Emscripten pass uint8_t array to javascript?

Question

Trying to display a uint8_t* rgb image data buffer to an HTML canvas that was process in C via WASM.

In C I have the following external method:

extern void JS_DisplayRenderData(uint8_t* data, int dataLength);

Then I call the extrnal method like so:

int size = 1280 * 720 * 3;
uint8_t data[size];
memset(data, 255, size);
JS_DisplayRenderData(data, size);

In javaScript I then try to display the buffer like so:

if (typeof mergeInto !== 'undefined') mergeInto(LibraryManager.library,
{
    JS_DisplayRenderData: function(data, dataLength)
    {
        alert("Data Length: " + dataLength);
        var c = document.getElementById("canvas");
        var ctx = c.getContext("2d");
        var imgdata = ctx.getImageData(0, 0, c.width, c.height);
        var imgdatalen = imgdata.data.length;
        var i2 = 0;
        for (var i = 0; i < (imgdatalen / 4); i++)
        {
            imgdata.data[4*i] = data[i2];    // RED (0-255)
            imgdata.data[4*i+1] = data[i2+1];    // GREEN (0-255)
            imgdata.data[4*i+2] = data[i2+2];    // BLUE (0-255)
            imgdata.data[4*i+3] = 255;  // APLHA (0-255)
            i2 += 3;
        }
        ctx.putImageData(imgdata, 0, 0);
    }
});

However all I get are black pixels even though it should all be white.

extern...and accessing data via index directly in JS side? Could you attach a complete source, including your emcc commands, the use Emscripten macros in C files, and JS file? To me this code should not able to run nor should not even able to compile in the first place. — Bumsik Kim
– Bumsik Kim, Commented Mar 31, 2019 at 23:39
Found the issue. Just needed "var a = HEAPU8.subarray(data);". Will post more info in an answer. — zezba9000
– zezba9000, Commented Apr 1, 2019 at 0:05

zezba9000 · Accepted Answer · 2019-04-01 00:07:34Z

1

Found the answer tnx to: Struct operations in Javascript through Emscripten

Just needed to state what the buffer type was via: "var a = HEAPU8.subarray(data);"

The method I'm using to invoke JS via C can be found: Webassembly calling JavaScript methods from wasm i.e. whithin c++ code

answered Apr 1, 2019 at 0:07

zezba9000

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