grayscale images in compute shader

Scripting Issues
5

Damn, i just came across a really strange behaviour.
My first test program was simply copying the hightmaps values into another image, using following code:

main.py:

from panda3d.core import Shader, ComputeNode, Texture, NodePath, ShaderAttrib, Filename

from panda3d.core import loadPrcFileData
loadPrcFileData("", "gl-debug #t")
from direct.directbase import DirectStart

# load source image
img_in = loader.loadTexture("hightmap.png")

img_in.setFormat(Texture.F_r16)
img_in.setName("img_in")
img_out = Texture("img_out")
img_out.setup2dTexture(8192,8192,Texture.T_float, Texture.F_r16)
img_out.makeRamImage()
img_out.setKeepRamImage(True)

shader = Shader.load_compute(Shader.SL_GLSL, "bw.glsl")
dummy = NodePath("dummy")
dummy.set_shader(shader)
dummy.set_shader_input("img_in", img_in)
dummy.set_shader_input("img_out", img_out)
sattr = dummy.get_attrib(ShaderAttrib)

base.graphicsEngine.dispatch_compute((8192/32, 8192/32, 1), sattr, base.win.get_gsg())
base.graphicsEngine.extractTextureData(img_out,base.win.get_gsg())
img_out.setFormat(Texture.F_luminance)
img_out.write(Filename("output.png"))

bw.glsl:

#version 430

// Declare the texture inputs
layout (binding=0, r16) readonly  uniform image2D img_in;
layout (binding=1, r16) writeonly uniform image2D img_out;
layout (local_size_x = 32, local_size_y = 32) in;

void main() {

  ivec2 texelCoords = ivec2(gl_GlobalInvocationID.xy);
  vec4 pixel = imageLoad(img_in, texelCoords);
  imageStore(img_out, texelCoords, pixel);
}

At the Beginning, all looked fine. When comparing the images in win media player, i saw the same image.
but somehow, output texture, althought in 16bit format, is just halph the size of the original file,
from 68867kb to 31068kb :neutral_face:
Any Idea how this could happen?