Using a texture as a LUT

[Yves Surrel]

Hi 

I try to use a lookup table (LUT) in a shader. (I went to that because I discovered that on my android device, I could not use a regular buffer 

In the shader, I added the lines:

{

// uniform float[256]; 

uniform sampler2D lut;

void main() {

    float color = texture2D(lut, tex_coord0).r / 255.0;

    gl_FragColor = vec4(vec3(color), 1.0);

}

expecting to have a horizontal gray variation as the LUT size will be (1, 256), increasing from 0 to 255.

and in my widget code I put:

{

    def __init__(self, **kwargs):

        # Instead of using canvas, we will use a RenderContext,

        # and change the default shader used.

        self.canvas = RenderContext(use_parent_projection=True,

                                    use_parent_modelview=True,

                                    use_parent_frag_modelview=True)

                                    

        # call the constructor of parent

        # if they are any graphics object, they will be added on our new canvas

        super(ShaderWidget, self).__init__(**kwargs)

        # Linear LUT

        self.lut = range(256)

        

        self.tex = Texture.create(size=(1,256), colorfmt='luminance')

        BindTexture(texture=self.tex, index=1)

        self.tex.add_reload_observer(self.populate_texture)

        self.populate_texture(self.tex)

        self.canvas['lut'] = 1

        

    def populate_texture(self, texture):

        # data = b''.join(map(bytes,self.lut))

        data = struct.pack('h'*len(self.lut), *self.lut)

        texture.blit_buffer(data, size=(1,256), colorfmt='luminance')

        

}

My widget remains desperately black. What am I missing ? (Elsewhere in the code I set the shader, this has already been tested without the LUT and is working fine.)

It is not clear what the index in BindTexture should be. Could it be arbitrary (e.g. 4 in this example) ? I think that at least it should be the same as in the line self.canvas['lut'] = 

Thanks in advance.

[Gabriel Pettier]

I *think* the multitexture example might help you :)

https://github.com/kivy/kivy/blob/master/examples/canvas/multitexture.py

from a quick look, i think you need to put the BindTexture in a canvas
block.

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[Yves Surrel]

Hi tshirtman

Yes, you are perfectly right.

I indeed had solved my issue after comparing with multi texture.py, and yes, the BindTexture has to be put within a canvas block.

There were two other flaws: the string to use in struct.pack() should be 'B' (unsigned byte) and not 'h', and in the shader the color returned by texture2D does not need be divided by 255, as it is already normalized to 1.0

Anyway, thanks for your time.