPosition");
    GLES20.glEnableVertexAttribArray(maPositionHandle);
    checkGlError("glEnableVertexAttribArray maPositionHandle");

    mTriangleVertices.position(TRIANGLE_VERTICES_DATA_UV_OFFSET);
    GLES20.glVertexAttribPointer(maTextureHandle, 2, GLES20.GL_FLOAT, false, TRIANGLE_VERTICES_DATA_STRIDE_BYTES, mTriangleVertices);
    checkGlError("glVertexAttribPointer maTextureHandle");
    GLES20.glEnableVertexAttribArray(maTextureHandle);
    checkGlError("glEnableVertexAttribArray maTextureHandle");
    Matrix.setIdentityM(mMVPMatrix, 0);
    mMVPMatrix[0] = scaleX;
    mMVPMatrix[5] = scaleY;
    GLES20.glUniformMatrix4fv(muMVPMatrixHandle, 1, false, mMVPMatrix, 0);
    GLES20.glUniformMatrix4fv(muSTMatrixHandle, 1, false, mSTMatrix, 0);

    GLES20.glDrawArrays(GLES20.GL_TRIANGLE_STRIP, 0, 4);
    checkGlError("glDrawArrays");

    GLES20.glDisableVertexAttribArray(maTextureHandle);
    GLES20.glDisableVertexAttribArray(maPositionHandle);
    GLES20.glBindTexture(GLES11Ext.GL_TEXTURE_EXTERNAL_OES, 0);
    GLES20.glUseProgram(0);
}

score 2 · Answer 1 · answered Jun 29 '16 at 16:15

2

Just change your texture coordinates (to pick what you want to appear on screen), and your vertex coordinates (to decide where on screen you draw).

answered Jun 29 '16 at 16:15

solidpixel

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score 2 · Accepted Answer · answered Jul 07 '16 at 14:12

Okay, I'm nut sure this is the right way to crop a texture, but it does what I need. I just set negative values to the starting coordinates of the view port and resized the resolution according to the output I need:

public void drawFrame(int sourceWidth, int sourceHeight, int targetWidth, int targetHeight, boolean flipHorizontally, boolean flipVertically) {

    int offsetX = 0, offsetY = 0;
    float scaleX = 1, scaleY = 1;
    if (flipHorizontally) scaleX = -scaleX;
    if (flipVertically) scaleY = -scaleY;

    float sourceAspectRatio = (float) sourceWidth / sourceHeight;
    float targetAspectRatio = (float) targetWidth / targetHeight;

    //Pull the image off the screen if needed, based on the aspect ratio diff.
    if (sourceAspectRatio > targetAspectRatio) {
        int scaledTargetWidth = (int) (targetHeight * sourceAspectRatio);
        offsetX = (scaledTargetWidth - targetWidth) / 2;
        offsetX = -offsetX;
        targetWidth = scaledTargetWidth;
    } else if (sourceAspectRatio < targetAspectRatio) {
        int scaledTargetHeight = (int) (targetWidth / sourceAspectRatio);
        offsetY = (scaledTargetHeight - targetHeight) / 2;
        offsetY = -offsetY;
        targetHeight = scaledTargetHeight;
    }

    checkGlError("onDrawFrame start");
    mSurfaceTexture.getTransformMatrix(mSTMatrix);

    GLES20.glViewport(offsetX,offsetY,targetWidth,targetHeight);

    //TODO....
}

This is very helpful, thank you! – Michael N Dec 05 '21 at 22:17 — Michael N, Dec 05 '21 at 22:17

Crop GLES texture before drawing

2 Answers2