Fix issue with glReadPixels() not taking into account the GL_PACK_ROW_LENGTH

src/lib/libwebgl.js

@@ -330,6 +330,10 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
     unpackAlignment: 4, // default alignment is 4 bytes
     unpackRowLength: 0,
 
+#if INCLUDE_WEBGL1_FALLBACK && MAX_WEBGL_VERSION >= 2
+    packRowLength: 0,
+#endif
+
     // Records a GL error condition that occurred, stored until user calls
     // glGetError() to fetch it. As per GLES2 spec, only the first error is
     // remembered, and subsequent errors are discarded until the user has
@@ -1289,6 +1293,10 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
       GL.unpackAlignment = param;
     } else if (pname == {{{ cDefs.GL_UNPACK_ROW_LENGTH }}}) {
       GL.unpackRowLength = param;
+#if INCLUDE_WEBGL1_FALLBACK && MAX_WEBGL_VERSION >= 2
+    } else if (pname == {{{ cDefs.GL_PACK_ROW_LENGTH }}}) {
+      GL.packRowLength = param;
+#endif
     }
     GLctx.pixelStorei(pname, param);
   },
@@ -1773,7 +1781,12 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
     }
 #endif
 #if INCLUDE_WEBGL1_FALLBACK
-    var pixelData = emscriptenWebGLGetTexPixelData(type, format, width, height, pixels);
+#if MAX_WEBGL_VERSION >= 2
+    var stride = Math.max(width, GL.packRowLength);
+#else
+    var stride = width; // GL_PACK_ROW_LENGTH does not exist yet in WebGL 1.
+#endif
+    var pixelData = emscriptenWebGLGetTexPixelData(type, format, stride, height, pixels);
     if (!pixelData) {
       GL.recordError(0x500/*GL_INVALID_ENUM*/);
 #if GL_ASSERTIONS

src/struct_info.json

@@ -1317,6 +1317,7 @@
     {
         "file": "GL/gl.h",
         "defines": [
+          "GL_PACK_ROW_LENGTH",
           "GL_UNPACK_ALIGNMENT",
           "GL_UNPACK_ROW_LENGTH"
         ]

src/struct_info_generated.json

@@ -301,6 +301,7 @@
         "File::DirectoryKind": 2,
         "File::SymlinkKind": 3,
         "File::UnknownKind": 0,
+        "GL_PACK_ROW_LENGTH": 3330,
         "GL_UNPACK_ALIGNMENT": 3317,
         "GL_UNPACK_ROW_LENGTH": 3314,
         "ICANON": 2,

test/browser/webgl2_readpixels_pack_row_length.c

@@ -0,0 +1,31 @@
+// Tests that glReadPixels() works when GL_PACK_ROW_LENGTH > read width.
+#include <GLES3/gl3.h>
+#include <emscripten/html5.h>
+#include <assert.h>
+#include <string.h>
+
+int main() {
+  EmscriptenWebGLContextAttributes attrs;
+  emscripten_webgl_init_context_attributes(&attrs);
+  attrs.majorVersion = 2;
+  EMSCRIPTEN_WEBGL_CONTEXT_HANDLE ctx = emscripten_webgl_create_context("#canvas", &attrs);
+  emscripten_webgl_make_context_current(ctx);
+
+  // Clear framebuffer to red.
+  glClearColor(1, 0, 0, 1);
+  glClear(GL_COLOR_BUFFER_BIT);
+
+  // Read 8 pixels wide into a buffer with row length 32.
+  glPixelStorei(GL_PACK_ROW_LENGTH, 32);
+  GLubyte buf[32 * 2 * 4];
+  memset(buf, 0, sizeof(buf));
+  glReadPixels(0, 0, 8, 2, GL_RGBA, GL_UNSIGNED_BYTE, buf);
+
+  // Verify pixel at (0,0) is red.
+  assert(buf[0] == 255 && buf[1] == 0 && buf[2] == 0 && buf[3] == 255);
+
+  // Verify that pixel at (0,1) was read to address at stride 32 (pixels), so is red as well.
+  assert(buf[32*4] == 255 && buf[32*4+1] == 0 && buf[32*4+2] == 0 && buf[32*4+3] == 255);
+
+  return 0;
+}

test/test_browser.py

@@ -1255,6 +1255,10 @@ def test_webgl2_ubo_layout_binding(self):
   def test_webgl2_texsubimage3d(self):
     self.btest_exit('webgl2_texsubimage3d.c', cflags=['-sMIN_WEBGL_VERSION=2'])
 
+  @requires_webgl2
+  def test_webgl2_readpixels(self):
+    self.btest_exit('webgl2_readpixels_pack_row_length.c', cflags=['-sMIN_WEBGL_VERSION=2'])
+
   # Test that -sGL_PREINITIALIZED_CONTEXT works and allows user to set Module['preinitializedWebGLContext'] to a preinitialized WebGL context.
   @requires_graphics_hardware
   def test_preinitialized_webgl_context(self):