What range of values does my vertex shader need to output?

Question

I'm using openGL, and trying to draw a cube, but it won't show up. I used transform feedback and found out my vertex shader is outputting these values

    [0] -0.500000000    float
    [1] -0.500000000    float
    [2] -3.54020095 float
    [3] -4.50000000 float
    [4] -0.500000000    float
    [5] -0.500000000    float
    [6] -4.55025101 float
    [7] -5.50000000 float
    [8] -0.500000000    float
    [9] 0.500000000 float
    [10]    -3.54020095 float
    [11]    -4.50000000 float
    [12]    -0.500000000    float
    [13]    0.500000000 float
    [14]    -3.54020095 float
    [15]    -4.50000000 float
    [16]    -0.500000000    float
    [17]    -0.500000000    float
    [18]    -4.55025101 float
    [19]    -5.50000000 float
    [20]    -0.500000000    float
    [21]    0.500000000 float
    [22]    -4.55025101 float
    [23]    -5.50000000 float
    [24]    -0.500000000    float
    [25]    0.500000000 float
    [26]    -3.54020095 float
    [27]    -4.50000000 float
    [28]    -0.500000000    float
    [29]    0.500000000 float
    [30]    -4.55025101 float
    [31]    -5.50000000 float
    [32]    0.500000000 float
    [33]    0.500000000 float
    [34]    -3.54020095 float
    [35]    -4.50000000 float
    [36]    0.500000000 float
    [37]    0.500000000 float
    [38]    -3.54020095 float
    [39]    -4.50000000 float
    [40]    -0.500000000    float
    [41]    0.500000000 float
    [42]    -4.55025101 float
    [43]    -5.50000000 float
    [44]    0.500000000 float
    [45]    0.500000000 float
    [46]    -4.55025101 float
    [47]    -5.50000000 float
    [48]    0.500000000 float
    [49]    0.500000000 float
    [50]    -3.54020095 float
    [51]    -4.50000000 float
    [52]    0.500000000 float
    [53]    0.500000000 float
    [54]    -4.55025101 float
    [55]    -5.50000000 float
    [56]    0.500000000 float
    [57]    -0.500000000    float
    [58]    -3.54020095 float
    [59]    -4.50000000 float
    [60]    0.500000000 float
    [61]    -0.500000000    float
    [62]    -3.54020095 float
    [63]    -4.50000000 float
    [64]    0.500000000 float
    [65]    0.500000000 float
    [66]    -4.55025101 float
    [67]    -5.50000000 float
    [68]    0.500000000 float
    [69]    -0.500000000    float
    [70]    -4.55025101 float
    [71]    -5.50000000 float
    [72]    -0.500000000    float
    [73]    0.500000000 float
    [74]    -3.54020095 float
    [75]    -4.50000000 float
    [76]    0.500000000 float
    [77]    0.500000000 float
    [78]    -3.54020095 float
    [79]    -4.50000000 float
    [80]    -0.500000000    float
    [81]    -0.500000000    float
    [82]    -3.54020095 float
    [83]    -4.50000000 float
    [84]    -0.500000000    float
    [85]    -0.500000000    float
    [86]    -3.54020095 float
    [87]    -4.50000000 float
    [88]    0.500000000 float
    [89]    0.500000000 float
    [90]    -3.54020095 float
    [91]    -4.50000000 float
    [92]    0.500000000 float
    [93]    -0.500000000    float
    [94]    -3.54020095 float
    [95]    -4.50000000 float
    [96]    -0.500000000    float
    [97]    -0.500000000    float
    [98]    -3.54020095 float
    [99]    -4.50000000 float
    [100]   0.500000000 float
    [101]   -0.500000000    float
    [102]   -3.54020095 float
    [103]   -4.50000000 float
    [104]   -0.500000000    float
    [105]   -0.500000000    float
    [106]   -4.55025101 float
    [107]   -5.50000000 float
    [108]   -0.500000000    float
    [109]   -0.500000000    float
    [110]   -4.55025101 float
    [111]   -5.50000000 float
    [112]   0.500000000 float
    [113]   -0.500000000    float
    [114]   -3.54020095 float
    [115]   -4.50000000 float
    [116]   0.500000000 float
    [117]   -0.500000000    float
    [118]   -4.55025101 float
    [119]   -5.50000000 float
    [120]   -0.500000000    float
    [121]   -0.500000000    float
    [122]   -4.55025101 float
    [123]   -5.50000000 float
    [124]   0.500000000 float
    [125]   -0.500000000    float
    [126]   -4.55025101 float
    [127]   -5.50000000 float

[0] is an x, [1] is a y, [2] is a z, [3] is a w, [4] is the next vertex's x, etc. What is wrong with these values, because nothing is showing up?

If it helps, it works when I manually divide by w in the vertex shader.

Edit More details are:

My vertex shader is

#version 410 core
uniform mat4 View, Model, Project;
in vec4 vPosition;
out vec4 position;

void main(){
    position=Project*Model*View*vPosition;
    gl_Position=position;
}

out vec4 position is being transform fed back.

My fragment shader simply colors any fragments red.

The matrices are

model

1,0,0,0
0,1,0,0
0,0,1,0
0,0,0,1

view

1,0,0,0
0,1,0,0
0,0,1,5
0,0,0,1

projection

1,0,0,0
0,1,0,0
0,0,-1.01005030,1.00502515
0,0,-1,0

The vPositions are

const GLfloat cube_positions[] =
{
    -.5f, -.5f, -.5f, 1.0f,
    -.5f, -.5f, .5f, 1.0f,
    -.5f, .5f, -.5f, 1.0f,
    -.5f, .5f, .5f, 1.0f,
    .5f, -.5f, -.5f, 1.0f,
    .5f, -.5f, .5f, 1.0f,
    .5f, .5f, -.5f, 1.0f,
    .5f, .5f, .5f, 1.0f
};

Indexed by glDrawElements and these in the Element Array Buffer

const GLushort cube_indices[] =
{
    0, 1, 2, 3, 6, 7, 4, 5,
    0xFFFF,
    2, 6, 0, 4, 1, 5, 3, 7
};

and 0xFFFF is the restart index

We require some more code like how you send this data to the shaders, how you setup and actually draw the cube. — Steadi, Aug 20 '14 at 12:51
I added more information. What I'm trying to ask is if these are the right values, and, if they are, what are some reasons they could be culled before they are drawn. — barbiedude97, Aug 21 '14 at 04:25

score 1 · Accepted Answer · answered Aug 24 '14 at 04:08

1

The range of values your vertex shader needs to output is [-1.0, 1.0] for the coordinates inside the view volume. This range is after the division by w.

It looks like your coordinates are within this range. For example for the first vertex:

[0] -0.500000000    float
[1] -0.500000000    float
[2] -3.54020095 float
[3] -4.50000000 float

Applying the division by w gives for x, y and z coordinates:

0.11111
0.11111
0.78671

This vertex is within the view volume, as well as all the other coordinates.

answered Aug 24 '14 at 04:08

Reto Koradi

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That is what I thought. Thanks. Do you have any idea why these could be culled? – barbiedude97 Aug 26 '14 at 14:34
Not sure. I have an article on my personal web site that lists some common reasons why rendering does not show up: http://retokoradi.com/2014/04/21/opengl-why-is-your-code-producing-a-black-window/. – Reto Koradi Aug 26 '14 at 15:17

What range of values does my vertex shader need to output?

1 Answers1