在opengl es片段着色器中，如何将纹理移动一些像素值？

Question

I am currently using multiple textures for a rendering job. All textures are passed to the same fragment shader for processing. One of the textures need to be shifted by some pixel values. How to achieve this in fragment shader code?

I have tried texture2D(tex, texCoord.xy + vec2(shiftx, 0.0)), where shiftx is a less than 1 float value. This does not work properly. Ideally, the area where the texture shift away should be blank, but the shader uses the last pixel color to fill the area. If this area can be cleared, this could be one method. But is there any other solutions?

Thanks!

Answer 1

No. The primitive defines the drawing area so if you are using multiple textures in a single shader that is not possible (if only 1 texture is used then rather offset the primitive). So what you need to do is check the coordinate and if the values are out of bounds (less then zero or more then one) do a separate logic as in not including the texture there.

Some source code might be helpful to suggest you a proper solution.

Just a note here it usually makes sense to do the coordinate translation in the vertex shader. You need to use another varying coordinates like varying lowp vec2 offsetedTexCoord for it then.

Answer 2

There's more than one way to go about this. The easiest is to check if one of the wrap types will do what you need. This one fills with a solid color instead of the last pixel which you said was causing trouble.

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_BORDER);

If a wrap doesn't give the desired effect, you already know where the cutoff for the shift is. So discard the fragment or set it to any color you want.

if (texCoord.x < shiftx) discard;