[英]glTexImage2D data not filled as expected
Willing to understand glTexImage2D, I wrote following test code in Python:为了了解glTexImage2D,我在Python中编写了以下测试代码:
import numpy as np
import OpenGL.GL as gl
import glfw
# initiating context
glfw.init()
w = glfw.create_window(100, 100, 'dd', None, None)
glfw.make_context_current(w)
# prepare sample image
width, height = 3, 3
image = np.array([i for i in range(width * height * 3)], dtype='ubyte')
# prepare texture
t = gl.glGenTextures(1)
gl.glBindTexture(gl.GL_TEXTURE_2D, t)
iformat = gl.GL_RGB
# upload texture
gl.glTexImage2D(gl.GL_TEXTURE_2D, 0, iformat, width, height, 0, iformat, gl.GL_UNSIGNED_BYTE, image)
# read texture
p = gl.glGetTexImage(gl.GL_TEXTURE_2D, 0, iformat, gl.GL_UNSIGNED_BYTE)
# display pixel values
pixels = iter(p)
for x in range(width):
for y in range(height):
print([next(pixels), next(pixels), next(pixels)])
and unexpectedly, printed result was following:出乎意料的是,打印结果如下:
[0, 1, 2]
[3, 4, 5]
[6, 7, 8]
[12, 13, 14]
[15, 16, 17]
[18, 19, 20]
[24, 25, 26]
[0, 0, 0]
[0, 0, 93]
Obviously some values are missing.显然有些值是缺失的。 Why is this happening?
为什么会这样? Am i missing something?
我错过了什么吗?
If an RGB image with 3 color channels is loaded into a texture object, GL_UNPACK_ALIGNMENT
must be set to 1:如果将具有 3 个颜色通道的 RGB 图像加载到纹理 object 中,
GL_UNPACK_ALIGNMENT
必须设置为 1:
gl.glPixelStorei(gl.GL_UNPACK_ALIGNMENT, 1)
gl.glTexImage2D(gl.GL_TEXTURE_2D, 0, iformat, width, height, 0, iformat, gl.GL_UNSIGNED_BYTE, image)
GL_UNPACK_ALIGNMENT
specifies the alignment requirements for the start of each pixel row in memory. GL_UNPACK_ALIGNMENT
指定 memory 中每个像素行开始的 alignment 要求。 By default GL_UNPACK_ALIGNMENT
is set to 4. This means the start of each line of the image is assumed to be aligned to an address which is a multiple of 4. Since the image data is tightly packed and each pixel is 3 bytes in size, the alignment has to be changed.默认情况下
GL_UNPACK_ALIGNMENT
设置为 4。这意味着图像的每一行的开头都假定与 4 的倍数的地址对齐。由于图像数据被紧密打包并且每个像素大小为 3 个字节,因此alignment 必须更改。
If the width of the image is divisible by 4 (more exactly if 3*width
is divisible by 4) then the error is not noticeable.如果图像的宽度可被 4 整除(更准确地说,如果
3*width
可被 4 整除),则错误不明显。
Since you are reading back the texture from the GPU, you also need to change GL_PACK_ALIGNMENT
:由于您正在从 GPU 读回纹理,因此您还需要更改
GL_PACK_ALIGNMENT
:
gl.glPixelStorei(gl.GL_PACK_ALIGNMENT, 1)
p = gl.glGetTexImage(gl.GL_TEXTURE_2D, 0, iformat, gl.GL_UNSIGNED_BYTE)
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